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Institutional Archive of the Naval Postgraduate School
Calhoun: The NPS Institutional Archive
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Theses and Dissertations l. Thesis and Dissertation Collection, all items
1985-09
A VHF-FM Digital Selective Calling System
mathematical model using grade of service criteria
Decker, James Wade.
http://hdl.handle.net/10945/21618
Downloaded from NPS Archive: Calhoun
Calhoun is the Naval Postgraduate School's public access digital repository for
¿ (8 D U DLEY research materials and institutional publications created by the NPS community.
| Calhoun is named for Professor of Mathematics Guy K. Calhoun, NPS's first
th
KNOX appointed — and published — scholarly author.
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DUDLEY KNOX LIBRARY
NAVAL POSTGRADUATE SCHOOL
MONTEREY, CALIFORNIA 95945
NAVAL POSTGRADUATE SCHOOL
Monterey, Galifornia
THESIS
A VHF-FM DIGITAL SELECTIVE CALLING SYSTEM
MATHEMATICAL MODEL
USING GRADE OF SERVICE CRITERIA
by
James Wade Decker
SEPTEMBER 1985
Thesis Advisor: E nman Perry
Co-advisor: Carl R. Jones
Approved for public release; distribution is unlimited.
1224125
Unclassified
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REPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM
4. TITLE (and Subtitie)
A VHF-FM Digital Selective Calling System
5. TYPE OF REPORT & PERIOD COVERED
Master's Thesis
Mathematical Model l September 1985
Using Grade GE Service Criteria 6. PERFORMING ORG. REPORT NUMBER
. AUTHOR(e) 8. CONTRACT OR GRANT NUMBER( s)
James Wade Decker
. PERFORMING ORGANIZATION NAME AND ADORESS : E t T ASK
Naval Postgraduate School
Monterey, California 93943-5100
. CONTROLLING OFFICE NAME AND ADDRESS | 12. REPORT OATE |
Naval Postgraduate School september 1985
Monterey, California 93943-5100 13. NUMBER OF PAGES
117
: MONITORING AGENCY NAME & ADDRESS(If different from Controlling Office) 15. SECURITY CLASS. (of this report)
Dnelicscit lca
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17. DISTRIBUTION STATEMENT (of the abetract entered in Biock 20, if different from Report)
18. SUPPLEMENTARY NOTES
19. KEY WORDS (Continue on reveree side if necessary and identify by block number)
Digital Selective Calling DoE
SFLCALL VAF
Carrier Sense Multiple Access Grade of Service
Random Access Channel ALOFA
Maritime Communications Marine radio
20. ABSTRACT (Continue on reveree side if neceseary and identity by block number)
This thesis presents a mathematical model of a maritime VHF-FM Digital
Selective Calling (DSC) System using grade of service as a criterion to
determine if a single DSC channel can accommodate both distress and
commercial calling. The model calculates the probability of a call being |
delayed, the average delay of a call, the probability of a call being
answered within a certain time frame, and the throughput for the random
access calling systems of ALOHA, Slotted ALOHA, Slotted ALOHA with Capture,
"s
DD E: 1473 EDITION OF 1 NOV 65 Is OBSOLETE
S/N 0102+ LF- 014-6601 i
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SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered
20. -Abstract continued)
Nonpersistent Carrier Sense Multiple Access (CSMA), and 1-Persistent CSMA,
Analysis of the results indicates that all VHF DSC calling can be made on
a single channel. Without regard to a cost-benefit evaluation, it was also
determined that 1-Persistent Carrier Sense Multiple Access was the superior
random access calling system to utilize for the VHF-FM Digital Selective
Calling system.
` ROSA E sx t
A EE
Uses eet ed
— ÉL LMEMAXSLLILOLLULLeLUO7UN'GM n .—————— HU NN
SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered)
Approved for public release; distribution is unlimited.
A VHF-FM Digital Selective Calling System
Mathematical Model
Using Grade of Service Criteria
by
James Wade Decker
Lieutenant, United States Coast Guard
B.S., United States Coast Guard Academy, 1976
Steeda partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN TELECOMMUNICATIONS SYSTEMS MANAGEMENT
fron the
NAVAL POSTGRADUATE SCHOOL
September 1985
ABSTRACT
This thesis presents a mathematical model of a maritime
VHF-FM Digital Selective Calling (DSC) System using grade of
service as a criterion to determine if a single DSC channel
can accommodate both distress and commercial calling. The
model calculates the probability of a call being delayed, the
average delay of a call, the probability of a call being
-answered within a certain time frame, and the throughput for
the random access calling systems of ALOHA, Slotted ALOHA,
Slotted ALOHA with Capture, Nonpersistent Carrier Sense
Multiple Access (CSMA), and 1-Persistent CSMA. Analysis of
the results indicates that all VHF DSC calling can be made on
a Single channel. Without regard to a cost-benefit
evaluation, it was also determined that l-Persistent Carta
Sense Multiple Access was the superior random access calla
system to utilize for the VHF-FM Digital Selective Cadm
system.
iB.
TABLE OF CONTENTS
Wu MEME MOBNWNE DIGITAL SEBECTIVE CALLING SYSTEM --
A. BACKGROUND -~----------------------------------
B. CURRENT MARITIME COMMUNICATIONS METHODS -------
1. VHF-FM Radiotelephone ---------------------
MEN NM ui cNSudewDund Radio -----------------
3. EHF MARISAT/INMARSAT ----------------------
C. DIGITAL SELECTIVE CALLING (DSC) SYSTEM --------
CHARACTERISTICS OF A DSC SYSTEM -------------------
A. OPERATIONAL CHARACTERISTICS ----------~--------
ls Digital Selective Call Sequence Format ----
2 eee pes OL NSC "Call Sequences ——-—-—-——------~--
LE TM T S RACTHMISTICS ————-.-----....------
l. Technical Format of the Call Sequence -----
IMEEM Ncc .cLLLl---
C. METHOD OF OPERATION ---------------------------
RANDOM ACCESS CALLING SYSTEMS ---------------------
O SDN Lee LL 2 2 NMLL
B. ALOHA TYPE CALLING SYSTEMS --------------------
IMEEM MM ee e o 2
Petia cee MINUM = ee
IS uWMNEUNMONM weh Capture ----------------
SA SERLE AQOESS O ====-=-==========-
pers sente COMA Pretocol ---------------
20
zs
2.9
ZS
24
26
30
30
32
S
40
44
46
48
2 l-Persistent CSMA Protocol ---------------- 49
3. P-Persistent CSMA Protocol ---------------- 3m
EV CURRENT VHF-FM DIGITAL SELECTIVE CALLING MODEL ---- 55
A. INTRODUCTION ---------------------------------- 25
B. TRAFFIC ANALYSIS ------------------------------ 56
C. OTHER FACTORS INFLUENCING THE NUMBER OF SGALLIS
CHANNELS -------------------------------------- Sy
l. Duration of DSC Call Sequences ------------ 58
2. Number and Duration of VHF DSC Sequences -- 59
3. Use of Scanning Devices ------------------- 60
4. Position of Coast and Ship Stations ------- 61
D. CCIR INTERIM WORKING PARTY 8/10 VHF DSC SYSTEM
MODEL ----------------------------------------- 62
E. SCANNING WITH A 1-SECOND DOT PATTERN ---------- 70
F. INTERIM WORKING PARTY DSC MODEL CONCLUSIONS --- 71
UE PROPOSED VHF-FM DIGITAL SELECTIVE CALLING MODEL --- 73
A. GENERAL MODELING REQUIREMENTS ----------------- 73
B. SPECIFICATIONS OF THE PROPOSED MODEL ---------- 74
l. | Format and Duration of Call Sequences ----- 75
2. Channel Loading --------------------------- Do
3. Grade of Service -------------------------- 80
C. APPLICATION OF THE PROPOSED MODEL ------------- 81
VIR CONCLUSIONS AND RECOMMENDATIONS ------------------ - Gis
A. CONCLUSIONS ----------------------------------- 106
B. RECOMMENDATIONS ------------------------------- TIO
LIST OF REFERENCES -------------------------------------- BES
INITIAL DISTRIBUTION LIST ------------------------------- 12106
Pa INIM BEBE DIGITAB SELECTIVE CALLING SYSTEM
The establishment of communications between shore and
ship Stations, and between ships has become markedly
difficult due to increased maritime activities, limited radio
officer watch periods, radio propagation characteristics, and
Himited availability of radio frequencies [Ref. 1:p.l!].
Delays of 6 to 18 hours are not uncommon, The Digital
Selective Calling (DSC) System has been developed to
alleviate the Torie of establishing communications while
providing means for a global maritime distress system. The
current recommendation for a VHF-FM DSC system has designated
one radio channel to be used exclusively for distress and
safety calling, and another hcm to be used for commercial
calling. The recommendation, based on a maximum channel
loading (6) of 0.1 Erlang [Ref. 2:p. 2], may not be practical
since there is:only limited availability of radio frequencies
and the use of one exclusively for distress may be
inefficient. This thesis develops a mathematical model of a
VHF-FM Digital Selective Calling System based on several
grade of service criteria, instead of channel loading, to
determine if a single DSC channel can accommodate both
distress and commercial calling.
A general history of digital selective calling, as well
as current communications methods used by the maritime mobile
service, is presented in this chapter. The operational and
technical characteristics of the proposed international DSC
system follow in )@nhaprer jie Chapter III contains a
description of various methods used to gain access to a radio
channel called random access calling systems. The current
model used to determine the number of required calling
channels is shown in Chapter IV. The author's’ proposed
mathematical model, along with various grade of service
calculations, are presented in Chapter V. Finally, Chapter
VI contains the.conclillis rons and recommenda rions
A. BACKGROUND
Communications are carried out over frequency bands
allocated to the maritime mobile service by the International
Telecommunication Union (ITU). Morse Code telegraphy (CW)
has been used since the beginning of maritime communications
because of its ability for transmission and reception under
almost any atmospheric conditions. However, CW has fallen
out of favor because of three ma jor reasons: [Ref. 3:p.77]
(1) The transmission of a message is a slow, manual
process.
(2) It may take a long period of time to make the initial
contact with a ship in order to send a message.
(3) It requires the use of separate distress frequencies.
The development of a system to improve communications
calling has long been recognized as being essential to the
maritime community. Studies into this area have revolved
around the use of autoalarms, radio-teletype (RTTY), tone-
selective calling (SSFC), and satellite communications. While
the bulk of future maritime communications traffic is
expected to be carried by commercial satellites, the need for
a global emergency system which did not require human
monitoring and was simple to use predicated the development
of a EE ced selective calling system [Ref. 3:p. 77].
Digital selective calling is designed to use the
techniques of digital transmission for the establishment of
communications, or for the passing of distress and safety
information. Once the receiving station has been alerted by
me DSC call that traffic pends, further communications are
conducted on designated "working" channels. These "working"
channels could be another DSC channel or they could be any
other type of communication method available, (e.g. HF voice,
VHF-FM radiotelephone, etc).
The current CCIR recommendation for a VHF Daneel
Selective Calling system has designated one BE. to be
used exclusively for ess and safety calling, and another
channel to be used for commercial calling. The exclusive
channel for distress and safety calling is based on the
ecotablishment sc 0.1 Eclang aswuhe maximum traffic loading
for a DSC calling channel in order to achieve the grade of
per vice eQUudeecd Meme distress calling [Ref. 2:p.2]. The
maximum channel loading was chosen as O.1 Erlang because, for
random access calling systems such as the University of
Hawaii's ALOHA system, a call becomes delayed an
exponentially increased period of time as the channel load
increases beyond about 0.1 Erlang. Figure 1 shows this delay
versus channel loading situation for two random access
calling systems, ALOHA and Slotted ALOHA.
D. Delay
ALOHA S ALOHA
200
150
100
SQ
Figure l. Delay as a Function of Channel Load
Submissions to the Interim Working Party of the TITUS
International Radio Consultative Committee (CCIR) theorized a
worst-case situation based on the number of VHF working
channels indicated in Appendix 18 to the ITU Radio
10
Pepmbatvons (eng. 28 working channels in total) [Ref..2:p.8].
It was indicated that the maximum capacity of a single
working channel would be 0.8 Erlangs [Ref. mp. 18].
Preliminary calculations based on the Erlang channel loading
factor suggested that this level of operation might be
supported by a single DSC channel [Ref. 4:p.22]. However, the
traffic intensity should not dictate the grade of service
(probability of finding the calling channel busy) because in
some circumstances it prevents efficient channel utilization,
For example, designating the channel loading limits the
number of calls that may be placed on that channel. If more
callers desire access to the calling channel than the loading
permits, two or more channels would have to be allocated to
service their needs, regardless of any other Eb ue SUR
as the delay on calls through the calling system, or the
probability that a call will reach its destination. The
grade of service parameters listed in Table 1 can be
important in designing a VHF-FM DSC system and as such will
dictate the channel loading allowed on the channel.
Therefore, a mathematical model for a VHF DSC system will be
formulated based on the grade of service factor. This model
en be used to calculate the maximum allowable migajas
pi
volume on a radio calling channel given a particular grade of
Service.
LADA
GRADE OF SERVICE PARAMETER
l. Probability of delay exceeding a specified amount
of time.
2. Probability of delay.
3. Average delay of all calls through the system.
4. Channel Throughput
b. CURRENT MARITIME COMMUN@GAT TONS METHODS
At the present time, there are three generally used
classes of communications emissions. These are VHF-FM marine
radiotelephone, HF single side band radio, and EHF satellite
radio. Morse Code, which used to be the mainstay of maritime
communications, is, in the author's opinion from experience
with Coast Guard communications stations, infrequently used
in today's mobile maritime service.
1. VHF-FM Radiotelephone
The range of a VHF-FM marine radiotelephone is
limited to 50-60 miles [Ref. 5:p. 2]. The ship station must
be within that distance of a coastal station in order to have
telephone communications ashore. The basic coastal stations
used for Pane ELE ashore are marine radio operators’) and
U.S. Coast Guard stations. VHF-FM usage has grown so fast
Nez
that there are insufficient ITU Radio Regulation Appendix 18
frequencies for today's environment [Ref. 6].
In addition to voice communications, VHE-FM is being
used for tone-generated selective calling in some limited
geographical areas such as the Great Lakes and Mississippi
River. This selective calling system, Sequential Single
Frequency Code (SSFC) was placed into service in 1972, but
never became widely used because of the severe limitations
wered below [PRef. 3:p. 76]:
(1) The potential number of future subscribers can not be
accommodated.
(2) SSFC is limited to full voice bandwidth operation.
6) call can not be prioritized.
(4) SSFC is unable to propose the use of other : working
frequencies while the call is in remote control at the
receiving end.
2. HF Single Side Band Radio
APS currently used for radio teleprinting (SITOR),
radio telephony, and automatic distress and safety alerting
(AUTOALARM). HF signal characteristics have made it an
excellent vehicle for long haul communications, filling the
gap between VHF-FM range and a range of up to several
thousand miles. However, HF is limited by propagation
phenomena and atmospheric disturbances which can affect its
performance. There are also few coastal stations operating on
HF frequencies and the number of ship stations wanting to use
E3
them is in the thousands. The Coast Guard indicated in a
report that for ships on the high seas, the average time
required to contact a ship station was in excess of 6 houk:
[Ref. 5:9. A2].
3. EHF MARISAT/INMARSAT
The Maritime Satellite Organization (MARISAT) and the
international Maritime Satellite Organization CINMARSAT)
provide satellites necessary for improving maritime
communications, thereby assisting in improving distress añd
safety communications, efficiency and management oí ships,
and maritime public correspondence services [Ref. Sap. 2].
Using EHF £requencies and an installed satellite
terminal, ship stations are connected into the papas
switched telepnone network and Telex services via a sateilite
and through a number oi coastal earth stations. Three
orbiting satellites provide coverage in the Paci aaa
Atlantic, and Indian ocean regions. It was projected that
over 2100 ship subscribers would be operating on this system
by 19368 | BHO 520 RENE
tJ
he high initial investment reauired
to purchase terminals and equipment prohibits many potential
customers from using MARISAT/INMARSAT [Ref. 9].
C. DIGITAL SELECTIVE CATELNG (DSC) SWSUTEM
Selective calling systems were described as ear!v as the
ITU's Ninth Plenary Assembly of the International Radio
Consultative Committee (CCIR, 1959). Improvements to the wars
in which maritime calling was performed were clearly
required. As a result of the CCIR assembly, the Federal
Republic of Germany and the United States conducted tests on
selective calling systems. These tests and further tests were
studied in the 1963 and 1966 Plenaries by a study group of
Mie CCTR [|Ref. l:p. 2].
The World osa tae Radio Conference (WARC, 1967)
amended the ITU Radio Regulations to permit the use of
maritime selective calling and adapted the German-developed
Sequential Single Frequency Code (SSFC) system to meet
immediate operational requirements. The United States did not
Support the SSFC system because it was not suitable for all
classes of radio emissions, e.g. HF and VHF, in the maritime
Mmepale sservices|Ref. lisp. 2]. The ITU also called for’ the
development of a system with greater capability for future
Maritime use. The four major areas of concern using the SSFC
tone-generated selective calling technique were [Ref. 3:p.
po] :
(D> the potential number of future subscribers can not be
accommodated.
(2) SSFC was limited to full voice bandwidth operation.
Ao itizingla call could mot be done.
(4) SSFC was unable to propose the use of other working
frequencies while the call was in remote control at
the receiving end.
The United States proposed at the 1967 World
Administrative Radio Conference and again at the CCIR Interim
15
Meeting in 19/72 that digital selective calling techniques
should be adopted. Because the United States could not
produce experimental data supporting its claims, the proposal
was denied. After the 1972 CCIR meeting, the U.S. Maritime
Administration initiated the development and testing of a DSC
system. The results of this effort are listed in Figure 2 and
show that digital selective calling is capable of
satisfactory operation in the maritime frequency bands of HF,
MF, and VHP [Ref mk p N
The CCIR, in its 1974 meeting, recommended the adoption
of a DSC system much bike the one proposed by the United
States. Later that year, the 1974 WARC established dedicated
DSC frequencies. The ITU's Fourteenth Plenary Assembly of the
CCIR © TOGO 1978) approved Recommendation 493-1 which
contained the operational and technical characteristics of
the DSC system [Ref. 10:p. 2-2]. Finally, the Fifteenth
Plenary Assembly of the CCIR (CCIR,1982) adopted the DSC |
operational and technical characteristics contained T GCIR
Recommendation 493-2 and the DSC operational procedures in
CCTR Recommendation E [Ref. POCA These last
recommendations were slight modifications of the DONIS
approved operational and technical characteristics.
A parallel study in the area of maritime distress
communications had been ongoing since 19/79 when an
International Conference on Search and Rescue adopted a new
16
SELECTIVE SELECTIVE
CALLS
TLV
FREQUENCY LOCATION | a enr F M z | RECEIVED >
DIRECTION OF TRANSMISSZON (Baud) SENT (OP adrsawes (1 or 2
addresses
compare)
compare) |
HICH FREQUENCY
KMI-S.$. MONTEREY
Shore-to-Ship 75 876 615 70 647 74
150 6,635 3,495 74 3,384 | 84
TOTAL 75 & 150 §,491 4,020 73 4,531 83
Ship-to-Shore 7S 255 224 88 239 94
150 2,429 ATL 75 2,057 85
TOTAL: 75 & 150 2,684 2,055 77 2,296 | 86
NMH-USCG CUTTERS
Shore-co-Ship 75 93 57 61 66 TL
: 150 6,945 5,244 76 5,613 81
TOTAL 75 & 150 7,038 2:301 75 5,579 81
Ship-to-Shore 150 4,710 3,289 70 3,699 79
HF TOTAL 75 15224 |. 896 73 952 | 78
150 18,699 13,769 74 15,233 az
75 & 150 192923 14,665 74 16,205 -8l
MEDIUM. ERECUPNC?
NMF-USCG CUTTERS
Shore-to-Ship
Shíp-to-Shore
MF TOTAL
VERY HICH FREOUESCT
NMH-USCG CUTTERS
Shore-to-Ship 600
600
1,200
600 & 1200
Shíp-to-Shore
TOTAL
VHF TOTAL 600
1,200
600 & 1200
PIGURE 2,
Summary of DSC Test Results
Iz
search and Rescue Convention, This Conference asked the
International Maritime Organization (IMO) to develop a global
maritime diste and safety system to provide the
telecommunications for the International Convention on
Maritime Search and Rescue [Ref. 12]. In the fall of 197205
the IMO proposed its Future Global Maritime Distress and
Safety System (FGMDSS). The FGMDSS was to be developed on
the principles of digital selective calling which meant that
short-wave CW communications would no longer be used for
distress [Ref. 3:p. 78]. IMO's announcement concerning the
FGMDSS provided a strong impetus for the development of a DSC
system.
In the fall of 1983, worldwide testing of the final DSC
system was conducted on the HF frequencies. Figure 3
provides the results of the testuncs CCIR Interim Working
Party (IWP) 8/10, which has overall responsibility fone
development and implementation of the DSC system, considered:
the results of the trials impressive. The IWP also
pronounced that the DSC system was a viable system, for
alerting distress cases by terrestrial radio-communication
[Ref e.
This introduction and short history of digital selle
calling should leave the reader with a basic understanding of
why DSC is an important step forward in the area of maritime
communications. In the next chapter, the operational and
technical aspects of the DSC system will be discussed.
18
CSC Success Probaoilities During Trials
| Call atscaeroes
|
Teansmi tting | Sates
Sinale Freazency (MHz) IMuiti- | |
Stacion mae |e lL ni... | i1 m | is | 17 frequency! Comoosi te!
Rogaland 177/19 46/38 4941/36 414/46 $343/46 _40+3/46 229/230 35/26
Portsmouth -18/10 3/0 9+3/24 11+1/2% 20+1/24 21/2% 9%+9/120 24/24
Sydney 20/10 32/48 27+1/%0 6/46 38/40 37/60 217«41/220 45/36
Tokyo 21/10 15/18 1551/18 16/18 15/16 1341/18 80/80 16/16
- Guan 22/10 3/0 46/36 44/46 46/46 42+1/23 230/239 46/46
Portsmouth 24/10 Q/Q 2L+1/31 2741/31 31/31 23+2/30 155/155 31/31
Somerz3n 25/0 $3/43 43/43 43/13 43/33 4042/42 221/221 42/42
Tokyo 27/10 18/18 18/18 18/16 16/16 14/18 80/8Q 16/15
Guam 29/10 .- 0/0 .. 38/42 31/42 39/42 38+1/42 200/210 40/42
Somerton ^ 31/10 45/58 48/45 46/46 46/46 "36/46 228/228 “43/43
[2u = L 10/15 13/18 - 15/18 16/16 . 1951/18 30/80 18/18
Izu 4/11 16/16 16/16 16/16 16/16 16/16 80/80 16/16
~ Rogaland 5/11 46/46 46/46 4541/45 4242/46 38+7/46 230/230 46/46
Norddeicn 7/1] 48/48 46/46 43+1/44 41»2/44 J8+3/44 229/229 46/36
Somertsn 95th 42/42 42/42 82/42 42/52 42/52 Zecreue 2/5
Ivan Chernykh 9/11 1/1 3/3 2/2 4/5 2+1/3 14/14 0/0
Totais 353 47347 . 44448 49943 464+22 2578+10 S15
By $18 322 513 S13 2627 518
2 of calls received
without errors 95.91 — 91.31 85.06 25524 90.10 98.13 99.32
Additional % of
calls received «ith
errors Sut no errors
in ID or coordinates #1.35 — *1.53 *1.74 +4 .27 *0.38
— C -—À——— Á—————————————o ——— ' ^ ——
Entries: Call attemats success?ui/cail actemo?ts transmittad. A + indicates aadit?onai
numoer of calls received with errors, but a0 errors in [D or coordinates.
Rotas:
1) The USA stations (Portsmouth and Guam) did not transmit on 4 MHZ. i
2) In cases wnere an analysis of receivers logs casts doubt upon a given transmitter
performance, the transmitter was, nevertheless, assumed to have performed correctly
and the resulting low reception success rates were included in the above table.
This table, therefore, represents a “worst case” analysis of the trial results.
[t is probable that these results underestimate the performance of the participating
receiving stations. e
The combined logs of all receiving stations indicated that there was a problem
with 3 dney's scheduled transmissions an 3-11-1983. There were no propagation
anomalies reported for that day, however no station received calls on 4, 6, 3, or
13 MHz. Only Japan received a few calls on 17 MHz during the early mornin
(UTC). Therefore, the [WP disreaarded the data on 3-11-1983.
3)
g hours
Figure 3. Wortdwide HF DSC-Testing Results
19
II. CHARACTERISTICS OFM ATE a
Aw OPERATIONAL CHARA MERTSI CS
The operational characteristics of a digital selective
calling system must be compatible internationally, thus
requiring a standardized format for messase Crk ai The
recommended format of a call sequence (message format)
adopted by the CCIR for the international DSC system consists
Ot a format specifier, address, category, self-
identification, message data area, and end-of-sequence block,
each of which is explained below [Ref. 14:p. 1]. The format
of a call sequence is shown in Figure 4.
format specifier self-identification
| omen d | message 2]... | end of sequence
Figure 4. Format of a Call Sequence
A DSC call sequence is a one-way radio transmission
and can be one of two types. A call alert sequence is
initiated by a sending station indicating a desire to pass
communications. A call acknowledgement sequence is sent by a
receiving station, acknowledging receipt of a call alert
20
sequence. If the sending station has additional
communications to pass, both the sending and receiving
stations shift to a "working" channel to complete the
transmissions.
1, Digital Selective Call Sequence Format
a. Format Specifier
The format specifier indicates whether the call
Pe Uu weal), an all ships’ call, or a call to a
selective group of ships or stations. The selective group
could consist of one ship T station, a group of stations
having a common interest, or a group of ships in a specified
geographical area. In addition, the format specifier might
indicate the call is a special sequence for semi-automated or
automated VHF/UHF services (e.g. marking Sequences,
Signalling sequences).
b. Address
The address block specifies for whom the call is
designated. The address information of distress and all
ships calls will be contained in the format specifier block.
For all other types of calls, the address will be the unique
identification assigned to each called ship or station, or
special identifiers assigned to a group of ships having a
common interest or ships operating in a particular
geographical area.
21
The digital selective calling system has been
designed to have the selective address capacity tron
quantitatively meeting all of the international maritime
selective call requirements for the foreseeable future.
c. Category
The category block of information defines the
degree of priority of the call sequence. For a distress call
this information is again contained in the format specifier
block. All calls are prioritized within one of six
categories:
(1) Distress:
(2) MU Scenic
(3) Vital safety.
(4) Important safety.
(5) Ship business priority (the proposed definition is a
call authorized by the ship owner or agent requiring
immediate handling on the ship).
(6) Routine.
ag Self-Identification
This block simply contame rene ala station's
unique identification symbols.
e. Messages
There is a limit of 3 general messages allowed in
a call sequence, with the exception of distress calls which
have 4 messages. For a distress, call, the distress
22
information is contained within four messages and appears in
the following order:
(1) An indication of the m of distress.
(2) An indication of the distress position.
(3) The time that the distress position was valid.
(4) The type of communication which is preferred by the
ünit in distress for subsequent exchange of
communications.
of DSC Call Sequences
N
E
(D
0
If the call sequence is not part of a distress call,
then it will either be a telecommand call alert sequence or
an acknowledgement call sequence. A telecommand call alert
sequence contains information for setting up communications
on another channel, or may include terminal Gomer ol
functions, transmitter and receiver control functions, or
special purpose functions. An acknowledgement call sequence
is simply a call to a sending station verifying that a DSC
call alert sequence has been received.
me eo NCAL CHARACTERISTICS
Paralleling the operational characteristics of the
proposed DSC system are its technical characteristics. The
technical characteristics maintain standardization in the
international community in the areas of electrical
engineering and communications by specifying the bit patterns
HUND COS ichsorsthe format blocks [Ref. 14:p.7].
23
l. Technical Format of the Call Sequence
The recommended technical format of a call sequence
contains a dot pattern, phasing sequence, format specifier,
address, category, self-identification, up to four message
areas, an end-of-sequence block, and an error-checking
character [Ref. 1l14:p. 23]. This format, with the exception
of the dot pattern, phasing sequence, and error-checking
character is the same as the operational characteristics
format. The technical format of a typical routine message is
presented in Figure 4.
a. Dot Pattern
As investigations continued into DSC, scanning
devices were proposed by the Japanese so that a receiver
could listen to more than one frequency [Ref. .10:p. 3M
make use of a scanning device possible, it was necessam
add a signal which told the receiver to stop scanning. This
is called the "dot pattern" which switches between "zero" amd
"one" bits for a period of 2 seconds. HF and MF "dus
selective calling signals begin with a two-second dot
pattern. Dot patterns will not be used in acknowledgement
sequence calls, ship-to-shore calls with the exception of
distress calls, or VHF calls, unless more than one Dae
channel is being used [Ref. CCIR 14:p. 9].
b. Phasing Sequence
The phasing sequence provides information to the
receiver to permit correct bit phasing and unambusemMN
24
Mx uario ot the positions of the signals within a call
sequence.
The receiver phase synchronizes with the
transmitter by recognizing a specific pattern of symbols
Euer than keying on a change in the dot pattern. This
method of phasing was adopted in order to reduce false
Enronjzatwon caused by a bit error in the dot pattern.
DX/RX
Format Called
Dot pattern Phasing specifier part y Category
sequence address
2 symbols 5 symbols ] symbol
Telecommand Frequency | Frequency | End of
message message message sequence
2 symbols 3 symbols | 3 symbols | 3 symbols
Figure 5. Technical Format of a Call Sequence
c. Error-check Character
The digital selective call is a synchronous call
using a ten-bit error-detecting code. The first seven bits of
the code are information bits representing the 128 symbols of
daN S code or the ASCII code: Bits 8,9, and 10 are
used for error detection.
Ae laine: Dagar sassy
With the exception of the dot pattern and phasing
Signals, each signal in the DSC sequence is transmitted twice
25
in a time-spread mode. Imperfect propagation, interfering
signals, and coincidence with other calls require this
"echoing" effect to achieve satisfactory reception of the
desired signal. The first transmission of a specific (ee
signal could be followed by the transmission of four other
DSC signals before the original signal is re-transmitted. The
time interval, thus providing for time-diversity reception,
for the signals is 400 milliseconds (msec) for HF and MF
channels and 33 1/3 msec for VHP chanmel=s [Ret. IDO
The frequency shifts and modulation rates for the
three modes of transmission are:
diio HF and MF channels will use a frequency shift of 1/0
hertz at a rate of 100 baud. When frequency-shift
keying is effected by applying audio signals to the
input of single-sidebanded transmitters, the center.
of the audio-frequency spectrum at the transmitter is
1./O00mhertz-
(2) VHF channels will use a frequency shift of 800 hertz
at 1200 baud. The modulation technique is proposed to
be audio (frequency-modulation) with frequency-shift
keying of the modulating carrier frequency of 1/00
hertz. Carrier Sense Multiple Access (CSMA), will be
the access method utilized. CSMA access techniques
are discussed in Chapter III.
C. METHODEOF F ORERAT TON
Digital selective calling (DSC) equipment operates in
conjunction with the ship's radio equipment when installed
onboard a ship, or in conjunction with the shore radio
equipment when it is installed at a shore station. Although
the exact description of the DSC unit will depend on eee
2:6
manufacturer, the equipment will generally be self-contained
including all necessary power supplies, modems, keyboards,
controls, and displays for operation with the installed radio
equipment.
The oee mea description of a DSC unit and its
operations is based ona DSC unit developed by the GTE
meena Corph ation (Ref. l:pp. 9-12]. The DSC unit for a
mobile maritime station incorporates a microprocessor, the
primary element of which is a central processing unit
contained ona single integrated circuit. The operation of
the unit is cOnmtErolled@by a program stored in the
microprocessor's programmable read-only memory. Message
processing and storage TS accomplished with the
microprocessor read/write memory. A functional block diagram
of a Digital Selective Calling unit is depicted in Figure 6.
A selective call message that is to be transmitted is
composed using a typewriter-like keyboard and is displayed on
a cathode-ray tube terminal. The m has full editing
capability and can make corrections prior to message
transmission. After the message has been prepared in the
proper operational and technical formats of (OCT
Recommendation 493-2, it is transmitted as a serial data
stream. The DSC output is a binary Audio Frequency Shift
Keyed (AFSK) signal that is encoded with the message. The DSC
unit automatically initiates transmitter keying and following
detection that the channel is clear, the AFSK signal is
2/
SUOLVINGOWAC
IVNOILIGGV ALVA du vOd ATH AV'Id SIG
-OWNODDV OL SANTI
TART 21901 dTNHVdS 77777 077
:ONHOET
SHHJAINOSMH > SAOLVINGOWACG þa amo a amo
HOYA AJA TADA danva VOL
anvg C6zt LINA
HOSSAJOAA
TVYULNAD
SUALLINSNVUL SUOLVIAGONT™ anvya 0071
OL WALLINSNVUL
anva ooct
AYOWAW AYONAH
XINO SS322V
qva WOUNVY
SLOVINOO
/
AV TAY
Functional Block Diagram of a DSC Unit
28
Figure 6.
applied to the transmitter output. Modems are interfaced with
the standard communications transmitters and receivers. These
modems are switch selective for transmission, accommodating
all classes of emission or frequencies assigned for mobile
maritime communications.
Receiving the DSC AFSK signal is essentially the reverse
of transmitting a message. The modem outputs are continually
monitored for the presence of the dot pattern or phasing
sequence. When these are detected, the DSC unit decodes the
signal and temporarily stores it in random access memory. At
present, the buffer is designed to store two incoming DSC
messages [Ref. 15]. The address block is compared to the
station's address or one of the special identifiers assigned
M 1 cogo stations. “Ir the comparison indicates the
message is destined for the station, appropriate aural or
visual alerts, or remote telecommand functionings are
enabled. If the message is not intended for the receiving
station, it is cleared from the buffer and the DSC unit
stands ready to receive the next message.
In order for the message/call sequence to be transmitted
and received ina broadcast radio network, as the maritime
mobile service environment is, there are numerous methods
available that the transmitting site could use to gain access
to the transmission medium. These methods, called random
access calling systems, are discussed in the next chapter.
2
III. RANDOM ACCESS CALLINGR PE PELL
AFS N PRODUC ETON
Broadcast networks in general, and packet radio networks
in particular, are composed of transmitter and receiver sites
which communicate on a,common frequency. The signal strength
of one site is usually of such power that it can only
transmit to a small subset of the other sites in the network.
The typical broadcast network site operates in the
asynchronous mode by transmitting to every site within its
range whenever it has a message. When a receiving site is in
the range of two transmitting sites, a collision of the two
messages occurs and neither message is correctly received.
Similarly, a transmitting site cannot send and receive a
message concurrently. For most broadcast networks, in which
every site can transmit directly to all other sites, the
o are constrained to have at most one successful radio
transmission at a time. Networks in which multiple users
share a common communications channel in a way that can lead
to conflicts are widely known as contention networks. The
best known network of this type is called the ALOHA network.
Allocating a single communication channel among competing
users happens in many different types of networks. One
common situation in which a single channel is shared is a
30
multidrop line connecting a group of users. A user is polled
by a central site giving the user a chance to send any
messages it might have. Which users the central site should
pol 1s a Major problem in this type system. Riana po. bine
network has hundreds or thousands of users, only a small
fraction “might have a need to transmit at any given moment.
Frequency-division multiplexing (FDM) is another method of
allocating a single channel among a number of users. Iii
there are N users, the channel bandwidth is divided up into N
equal portions, with each user being assigned to one of the
portions. Since each user has his own frequency to transmit,
there is no interference between users, and the network can
operate without any collisions. However, when there is a
large number of stations which come and go as they please,
FDM poses a few major problems. A large piece of the channel
bandwidth may be wasted if less than N users are using the
channel, and if more than N users want to use it some of them
will be denied access. In addition, most computer data
uu sccumemely bursty (peak traffic to mean traffic
ratios of 1000:1 are common) and as such most of the channel
will be idle most of the time [REF. 16:p. 252]. A last
example of how a channel might be allocated among competing
users is the Asynchronous time-division multiplexing (ATDM)
concept. Each user is given its own dedicated port into a
concentrator which then feeds into the central site. If two
users send traffic simultaneously, the concentrator handles
ul
the situation by placing each user's data into diffemenm
memory locations within the concentrator. The data is
transmitted from the concentrator when the channel is clear.
In an environment such as the maritime mobile service, with
uncoordinated, geographically dispersed users who have only a
single shared channel to communicate on, there is no private
port, and two simultaneous transmissions will collide.
Giving each user a portion of the bandwidth to work with is
just the FDM technique which was discussed earlier, which was
determined to be too inefficient with many bursty users.
Therefore, the aforementioned techniques are not suited for
the VHF-FM DSC system.
B. ALOHA TYPE CALL IG SYSTEMS
l. Pure ALOHA
In the early 1970's, Norman Abramson and hicem
workers at the University of Hawaii developed the ALOHA
system to solve the problem of allocating a single
communications channel among competing users [Ref. 17].
ALOHA was the first computer system to employ radio
techniques instead of point-to-point wires for its
communication facility. The ALOHA network works on the
following principles users are allowed to transmit on a
totally random basis (Pure ALOHA) whenever they have data to
pass. A sketch of packet generation in an ALOHA network is
B
given in Figure /. A packet is synonymous to a DSC call
Sequence. All of the packets are the same length because it
has been shown that the throughput of ALOHA networks is
maximized by having a uniform packet size rather than
allowing variable length packets [Ref. 18]. Some of the
message packets will collide with one another, destroying
them. However, due to the perfect feedback property of
packet broadcasting, the sender of a packet can always find
out whether or not his packet was destroyed [Ref. 16:P. 253].
If the packet was destroyed, the sender waits a random amount
of time before re-transmitting the same packet. The
vulnerable period of a packet is shown in Figure 8.
-
User
A E) um
B =
C ES O =
D ere) E =
E C C E =
Figure 7. Packet Generation in an ALOHA Network
a. Capacity and Delay of the ALOHA Network
Iu cms Go) will first be determined by
Sool Ena channie loading (G) is the total number of
messages or packets per second to be delivered. This value
5)
is the current packet demand on the channel, measured in
packets per second. Packet time will be defined as the
amount of time required to transmit the fixed-length packet.
Again, the fixed-length packet is necessary for maximum
throughput. The user population is assumed to be an infinite
set, generating K new packets according to a Poisson
distribution with mean S packets per packet time. (The
infinite-population assumption is necessary to ensure that S
does not decrease as users become blocked.) In addition to
new packets, the users also generate retransmissions of
packets destroyed by collisions. The probability of kK
f
i
$ — i ie e P
4
|
|
Collides with Collides with
the start of ! the end of
the shaded the shaded
——— t————
packet packet
|
| qt Time —
re
Q
t +t t +2t
o 5
re
Q
ee - 4
Figure 8. Vulnerable Period of a Packet
packets arrivals per packet time, is also Poisson
distributed, with a mean of G packets per packet time. At
low load conditions (S®0), there will be few collisions, few
retransmissions, so G^&sS. Under heavy loading conditions
there will be numerous collisions so G > S. Under all loads,
34
the throughput is just the offered traffic, G, times the
probability of a transmission being successful (S = ES
E M chc pmobability that a packet does not suffer a
collision.
A packet does not suffer a collision unless
meter packet is generated during the time the first packet
is being transmitted. The probability UM K: packets are
generated during a given packet time is Poisson distributed.
This indicates that the probability of zero packets being
generated during a given packet time is also Poisson
distributed and is equal to exp (-G), obtained by replacing K
with zero in the equation below. The use of (exp) in this
thesis will mean the exponential function. The expression
exp (-G) means the exponential function raised to the (-G)
power. Ihe Poisson process is based " the assumption that
there is an infinite population of statistically unrelated
calls and/or callers. Telecommunication network theory uses
the Poisson process frequently to describe the generation and
interarrival of calls within the system [Ref. 19:p. 64]. The
Poisson equation is given below where G is the channel
loading measured in Erlangs.
K
PCR) = G ex -G
K !
In an interval of two packet times (refer to
Figure 8), the mean number of packets or channel loading
29
generated is 2G. Ihe probability of no other traffic bes
generated during the 2 packet length time is again given by
the Poisson equation: (Rer. loo
P(0) = P(K=0) s» exp (-26)
Using S = GP , the throughput is then calc umos
to be:
S = G exp (-2G)
This result was first derived by Abramson in
PODA The value G in the last equation is the total traffic
seen by the communication channel which includes not only the
current packet demand on the channel but all the packets re-
transmitted because of collisions as well. This last
probability, P(K=0), is that of a particular user's PARAS
being successfully transmitted to the receiver site.
The throughput-offered traffic load relation is
shown in Figure 9. As the channel traffic begi TEENE
increase, the useful th roweie we begins increasingly
relatively quickly. The probability of collisions’ also
increases rapidly resulting in a lower probability of
successful transmission. At a value of G equal to one-half,
3/0
E ener increase in traffic creates collisions with such
a high probability that the useful throughput is actually
reduced. The point of maximum useful throughput, known as
the ALOHA channel capacity, - occurs at a value of channel
praec of G = 0.5. The useful channel throughput is S =
1/(2e) = 0.184. Mas states that the best channel
utilization obtained with Pure ALOHA is 18.4% of the original
channel bit rate. Moe Els when the channel is filled to
0.6
0.5
0.4
0.2
0.5 1.0 13 2.0
Figure 9. Channel Throughput Versus Channel Load
in an ALOHA Calling Channel
Peo Cce=sO,5)ot ite bat rate with transmitted traffic [Ref.
ioe. 223]. Although the useful throughput is only 18.42,
2:7
the ALOHA network throughput is composed of only useful
delivered information. A 50,000 bit per second channel would
have a useful throughput of about 9200 bits per second, and
if the average di of a single user was only a few bits
per second, the channel could support possibly as many as
5000 users [Ref. 19:p. 223].
Delay through a Pure ALOHA network is defined as
the time interval from when a user node is ready to transmit
a packet until when it is successfully received by the
receiving node. This delay REEL E the queueing delay,
propagation delay, and transmission time. In ALOHA systems
the queueing delay is equal to zero because a user transmits
immediately when it has a packet to send. However, because
of collisions, the queueing delay time may be considered the
total time consumed prior to a successful transmission, (i.e.
the total time spent in unsuccessful transmissions). Fl tise
the value of the expected number of transmissions per packet
is necessary. This simply is the channel load divided by the
throughput O The value of S is the amount of trama
that is ultimately delivered successfully, regardless of how
many times it has to be repeated. The value of G is the
total traffic in the channel, which is the succes
throughput plus all the previously unsuccessful packets. The
ratio of G/S then represents the average number of times each
packet has to be transmitted before being successfully
DIS
delivered. Since there is only one successful transmission
required, the number of re-transmissions per packet is the
average number of transmissions (G/S) minus 1. successful
transmission. The full equation is:
CS- 1 = AE) — 1
The delay through the system is derived from this
value. The average number of re-transmissions is multiplied
by the average delay encountered for one re-transmission.
This average delay encountered by one re-transmission is
developed as follows: a common normalized algorithm used for
ALOHA is to re-transmit after a time selected from a uniform
distribution of from 1 to K packet-transmission times [Ref.
20:p.294]. The average delay is then (k + 1)/2. To this is
added the amount of time a station must wait to determine
that its packet was unsuccessful. mae nis just the time it
takes to complete a transmission, (1 + N), plus the time it
takes for the receiver to generate an acknowledgement (w)
plus the propagation time for the acknowledgement to reach
re Swoon el Pure ALOHA, the vulnerable period is 2
Packet lengths, 2(1 + N)w The throughput equation and delay
equation after all this look like:
S = G exp [ -2(1+N)G]
39
Delay = [ exp 2(14N)G - 1 ] [1 + 2N + w + (k+1)/2] + N + 1
where
N = propagation time
transmission time
In a typical packet radio network NES
extremely small, 10° or less. The term
can be safely tenored,.
w = time required to generate an
acknowledgement
k = packet retransmission protocol delay
24 Slotted ALOHA
Although the Pure or Basic ALOHA network comprised
only useful deliverable information, it was considered too
inefficient (throughput 18.4%) for many applications. This
ine ff ieee mien, created a lot of research into ways
achieving greater capacity while keeping the simplicity of a
random broadcast system. One of the solutions was to
establish a slotted channel (a channel with discrete time
slots in which users may transmit their information). The
so-called Slotted ALOHA broadcast network [|Ref. 209
considerably reduce the vulnerable period of Figure 8 when a
packet of information is likely to create interference.
Slotted ALOHA divides the channel into organized,
uniform slots whose size is equal to the transmission time of
the packet. A central clock or other timing mechanism is
used to synchronize all the users on the system. When a user
has information to transmit, he or she must wait unt 7 eee
40
beginning of the time slot before sending the traffic. Under
this scheme, only packets which overlap completely will
collide and be destroyed.
a. Capacity and Delay of the Slotted ALOHA Network
Calculating the lo mea pu t is mall y
straightforward. The number of packets that are transmitted
during atime slot is equal to the number that was generated
during the previous time slot and which had to wait for
transmission, The probability that no other packets were
generated during the breves time frame is exp (-G).
Stating it another way, exp (-G) is the probability that a
packet being transmitted will be successful in reaching its
destination because no other packets were generated to
collide with it. Therefore, probability theory says that the
probability an individual packet will suffer a collision is
l - exp (-G). ni tneuthnrouwehiput of a Slotted ALOHA
channel is the channel load (G) times the probability that
the packets are successful in reaching their destination:
S = G exp (-G)
The throughput-offered traffic load relation for
the Slotted ALOHA is shown in Figure 10. Similar to the Pure
O Droa at network, the channel traffic in Slotted ALOHA
increases quickly, reaching maximum capacity (G=1/2) at a
mceful throughput of 0.368. The channel utilization of
4]
Slotted ALOHA can be twice that of the Pure ALOHA system.
Maximum throughput is based on the assumption that each
transmission contains a full packet. If, on the average, the
packets were only half full, the actual throughput will be
that of the Pure ALOHA since no other user can transmit
during the empty portion of the Slotted time interval. [{Ref.
Soper a
Delay on a Slotted ALOHA broadcast channel is
estimated in the same manner as Pure ALOHA. The only
difference is that, on the average, each time a user is ready
to make a transmission he or she has to wait one-half of a
packet time until the beginning of the next slot interval
before the packet can actually be transmitted. The delay
equation is: [Ref. 20:p. 295]
Delay = [exp (G) - 1] [ 1 + 2N + w * (k * 1)/2 ] +N
where N = propagation protocol delay
w = time to generate an acknowledgement at
FEVT l
k = retransmission protocol delay
Under heavy channel loading conditions the delay
equations for both Pure and Slotted ALOHA confirm the
instability of contention-based protocols. As the rate of
new packets increases, so does the number of collisions.
Both the number of collisions and the average delay grow
exponentially with the offered load. Figure ll-a shows that
delay increases exponentially with the offered load. Figure
ll-b indicates that delay increases with throughput up to the
42
maximum possible throughput. Beyond that point, although the
throughput declines because of the increased collisions, the
delay continues to increase.
0.6
0.5
0.4 Slotted
0.2 E Unslotted
p E
Y —
Q. 1 / prm
/ —X
0.0 0.5 1.0 1:5 2.0
Figure 10. Plot of Channel Throughput Versus Channel
Matic Amores co etted and Pure ALOHA
c. Disadvantages
Even though Slotted ALOHA has the capability of
improving the throughput to twice the value of Pure ALOHA,
two potential problem areas can limit the usefulness of the
network. Proper clock synchronization is the whole key to
the Slotted ALOHA network. The potential exists for the need
of highly sophisticated user equipment, not onl» to
synchronize the clocks but also to allow for the variation in
actual distance between the users and the master clock.
Secondly, a user is limited in the amount of data that can be
43
sent at any given time since the packet length and time slots
ame fixed.
ALOHA S- ALOHA
S ALOHA
200 i
ISU 20
100 `
ALOIA
c 10
G 6
0 | à i Ut 1 Os “Oe
e le
(a) (b)
Figure ll. Delay as a Function of Channel Loading (6)
3. Slotted ALORA Swath Csr iis
It was assumed in the cases of Pure and Slotted ALOHA
that whenever two or more packets overlapped, all were
destroyed and required retransmission. If one of the NBN
signals happened to be strong enough to "capture" the
receiver and block the other signals, the packet could be
transmitted correctly and accurately. This situation eqns
occur when one transmitter is closer to the receiver th
44
others or when one transmitter uses more power. Fes
possible then to allow users with a need to transmit, based
on some priority scheme, to increase their power output.
This creates a higher probability of being received correctly
even in the presence of interfering packets. Rosner [Ref.
19:pp. 238-240] has analyzed this type of situation and it is
summarized below.
a. Capacity of Slotted ALOHA with CAPTURE Network
Rosner assumed for simplicity that, based on
sufficient random fluctuations in received signal levels, for
any pair of users there would be a one-half probability that
one of the users would capture the receiver and transmit
@onrectly. If three or more users transmit at the same time
one has power enough to une the receiver and all
packets are destroyed. Using a slightly modified version of
the Slotted ALOHA throughput equation, Rosner concluded that
a Slotted ALOHA channel with CAPTURE increased the channel
capacity to IM carl aros increase over Slotted ALOHA.
The throughput equation is:
S = G exp (-G) x (1 + G/2)
To analyze the probability of capture for
Ce e ae ter Conditions, the denominator in the last
term of the throughput equation can be changed. For example,
when two packets collide and the probability that one user
successfully completes the transmission is aed the
probability that the other user completes the transmission is
45
one-third, and the probability that neither packet is
received is one-third, then the last term becomes GC/3. MMS
assumes an equal probability distribution that each of the
transmitter sites will capture the receiver. Figure 12 shows
the effect of CAPTURE over Pure and Slotted ALOHA.
57
3
S 54
2e z
Slotted ALOHA
with capture
MG
e
S Slotted
ALOHA
l 18 Unslotted
2e f ALOHA
! |
Ya 1 Ne 1
Figure 12. Plot of Throughput Versus Channel Traci
for Slotted, Pure, and Capture ALOHA Systems
C. CARRIER SENSE MUI: AnS
Terrestrial ALOHA broadcasting networks such as packet
radio systems inherently have extremely short propagation
delays. This feature allows the ALOHA channel utilization to
be pushed far above the l/e limit imposed by Slotted ALOHA.
46
When the propagation delay is short compared to the
transmission of the information packet, a user can listen to
the channel before sending traffic. This method of operation
is called Carrier Sense Multiple Access (CSMA).
Networks in which the station-to-station propagation time
is large compared to the data transmission time have many
packet collisions. A significant block of time elapses
between the time one user sends traffic and the time other
users on the network know about it. During that time frame
another user may transmit a packet .thinking the channel was
clear. The result is that both packets collide and are
destroyed. In the reverse situation where the propagation
time is very small compared to the packet time, every user on
the network knows immediately when another user has sent
fic. À user wishing to send traffic can listen first to
the channel and base his actions on whether the channel is
busy or not.
F.A. Tobagi analyzed several of the CSMA protocols of
which a few of his results are presented below. ALI Tof
Tobagi's analyses are based on the following assumptions:
[Ref. 16:p. 289]
(1) All packets are of constant length.
(2) There are no errors, except those caused by
collisions.
MI ono feepreure effect.
47
(4) The random delay after a collision is uniformly
distributed and large compared to the packet
transmission time.
(5) Packet generation attempts (original calls plus re-
transmissions) from a Poisson process with mean G
packets per packet time.
(6) A station may not transmit and receive simultaneously.
(7) Each station can sense the transmissions of all other
stations.
(8) The propagation delay is small compared to the packet
transmission time, and identical for all stations.
(9) Sensing the state of the channel can be done
instantaneously.
l. Nonpersistent CSMA Protocol
There are three general CSMA protocols to examine.
The first protocol is called Nonpersistent CSMA. A user
desiring to transmit listens to the channel and obeys’ the
following rules [Re m22 LE
(1) If the channel Ws wWwudle, transmit:
(2) If the channel is busy, wait a random delay period
and repeat step l.
If there are several stations that have traffic to
pass, there is likely to be some wasted idle time fol lowe a
prior transmission.
a. Throughput of a Nonpersistent CSMA Network
For the case of zero propagation delay, both the
Pure and Slotted ALOHA versions of Nonpersistent CSMA give
the same throughput as a function of demand:
S = G/(1 + 6)
48
b. Delay Through a Nonpersistent CSMA Network
[Wee Scateomliation for Total Average Delay @r A)
through a Nonpersistent CSMA network follows a rigorous
approach and only the final equation is presented here. FOr
an in-depth view of the mathematics, the reader is directed
to Kleinrock and Tobagi's research on CSMA modes and their
throughput-delay characteristics [Ref. 23:pp. 1414-1415}.
TAD 2 [ (G/S - 1) R ] + 1
where C channel loading
Æ = channel: throughput
R = total time required to transmit a
call alert and call acknowledgement
sequence, to include re-transmission
delay protocol
2. l-Persistent CSMA Protocol
The second protocol, which avoids this idle -time is
labeled l-Persistent CSMA. Again, a user wishing to send a
packet listens to the channel and obeys Ste following rules
BEC D. 472]:
(TE the channel is quiet, transmit.
MI the Chanmel Ss busy continually sense the channel
until it becomes idle, then transmit immediately.
oo) e res cado llaision, walt asrandom period of time
and then repeat step l.
Although this protocol avoids the channel idle time,
more collisions result. If two users become ready to
transmit while another is in the process, both wait until the
transmission has completed. Immediately upon hearing the end
49
of the present transmission, both users transmit their
packets, thereby resulting in a collision. Even so this
protocol is better than just using Pure or Slotted ALOHA
because the users desist from interfering with the third
user's packet.
a. Throughput of a 1-Persistent GSMA Network
Again, for the case of zero propagation time and
valid for both Pure and Slotted ALOHA systems, the throughput
is the following:
S = [G exp (-G)] (1 + G)
+ exp (-
b. Delay Through a 1-Persistent CSMA Network
Similar to the theory behind the Nonpersistent
CSMA Total Average Delay (TAD) equation, TAD for MM
Persistent CSMA network is also a complicated equation. The
reader may refer to [Ref. 23] if desired for the complete
calculations. The Total Average Delay equation is:
TAD = [ (G/S =- 1) CI 2a CORE r ) |] +r +a Pa
where G = channel loading
S = channel throughput
a = propagation delay
c - transmit time of call packet divided
by the transmit time of the
acknowledgement packet
F = average delay a packet waits before
transmitting after it sensed the
channel was busy
i = re-transmission delay protocol
20
O ess tente esta Protocol
The third protocol applies to Slotted ALOHA and is
called P-Persistent CSMA. This protocol attempts to reduce
both the idle time seen in Nonpersistent CSMA and the number
of collisions resulting from l-Persistent CSMA. Any users
having traffic to send listen to the channel and obey the
following rules [Ref. 22:p. 472]:
(1) If the channel is idle, transmit using a probability
POE a one time unit with probability q=(1-P).
The time unit is typically equal to the maximum
propagation delay.
(2) If the channel is busy, continually sense until the
channel is idle and repeat step l.
(3) If transmission is delayed one time unit, repeat step
(ee
With P-Persistent CSMA, arguments arise over what
value of p best. The main problem to avoid is one of
instability under heavy channel loading conditions. EE «ui
users have packets to send while a transmission is currently
taking place, the expected number of users that will attempt
to transmit when the channel becomes idle is NP. Multiple
users will transmit and a collision will result. As soon as
all these stations realize that they did not get through,
they will be waiting to transmit again. These
retransmissions, along with any new packet arrivals, further
increases the probability of a collision. Within a short
period of time, all users will be trying to send packets,
causing continuous collisions, and therefore, throughput is
sul
ZETOR The value NP must be less than one for the expected
peaks of N. Otherwise, during the peak periods, 100 percent
of the expected i out of users (NP) will attempt to trama
when the channel becomes idle. As P is made smaller, users
must wait longer to attempt a transmission but this reduces
the number of collisions. At low channel loading, however,
users will have unnecessarily long transmission delays.
a. Throughput of a P-Persistent CSMA Network
The analysis of the throughput of this’ protocol
is very complicated. For a zero propagation delay
assumption, the throughput is given as: [Ref. 16:p. 291]
G exp (-G 1 + PGA
G + exp (-G)
where
x K
X = » (qG)
K*
M C CP I
d Fl E
For all P > O, the throughput drops to OMM LL
approaches infinity. For P approaching O, the asylum
throughput is 1.0 but the delay becomes infinite. Figure 13
shows the throughput versus offered traffic for all three
protocols, as well as Pure and Slotted ALOHA. It is clear
that vast improvements can be made using the techniques of
Carrier Sense Multiple Access.
212
b. Delay Through a P-Persistent CSMA Network
The Total Average Delay (TAD) calculation in a P-
Persistent Network is also very complicated and the reader is
QUIE sient CSMA
0.9
ae Nonpersistent CSMA
0.7
0.6 0.1-persistent CSMA
Ss 05
04
0.3
0 ALOHA
l-persistent CSMA f} Sepervistent CSMA
0.1 Pure ALOLLA
7
D !
0 l Š 3 4 S 6 7 X T
Figure 13. Throughput of Various Random Access Calling
Systems with Propagation Delay = OQO
recuento Rer. 23:p. 1415] for an in-depth analysis. The
delay as presented by Kleinrock and Tobagi's paper is:
Cc) A Za + 6 +r ]) + 1 + ase
where G = channel load
S channel throughput
a propagation delay
f
r
retransmission delay protocol
= initial delay due based on the
probability p
59
Now that this thesis has discussed various random access
calling methods available for the transmission of a DSC call
sequence, the o will present the current mathematical
model advocated by the CCIR. This model is developed around
the l-Persistent CSMA random access calling system.
24
NAAA EM PGE TAL SELECTIVE CALLING MODEL
A. INTRODUCTION
ec etlay somo pmorethe CORR, Interim Working Party (IWP)
EN met in October 1984 to discuss, among other agenda
items, the VHF channel requirements for a digital selective
calling system. A DSC VHF model submitted to the study group
postulated a worst-case situation based on the maximum number
of VHF working channels allowed by Appendix 18 to the Radio
Regulations and the maximum channel loading capacity of those
channels. Based on those calculations, which are presented
below, it was recommended that all VHF DSC gray ll hatin eer
commercial and distress, should be made on a single frequency
VHF channel.
There are two types of commercial DSC calls. a
navigational call and a general call. Navigational calling
is used to transmit information regarding a vessel's position
and intended movement. A navigational DSC call requires a
DSC call alert sequence from the sending station and a DSC
call acknowledgement sequence from the receiving station. If
^an acknowledgement sequence is not received, the sending
station re-transmits the DSC call alert sequence. The
reception of a DSC call acknowledgement sequence terminates
the communications on the calling channel. Further
29
communications, if necessary, are carried out on a "working"
channel designated in the DSC call alert sequence.
A VHF-FM transmission that is not a distress or
navigational call is defined as a general call. A general
DSC call also requires a DSC call alert sequence and a DSC
call acknowledgement sequence. Once the call acknowledgencum
sequence is received, communications are either terminated or
shifted to a "working" channel.
Bo DESEAN SIS
In order to calculate the required number of VHF calling
channels, the predicted channel loading on those channels was
needed. Since actual VHF channel loading conditions were not
known, the developers of the mathematical model submitted to
Interim Working Party 8/10, which will be referred to as the
IWP model, determined that the maximum possible signa A
intensity at VHF would occur in densely populated areas.
Channel assignment schemes will preclude interference on
working channels except under severe ducting conditions.
Therefore, the maximum number of working channels for general
VHF calling within the possible range of a calling ship is
28, according to Appendix 18 of the Radio Regulations.
In addition to general calls, a VHF DSC channel (egg
also be used for navigational communications. These
communications are currently transmitted on the single-
frequency channels of which Appendix 18 of the Radio
56
Regulations allots 19 channels (excluding channel 70 which is
currently reserved for distress and safety calling only)
NET. ilsp. 7].
Statistics from the Federal Republic of Germany and from
Denmark indicate that a working channel has a maximum
capacity of 0.8 Erlangs, and that the average duration of a
commercial communication was 5 minutes [Ref. 4:p. 18]. These
channel capacity and call duration figures were given without
reference to how they were calculated. Me VHP traffic
analysis figures used in the IWP model are listed in Table 2.
TABLE eee
VHF TRAFFIC ANALYSIS
Maximum working channels 28
Maximum navigational channels ine
Maximum channel loading in Erlangs 0.8
Average call duration in minutes 5
eee! ORS TNELUENCING THE NUMBER OF CALLING CHANNELS
There are other factors which can influence the number of
calling channels required in the proposed VHF DSC system.
CCIR Report 493-2 and CCIR Interim Working Party 8/10 report
of November 1984 list several of these factors which are
presented below.
2
l. Duration of DSC Call Sequences
There are three distinct types of DSC sequences: 1)
commercial call alert, 2) commercial call acknowledgement,
and 3) distress call alert [Ref. 14:p. 8]. The first twompee
call alert sequences have a duration of 0.5/7 seconds each
while the distress DSC call alert sequence will be 0.38
seconds in length.
Channel loading characteristics are partially
calculated using the duration of a DSC sequence. Since the
Intergovernmental Maritime Organization (IMO) has placed the:
maximum permitted loading of a random access calling channel
at 0.1 Erlang [Ref. 2:p. 2], an increase in any of thc
call durations will adversely impact on the loading of the
channel, possibly increasing the number of channels required.
Channel loading is calculated using the number of calls
arriving per hour and the duration of a single call. For the
channel loading due to DSC calling, each individual cali
multiplied by the respective number of DSC sequences Ton
call. For example, a commercial DSC call actually consists
of 4 DSC sequences: 2 call alert DSC sequences and 2 call
acknowledgement DSC sequences. A distress DSC call consists
of 10 DSC sequences as explained below. The Erlang equation
is given as:
Erlangs = (calls) x (call duration) x (#7 of seguon e
3600 (seconds/hour )
D
2. Number and Duration of VHF DSC Sequences
The CCIR has recommended that for a commercial call
to be — NH Muse consist of a DSC call alert
sequence from the sending station and a DSC call
acknowledgement sequence from the receiving station back to
the sending station. Because of the time-diversification
requirement set forth by CCIR Recommendation 908, each DSC
sequence will actually be transmitted twice. Eor a
commercial call this translates into four DSC sequences: two
DSC sequences for the sender's call plus two DSC sequences
for the acknowledgement call. According to the IWP VHF-FM DSC
model, for a commercial call, a one-way DSC call sequence has
wae aon "or. 0757 seconds, and the total duration of a
successful call (call alert plus acknowledgement) is 1.14
seconds. The IWP calculation for the number of DSC sequences
per successful call was [Ref. 4:p. 18]
lx ue 29DSCoseq/call = 2 DSC sequences
Ia xx 0557 sec/seq = 1.14 seconds
A single DSC distress call, as defined by the CCIR,
will actually consist of four repetitions of the same call.
An analogy to the present voice VHF-FM system is that the
unit in distress transmits its complete distress call five
cimes, one might after the other, Returning to the DSC
model, each distress call is repeated five times, and each
Shy
DSC sequence is transmitted twice for time-diversification.
Pas results in the ultimate transmission of 10 Dae
sequences.
Distress call x 5 repeats/call x 2 DSC seq/call) = 10 DS
Each distress call DSC sequence is 0.38 seconds in
duration and the total transmission time for a distress call
is 3.8 seconds. No acknowledgement sequence is required for
a distress call.
Channel loading calculations are determined in part
by the length of a DSC call. Since a successful DSC call
sequence consists of several imdividual call sequence sm
higher traftic loading will resumir This may increase the
number of required DSC calling channels.
3. Use of Scanning Devices
The dot-pattern is used in DSC calling Lor
synchronization purposes. If more than one DSC channel is
being used for calling, a scanning device is required to
sample each channel for traffic. The scanner will halt at a
particular channel only if the dot-pattern is recognized.
Since some time is required to determine if a call is
destined for another station, there is a non-zero probability
that some calls will be lost due to scanning. The use of
scanning devices will limit the number of calling channels.
60
At present only MF and HF DSC calls will utilize the dot-
pattern technique. |
4. Position of Coast and Ship Stations
In the calculation of the number of calling channels,
it was assumed that the position of the called station was
known. This permits the selection of the optimum frequency
bands, the correct channel in the selected frequency band,
and the optimum time to call the station. This pertains’ to
all DSC calling but may be of more importance in VHF calls
due to its limited calling range. Out of range calls will
result in retransmissions, ultimately increasing the traffic
loading on the channel.
Ship stations have an anomaly in communications in
that the ship-to-ship communication range is usually less
than the range between a ship and a coast station. This
situation creates the problem that not every call in progress
on a random access calling channel is necessarily heard by
possible interferers. Two ship stations within the same
coast station coverage area may possibly be out of range with
each other. Since both ships think the channel is clear,
they each access the channel to send traffic. The IWP model
of a VHF DSC system assumes that a particular ship will only
Ie a approximately 30% of the other ships in the
area. The end result is that the coast station may have
interference from 704 of the total ship station signalling
Seeurringes [Ret. 43 p. 19]
E
D. CCIR INTERIM WORKING PARTY 8710°VHF DSGesis TEM ori
The IWP model determined the worst-case required number
of VHF DSC calling channels, given a projected channel
Loading conditions The channel loading (offered load)
calculation is made in seconds, with the total number
call-seconds divided by 3600 (number of seconds in an hour)
to arrive at the number of Erlangs over the busy hour calling
period. The model of the VHF DSC system presented before the
Interim Working Party 8/10 group, with a large number of
independent calling stations, closely ro i a random
access Calling system such as those listed in Chapter III.
As such, the IWP model uses the Poisson process for the
generation of calls.
K
P (t) = (At) exp (-At) A AS O,
k!
where A = mean call@arrival rece
t = mean duration of the can
P (t) = probability of k calls arriving duna
time t
The first calculation was to determine the projected
channel loading on VHF. Referring to Table 2, the maximum
number of working channels available is 28, each having a
maximum capacity Wof OT eE lamer The call duration 159
minutes in length. Ihe maximum number of commercial
communications per hour from the coast stations which
62
simultaneously may receive the same DSC call is calculated
as:
Peecnanmels x 0.8 Erlang x 60 min/hour = 269 calls/hour
Simio call
Two DSC sequences (call alert plus acknowledgement) are
required to set up a general commercial communication on a
working channel. This will require:
269 calls/hr x 2 DSC seq/call = 538 DSC seq/hr
The duration of a general DSC sequence is 0.57 seconds
which corresponds to:
538 DSC seq/hour x 0.57 seconds/DSC seq = 0.085 Erlang
3600 seconds/hour
This Erlang figure represents the projected maximum
Single channel loading during the busy hour from general DSC
eal emo It corresponds to the hypothetical worst-case
scenario where only one coast station would receive all the
DSC calls necessary for establishing communications on the
total number of working channels allocated in Appendix 18 of
the Radio Regulations.
In addition to calls for general commercial
correspondence, the calling channel may be used for calls
63
establishing navigational communications, e.g. communications
on the single-frequency channels. Again, assuming a worst-
case Situations the calling channel must handle the
communications carried by the total number of single-
frequency channels of Appendix 18. This number, less channel
70 which is assigned for distress and safety calls only, is
I Channel 70 was designated exclusively for simplex CCIR
DSC distress and safety purposes by the Final Acts of the
WARC 1983 conference. [Ref. ll:p. 7]
The maximum number of DSC sequences per hour due to
navigational calls, assuming the same call set-up, duration,
and channel efficiency as a general commercial call, will be:
19 chnls x 0.8 Erle x 2 seq/call x 60 min/hr = 366 SOMA
5 min/call |
366 DSC seq/hr x 0.57 seconds/seq = 0.058 Erlang
3600 seconds/hour
The total load on the calling channel for general and
navigational calls is:
538 DSC sequences + 366 DSC sequences = 904 DSC sequences
904 DSC seq/hr x 0.57 seconds/se = 0.143 Erlang
3600 seconds/hour
64
Using the concept of a Basic ALOHA random access channel
and the Poisson process for call arrivals, the probability
that no new calls are generated while another call is in
p ogress is:
EC)
O
QUE COMES -0.143)
E NE A LL
OSO?
The probability that a new call is made while another is
in progress is, therefore:
Recalling from Chapter III and Figure 8, a call will
suffer a collision if any other call is generated within one
CAA of its start. The probability that a DSC call is
destroyed by another call because. of a collision is:
LL —2 x 06143) = 0.249
The developers of the IWP model consider the probability
ucc xhchelRet- 4:p. 19]. Shifting the attention to
the principles of Slotted l-Persistent CSMA, the IWP model
calculates tħe probability of a call colliding with another
65
eal ir This probability is equal to the probability thaw
calls are generated while one call is in progress:
Assuming that a ship station can only hear approximately
30% of the other ships in the area, this results in
interference at the coastal station from 702 of the total
ship. stations When the DSC calling channel is used
for general calling only (projected 538 call sequences per
hour maximum), disciplined access occurs for 350 DSC call
sequences (269 coast station call sequences plus 304 of the
ship station call sequences) while 188 DSC call sequences
enter the communications system randomly.
| The probability that a general call is generated while
another is in progress, resulting in a collision is:
P4 = 1 = exp ( -538 x 0257 ) =e 2
The probability that the general call will collide with a
randomly entered commercial call is:
P5 = 1 = exp (AAA ZO
66
The total probability of losing a randomly entered
general commercial call is then:
LEN PAF ( 1 PAI PS = 0.109
Eccc cs channel as also used for navigational
Ew. 70% of the navigational calls (256 DSC sequences) enter
the system randomly since they originate from ship stations
or low range land-based stations for port operations, pilot
stations, etc.
The combination ot 188 general DSC calls and 256
navigational calls (444 DSC sequences) access the DSC calling
channel randomly. The probability that a randomly entered
commercial call (general and navigational) will collide with
a commercial call in progress is:
LEM mexp ( —-904 x 0.57 ): = 0.133
The probability that this random commercial call collides
with another commercial call entered randomly is:
PS = | —- exp ( -444 x 0.57 ) = 0.068
67
Finally, for a DSC channel containing only commerce m
calls (general and navigational) ,the total probability eee
losing a randomly entered commercial call is:
P9 = P7 + ( 1 =927 See a
The IWP model considers this probability to be within
reason. The above calculations assume that only one coast
station would be receiving DSC calls for all coast station
working channels and all navigational channels in the Radio
Regulations. In practice, the maximum number of coast
stations in the Danish/Swedish waters which simultaneo
can receive the same DSC call is 9, with a total of 17
working channels at present (Ref. 4:p. 21]. The traffic
Statistics for these waters show a stagnation in the traffic
so it is unlikely that the channel loading will ever exceed
that corresponding to 20 working channels ( 0.06 Er Kom E
Therefore, a 502 margin is available to cope with conditions
of extraordinary ducting or ca Me one eo CANE
One other assumption was that the navigational DSC calls
will have the same duration as the commercial DSC calls. The
duration of a navigational call will only be 754 of a
commercial calir duracion: This fact will reduce the actual
channel loading.
It was shown that a single VHF DSC channel could meom
general and navigational commercial communications traffic
68
with a 319.25 probability of losing a randomly entered
commercial call. The IWP communications model concluded with
a discussion on the effect of distress calling if there was a
common VHF channel for DSC distress and commercial calls.
The probability that a commercial call is in progress
meme a distress call is initiated is:
CE 1 = exp -004 "0.57 ) = 0.7733
The probability of this random commercial call colliding
with another commercial call entering the system randomly is:
EME 1 — exp ( -444 x 0.57 ) = 0.068
Therefore, the probability that a distress call will
HMM de^wlth'"a commercial call is:
NU CI - PIO ) Pll = 0.192
Since a distress DSC sequence consists of 5 individual
e e babi lity of collision destroying all 5 calls by
5a Conmercia m call is:
5
ES ae) moe 262 or 1/3815
69
If only distress calls are allowed on the DSC channel,
the collision probability between any 2 distress calls within
One. NOMAS:
P14 s l- exp (.22 7x 28 Di O OOOO
On a VHF DSC calling channel where distress and
commercial calling are both present, the additional or
marginal loss of distress call attempts due to commercial
calling will be:
P15 = P14 + ( 1 =- PIS) PIOR
= 10. 000202
When using a single channel for both distress and
commercial calls the combined effect of the maximum possible
commercial traffic density can not exceed the risk that one
distress call attempt may collide with another one. [Ref.
4p. M
E. SCANNING ITH AS STIS DUNEDIN M
Is a dedicated channel is used for distress calls, the
use of a scanning device may be necessary. The scanning
70
process will produce lost calls and the dot-pattern will
increase the channel loading considerably.
The channel loading of commercial calls alone would be:
904 DSC seq x 1.57 seconds/DSC se = 0.394 Erlang
3600 seconds/hour
This figure clearly exceeds the IMO standard of 0.1
Erlang for the maximum channel loading permissible on a
mandom access Channel.,
B. INTERIM WORKING PARTY 8/10 DSC MODEL CONCLUSIONS
The conclusion drafted by the IWP model developers was to
recommend that all DSC calling be made on a single-frequency
VHF channel. This was based on the insignificant difference
between the probability of losing a single distress call
attempt ` on a separate calling channel and that using a
combined distress/commercial calling channel.
The author of this thesis feels that the current model
Icons ider all the criteria relevant to a combined.
distress and commercial calling channel. Although the
current model has taken the probability of losing a single
dn cms attempt (probability of delay) into account,
other grade of service aspects such as the probability of
answering a distress call within a certain time frame and the
actual time delay in the transmission of a call sequence need
to be considered. Therefore, the author has developed a
val
n
d
model which includes these grade of service parameters.
model is the subject of the next chapter.
a
NO DIGITAL SELECTIVE CALLING MODEL
In order to gain an understanding of the real-world
behavior of a VHF-FM Digital Selective Calling system, a
mathematical model can be effectively used to analyze how
various situations affect that behavior. As an example, when
the channel loading increases on the calling channel, the
analyst may wish to know what effect that increase has on the
probability of receiving and acknowledging a single call.
This chapter discusses general modeling requirements for the
VHF-FM DSC system, specific parameters to be used in the
model, and applies the model to the random access calling
systems of Chapter III.
AC GENERAL MODELING REQUIREMENTS
several aspects of the VHF-FM DSC system need to be
Slaw lee so that the development and application of the math
ar will be evident. The first aspect to be considered
concerns the generation and interarrival of calls in the
system. The Poisson process, used frequently ra
telecommunication network theory, is used in this model to
describe the generation and interarrival of calls at a VHF
reception facility. The Poisson process is based on the
assumption cnar there is an infinite population of
SaaS ea l e e ed calls and/or callers. The VHF-FM
95
Digital Selective Calling system fits into this category
because, although not a true infinite set, there is a
sufficiently large base of potential users in the maritime
industry, including recreational boaters, to make this
assumption. The assumption also includes that each user is
statistically independent of the other users. It has been
shown that for a Poisson arrival process, the time between
arrivals (interarrival time) is exponentially distributed
[Ref Mimo da
The second aspect of a VHF-FM DSC system pertains to the
types of calls, including their format and duration
characteristics, that are going to be placed on the system.
The new DSC system will have to accommodate the same types of
calls currently present on the voice VHF-FM channels. These
are the distress and safety calls, and the commercial calls,
which include both general and navigational calls.
The final aspects of the system to consider are the
parameters to be used for the model. These include but are
not limited to the specific levels of channel loading,
probabilities of a call being blocked, delays through the
system, and reliability.
B. SPECIFICATIONS OE THE PROPOSED
This section describes the assumptions used in the
proposed model for the VHF-FM DSC system. Following general
telecommunication network theory, the Poisson process will be
74
Used Low saelcu laeen the maritime traffic call -arrivals- and
interarrival times.
1. Format and Duration of Call Sequences
A single DSC channel is proposed to accommodate both
distress and commercial (general and navigational) calling.
Except for non-routine calls, safety calling will be
transmitted on a separate channel [Ref. 15:p. 3].
À calling channel is used primarily fom the
transmission of a short message to indicate that a calling
party desires to transmit further information on another
aanmel called a "working" channel. In the present voice
VHF-FM calling and distress system, a call contains a call
alert transmission from the sending station and a call
acknowledgement transmission from the .receiving station.
Once those transmissions have taken place, the two stations
switch to the desired "working" channel to complete their
transmissions. In the proposed VHF-FM DSC system, the term
DSC sequence is used to indicate a one-way transmission such
as a single call alert message or a Single call
acknowledgement message.
A successful commercial DSC call consists of one DSC
call alert sequence from the sending station plus one DSC
call acknowledgement sequence from the receiving station.
CCIR Report 908 specifies that each sequence will be sent
twice for time-diversification. [iveretore, each commercial
call generated will require four DSC sequences: two call
Um»
alert sequences and two call acknowledgement sequences. The
duration of each commercial DSC call sequence is 0.57
seconds.
A distress DSC call will actually consist of ten DSC
sequences. The original distress call alert DSC sequence
will be re-transmitted four times, one right after the other,
making a total of five DSC sequences. Because of time-
diversification, each of the five DSC sequences will be
transmitted twice, accounting for the distress call total of
ten (10) DSC sequences. The duration of each distress DSC
call alert sequence is 0.38 seconds. No acknowledgement is
required on a distress call. A distress call is successful
if further communications are established between the Siu
in distress and another station on the "working" channel
designated in the distress DSC @all alert sequen If
communications are not established on the "working" channel,
the station sending the distress shifts back to the DSC
calling and distress channel and re-transmits the’ complete
distress call (10 sequences), after waiting the required
period of time designated by the re-transmission delay
protocol:
The re-transmission delay protocol for all calls is
assumed to be forty-five (45) seconds in duration.
76
Z3 Channel Loading
The projected maximum channel loading due to general
calls on VHF is calculated from the sum total of all general
calling channels presently allocated (28 working channels).
Using 0.8 Erlang as the maximum channel loading capacity on a
single channel (Ref. 4:p. 18] and 5 minutes as the average
NNNM holding time tor a voice call [Ref. 19:p. 20}, the
maximum number of general calls expected is 269 calls per
hour. This figure represents original calls plus those calls
IGNEM-rTamesmdssmededaewmto collisions
28 channels x 0.8 Erlang x 60 min/hr = 269 calls/hr
5 min/call
Since 4 DSC sequences are required for a successful
general call, the number of DSC sequences is:
AS ME x 4eDSC sequences/call = 1076 DSC seq/hr
The duration of each general call DSC sequence is
0.57 seconds. The channel loading due to general calling is
calculated as follows:
"OOO sea hr sm 0 ¿Side secu DSC se = 02170 Erlame
3600 sec/hr
The projected maximum channel loading due to
navigational calls on VHF is calculated from the sum total of
all the single-frequency channels presently allocated (19
excluding channel 70 which is assigned to distress traffic
7E
omiy). Using 0.8 Erlang as the maximum single channel
loading condition and 3.75 minutes as the duration MS
navigational call, the projected maximum — of
navigational calls is 244 calls per hour. The developers of
the IWP model chose to use 5 minutes as the duration of a
navigational call, even though they stated that the duration
of a navigational call was 75 percent (75%) of that OES
general call. The actual load on a single-channel calling
system will increase, not decrease as suggested by the IWP
model developers, if the navigational call duration is in
fact 3.75 minutes. This can be seen in the equation below by
varying the duration of the navigational call (denominator)
between 5 minutes and 35.73 Me” At 5 minutes the unm
of calls on the 19 channels is 182 calls. Two DSC sequences
per navigational call brings the total number of DSc
sequences on the channels to 364 sequences, which translates
intoxm0- 095 A I ES cm When 3.75 minutes is used as the
navigational call duration, the number of calls on the
channels is 244, which translates into 0.0/7 Erlangs. In
this light, the proposed model will use 3.75 minutes in order
to obtain the worst-case channel loading conditions.
19 channels x 0.8 Erlang x CUA A a lc
3.7/5 min aM
Since each navigational DSC alert sequence is assumed
to have a corresponding DSC acknowledgement sequence, the
ji
total number of DSC sequences per hour is calculated as:
Peeewecalils x 4 DS€"sequences/call = 976 DSC seq/hr
The duration of each navigational DSC sequence is
0.57 seconds, the same as a general call DSC sequence. The
channel loading due to navigational calling is, therefore:
976 DSC seq x 0.57 sec/DSC se Ones Erlang
3600 sec/hr
The projected channel loading due to distress calling
Afines that distress traffic constitutes l percent (0.01) of
all calling on the channel. There is no information available
on the exact percentage of total calls that make up distress
calls so the author will use l percent as a base line figure
for the calculations presented below. Later in this chapter,
tables will be formulated showing distress percentage of
Manos from 1 tor lO percent of total calls and its
Mene different random access systems. Assuming the
average holding time for a voice distress call is the same as
BENT semeral call (5 mimutes), the total number of
distress calls per hour is calculated as:
269 general calls + 244 navigational calls = 513 total calls
Jls commercial @alils x 029€) = 6 distress calls/hr
Each distress call will actually generate 10 distress
DSC sequence transmissions as explained in section B.l. of
"n
this chapter. The resultant number of DSC sequences per hour
due to 14 distress calling coer
6 distress calls/hr x 10 DSC seq/distresse call = "60 seu BB
Each distress call DSC sequence is 0.38 seconds in
duration eto cS MN The channel loading due to 1Z
distress calling is:
60 DSC-seq/hr x 0.38 sec/segquenc e = 0. 00C Bride
3600 sec/hr
The worst-case projected channel loading due to
general calling, navigational calling, and 1% distress
calling ise 0.33bgE2d8 mm This is obtained by summing the
Erlang loading figures for each type of call.
0.170 (gen) + 0.155 (nav) + 0.006 (distress) = 0.331 Er Ramm
oe Grade of Servie
The grade of service is defined as the probability of
finding the VHF DSC system busy. Grade of service is the
central design element in telecommunication tra tae
engineering as it involves the ability of the system to
interconnect callers, and determines the speed with which the
interconnection is made. The grade of service can be
designated in various ways, some of which are the listed in
80
Table 3. The probability of a call being delayed longer than
a specified time is especially important when a single
ennel is used for both calling and distress traffic. The
TABLE 3
freee OF SERVICE PARAMETERS
1. Probability of delay
2. Average delay of all calls
3. Probability of delay exceeding a specified amount
of time
4. Channel throughput
probability of a successful distress call attempt within time
t, for t ranging from 1 to 5 minutes, will be another one of
the grade of service parameters used. The geometric
distribution is used to determine this probability because
the interest is in na the probability that the first
success will cams on any given trial. In this case the
first success occurs when any one of the 10 DSC distress call
alert sequences from a distress call arrives at the receiver.
A trial, for this model, is the act of sending the 10 DSC
sequences. The geometric distribution gives such a
probability. The other grade of service parameters listed in
Table 3 will also be calculated for the random access calling
systems of Chapter III.
81
C. |) APPLICATION OR THE FROPO EDENO
The grade of service parameters listed in Table 3 will be
calculated in this section using the proposed VHF-FM Digital
Selective Calling System model. The full set of calculations
shown are based on the Basic Aloha Random Access Calling
System, and use 0.1] Erlang as the channel loading factor (G)
and 0.01 (14) as the percentage of total calls that COMPRAS
distress calls. The tabulations for the remaining Basic
Aloha calculations, as well as the other random access
calling systems of Slotted Aloha, Slotted Aloha With Capture,
Nonpersistent CSMA, and l-Persistent CSMA, are listed in
Tables 4a through Table 6j for ease of reading. Since Basic
and Slotted CSMA throughput calculations are identical for
both Nonpersistent and l-Persistent CSMA, respectively, only
one matrix for each calling system is tabulated. P-
Persistent CSMA throughput and delay curves fall between
those of ens other CSMA calling systems (refer to Figure 13
of T III) and therefore will not be included. The
computer program used calculated precision to seven
significante ieit E
The Total Channel Load (G) figure is used to calculate
the random access calling system Throughput (S) and the Total
Average Delay (TAD) expected for a single DSC sequence.
S = (0.1) x exp 2 VD Re
TAD = [[exp (2 x 0 D) m]: T m eee) lee
82
210.08 seconds
The Probability of Delay (PCl) for any one DSC sequence
is calculated as the probable” "ot collision dte to the
channel loading. PSI is the probability of success for a DSC
sequence to reach the receiver. The probability that a DSC
sequence will not reach the receiver is PCl.
PS1 = Throughput / Load
= 0.082 / 0.1 = 0.819
EN - ] - PSl = 0.181
The probability of delay for a complete distress call
attempt is obtained using the probability of collision - fad
any one DSC M use. Since a distress call attempt consists
of ten DSC sequences, the probability that all ten sequences
are destroyed (PC5) by colliding with another DSC sequence
is:
PCS = ( PC! ) = ss) = 0.0000000383
PS5 = 1 - PCS = 0.9999999617
To calculate the probability that at least one of the
distress call alert DSC sequences will be successful within a
certain time frame, the transmission time for the complete
distress call attempt, including the re-transmission protocol
83
delay, is required. The duration of each distress DSC call
alert sequence is 0.38 seconds. Each distress call consists
of ten DSC alert sequences so the total time to transmit the
DSC sequences is 3.8 seconds. The re-transmission delay
protocol assumed is 45 seconds. Therefore, an unsuccessful
distress call attempt requires 48.8 seconds to elapse before
it will be re-transmitted. For each of the grade of service
times desired, ( 1 to 5 minutes), the total time required for
each unsuccessful distress call attempt is divided into that
time (in seconds). For example, if the probability of
success for a distress call to be completed within l minute
is desired, divide 60 seconds by 48.8 seconds. By rounding
down to the nearest whole number, this will give the maximum
number of complete distress calls that can be made in that
time frame. In this case only one call can be made, since
only whole distress calls can be made. The geometric
probability distribution is now used to determine the
probability that the first success will occur on or before
any given trial.
Geometric Probability
X xX-
Eu = > P (1-p) for x 9805.75 4 - a
l
where p = probability of success
x = number of complete ‘distress calls
within the desired time frame
For the proposed model calculations, the probability of
success for a complete distress call transmission was
84
determined above as PS5. hier ceometric distribution results
in the following probability of success for one distress DSC
sequence to be received correctly:
F(x) = (.9999999617) x (1 - 0.9999999617) zo MS ol
Since only one complete distress call could be made
within the desired time frame, this geometric probability is
also BEEN O Dala that the distress calli will be
successfully received within that time frame.
Tables 4a through 6j present the Probability of Delay
(PC1) for any one DSC sequence, the Total Average Delay (TAD)
Mis econds.) the Throughput (S), and the Probability of a
Successful Distress Call Attempt within a certain time
Eme. [P(T € t)]. The calculations are obtained by varying
HI xEnSunel loading from 0.1 to 0.5 Erlang, varying the
distress percentage of total calls from 1 percent (14) to 10
EpL 02). amd varying the time to complete a successful
distress call from 1 to 5 minutes.
The chapter concludes, following the tables, with
additional equations designed to calculate the marginal loss
of distress call attempts due to commercial calling. These
are presented for comparison purposes between the author's
model and the IWP model shown in Chapter IV.
85
TABLE 4a
BASIC ALOHA
1% Distress CSOT SUEZ S= 05%
Prob of Delay (POT 0.180 089550 OS: |
Average Delay (TAD) 10.08 Zl 34.71
PNT Omen pa econ) 0.089 (muro 0.165
PCtsl) 0.9999999 0.9999849 0.9996504
I CES 0.9999999 0.9999999 0.9999998
P(t<3) 0.9999999 0.9999999 0.9999999
P(ts4) 0.9999999 0.9999999 0.9999999
Der 0.9999999 0.9999999 0. 999999085
TABLE 4b
BASIC ALOHA
1% Distress emm ll ESO, 4 G=0%5
Prob Of. Delian iwc) 0.484 OR] SS
Average Delay (TAD) 39.48 2122205 71.45
Throughputets) QNEM Og 0.184
Bd 0.9992920 0.9974360 0.9898141
paucos 0.9999995 0.9999934 0.999805
pans 0.9999999 0.9999999 0. 99999085
PE 0.9999999 0.9999999 0.9999999
pete» 0.9999999 0.9999999 0.9999999
86
lm Distress
Prob of Delay (PCI1)
Average Delay (TAD)
Throughput (S)
EE
P(t€2)
E Ta)
P(t£4)
MESS)
I Dsrtress
Prob of Delay (P€1)
Average Delay (TAD)
Throughput (S)
Pat =i)
Pet=Z)
ERE: 3D
P@r=4 )
PEES )
=
on
4.31
2999
BU SSS
. 999 9999
. POIS
- 991010999
- 901010999
Saas Sa
G=
TABLE 4c
SLOTTED ALOHA
all
O9
TABLE
4d
E
".
DS
.164
EO JO
| OUO
PO
< 990199.9
. FOIOS
CONSEC C» c» c»
SLOTTED ALOHA
(oes 31
239
oD
. QIOIDIOIBIO 9
. 99.9999. 9
¿ PODIO
, TIO
8 /
181
.9999849
. 910191919 9 9
POSI O
: III 9 9 9
- J999
. 9990S /
LOI)
. 99 9099
99?
- IIS
20
£99 9 NMILT
-997009
< 9990009
. 9990099
. 200189629
TABT TER e
SLOTTED ALOHA Wii CAPTURE
172 Du ress
Prob of Delay (PCI)
Average Delay (TAD)
Throughput (S)
aC t=)
DET
PC tea)
PC re)
oop
G=
Oz
OBI
050
UNKNOWN
C ecco. CO. CÓ
-095
. 99999099
9999
Meee 2) 5S),
. 99 0I
. 99990
TDI E
IE
Or
099
UNKNOWN
O OO
LSO
IESO
SORA
« 9 OUI) 9
ICAO
m
SLOTTED ALOHA WITH CAPTURE
14 DUStre ss
Prob oi Delay (PCD
Average Delay (TAD)
Throughput (S)
P(t)
Bore
P(ts3)
PoE
Ets
Or
Gz0.331
163
UNKNOWN
O Tito Or
2207
.O09900
-999900
; 099
:9990 0E
-9999925
88
G=0.
Oa
4
196
UNKNOWN
O O Sita
152
OO
OOD
oD
19999999
DIRE
CU
De
148
UNKNOWN
O Dic
G=0.
Ox
. 208
9999
09995
.O09990 154
90999
9999
2
242
UNKNOWN
O DIS)
. BS
. 9997585
. 7 9 TS
. 9999505
90999]
9997
TABLE 4g
CSMAGNONPERSisd ENT
14 Distress G=0. 1 G=0.2 G=0.3
Peob of Delay (PC1) 0:09] OB O., 224)
Average Delay (TAD) Gu LOAD LS O
M ouüughput (S) 0.091 0-67 0572251]
P(tz1) 0.9999999 0.9999999 0.9999996
P(tsz2) 0.9999999 0.9999999 0.9999999
P(ts3) 0.9999999 0.9999999 0.9999999
P(ts4) 0.9999999 0.9999999 0.9999999
nee) ) 0.9999999 0.9999999 0.9999999
TABLE 4h
CSMA SHONPE RSs TEN T
14 Distress p [B G=0.5
Peeopmor Delay (PCl1) 0.249 0.286 Dias
Average Delay (TAD) 26.81 32 EUM 40.50
Timo Wei p we (S) DO 0.286 US
P( tear) 0.9999991 0.9999964 0.9999831
Dan 0.9999999 0.9999999 0.9999999
SED 0.9999999 0. 9999999 0.9999999
Poros) 0.9999999 0.9999999 0.9999999
porte 0.9999999 0.9999999 0.9999999
89
14 Di tress
Prob of Delay (PC1)
Average Delay (TAD)
ITnrowecehput (3)
P(t£1)
P(t#2)
P(t£3)
P(t£4)
PECES)
1% Di- tress
Prob of Delay (BC
Average Delay (TAD)
Throughput (S)
P(ts1)
I
Ies
P(ts4)
puts
TABLE 41.
CSMA ~ TEPER EROTENT
99990005
IOS
TABLE 4j
D DNA
0056
O Tr Ot Oi
52 00
Tro
19009999
IIS Y
PODAIS Y
IO
MO IO
CSMATISPERS T TENT
OF
SO
219
. 9999923
. 99090
. 9999009
-9999959
-9999959
eS 2 ©
G=0.331
O89
90
al
ee 29909
199719999
o SIME
-9099799
SIDO O
G=0.3
02,072
OOO E
6.54
. 20S
. I9 7 ES
. 99 9 QTE
. 99 Q0
. 99990
0998071
G=0.5
Oe
178
1729
SS: Di
cael
. 99 905
. 979 79006
. 97 PO
. 9999
. 99 JODE
TABLE 5a
BASIC ALOHA
54 Distress G=0.1 G=0.2 G=0.3
Prob of Delay (PCl) O WSL 050551 DOUBT
Average Delay (TAD) 103508 2 ARO Sau]
Throughput (S) DO USE 0.134 ONES
P(tsl) 0.9999999 0.9999849 0.9996504
pues ) 0.9999999 0.9999999 0.9999998
P(te3) 0.9999999 0.9999999 0.9999999
P(t<4) 0.9999999 0.9999999 0.9999999
P( t85) 0.9999999 0.9999999 0.9999999
TABERE 5D
BASIC ALOHA
54 Distress G=0.331 G=0.4 G=0.5
ProD of Delay (PC1) 0.484 O. IM Ono 2
Average Delay (TAD) 39.48 D S 71.45
TIhrousmput (S) Uo OTISO 0 SA
P(ts1) 0.9992920 () 09 7885 60 0.9898141
P(té2) 0.9999995 0.9999934 0.9998962
P(t£3) 0.9999999 0.9999999 0.9999989
P(ts4) 0.9999999 0.9999999 0.9999999
PM) 0.9999999 0.9999999 Oe
oa
5% Distress
Prob of Delay (ECT)
Average Delay (TAD)
Throushput (sS)
pem
DE p
DE
pats
POCED)
5% Distress
Prob of Delay (PCI)
Average Delay (TAD)
Throughput (5)
Pur
ue
P(t<3)
Pt =e )
PLt=5)
TABLE
Ec
SLOTTED ALOHA
CHORN
O.
4.31
-090
.999000
.9990 7
.09990 5
.0O99
79999909
O00000
G=
095
TABTE
Sd
G=0.2
oe
9 08
181
0.164
SAS E
S ODE
0 c9
MOS ye,
92
:. 9999999
oe
OE
-9090999
9979799
G=0.4
O.
330
20 35
CODI
LAOS
213249
-9999999
SOS 9
-9229099
-9999999
999
99995
9999
-9999999
9999999
TABLE 5e
SLOTTED ALOHA WITH CAPTURE
me Uistress
pro of Delay (PCT)
Average Delay (TAD)
Throughput (S)
P(ts1)
INE)
Bae )
P(ts4)
P(ts5)
SLOTTED ALOHA WITH
DONEDNSLtress
LED Delay (PCT)
Average Delay (TAD)
Throughput (S)
P(ts1)
P tes)
PI Le
ERU
pu pese
G=0.1
qa
050
UNKNOWN
Sar CC)
=
o
.095
IS
-9990999
IIS
- 9 TS
SNO O9 O9
TABLE 5f
O35 |
FO'S
UNKNOWN
OSOS
aT
IPD
9999
IST
II
-JOR 9
E
SZ
D
O99
UNKNOWN
OSO OD OO
abo
O O
19099999
209099
999
UO
CAPTURE
G=0.4
oF
196
UNKNOWN
DADA =
22
2999909099
20000109
IO
IR CIEGO
A O
ES
Om
148
UNKNOWN
OSO. O @
-290
a I 99
Me O
Mc oe,
ooo 9
E SNS
G=0..5
on
242
UNKNOWN
(55650069009 C) C5
Em
797993
. 99i I9 O
¡TECOS
_ 29 ee C)
MO
TABLE 5g
CSMA NONPERSISTENT
54 Distress C= eek G=0.2
Prob of Delay (PCI) 0.091 [DOMO
Average Delay (TAD) GIO 16.20
Thr omighput (S) OO] OO
POT 0. 9999999 0.9999999
PONES 0.9999999 0.9999999
P(t=3) 0. 9999999 0.9999999
P( t=) 0.9999999 0. 9999999
pod 0.9999999 0.9999999
TABLES
CSMA NONPERSISTENT
5% Distress G=0.331 EIE
Prob ol Delay (RCF 0.249 0.286
Average Delay (TAD) 2643 1 32.40
Throughput (S) 0.249 O. 280
=> 0.9999991 0.9999964
puc 0.9999999 0.9999999
ERES 0.9999999 0.9999999
parcs 0.9999999 0.9999999
EX J 0. 9999998 0.9999999
94
99/0 0E
999
. IIS
SD stress
EOD of Delay (PCI)
Average Delay (TAD)
Mirouehput (S)
B tel)
P(ts2)
ute)
P(ts4)
Ens»)
5% Distress
Prob ot Delay (PC1)
Average Delay (TAD)
Throughput (S)
[CER
PES)
¡MESS
Ea)
ECE
CSMA 1-PERSISTENT
G=
0.
Om /
no
EOM
. 979 9999
TIO OIDO
SI OD
IO
O TS O
TRBEE Di
oSI
009
TABLE 5j
EOM
9.
3.00
L1
990999
-9999999
POROS
UTOR O
27999999
O CRETO
036
CSMA 1-PERSISTENT
G=
B.
O
SO
e PITT
MOO oe 7
o
OUI
DIO
ere C3 C9
09 34
089
J5
G=0.4
Ok,
1525
1-150
(Co COMO C CC
p otn
OO
. 9929999
- PIO O
See oo!
POSI 99
CS
0.
6.54
n2
TITO
IO
SS
O
791990999
SS O.
O75
G02 5
0.
178
eel
DO OOO G
.411
-9992099
-992099
OUI,
¿IIS O
O
10% Distress
Prob of Delay (ECT)
Average Delay (TAD)
Throughput (5)
P( tel)
TAR)
PUTES)
P(ts4)
P(ts5)
102 Distress
Prob of Deday (Pea)
Average Delay (TAD)
Throwenput (5)
ECCE
pue)
Eee)
PCE=4 )
P(té£5)
TABLE Ca
BASIC ALOHA
GUT
Oc rod
10.08
2
O SUI
. 990
.O0999 M»
. 99 OE
TIT
SS)
G
0
2
CCS CO CSS O
TABLE 6b
BASIC ALOHA
=0.331
. 484
9.48
al
-99072020
. 999 OS
999
. 999
, IIS
96
. 9999849
IS,
79992009
SEPSIS SSNS
EIE
.9974360
-2999934
IO ISS
SIS
RIO
108
. 9996504
. 9999585
. 9999055
.O090 99
09998]
.184
.9898141
. O0 9/8
999°
9990
09909
10% Distress
Wob of Delay (PGI)
Average Delay (TAD)
lhswwehput (S)
i291)
pause?)
B tee )
D 54A )
pU tee )
102 Distress
Prot Delay (PCI)
Average Delay (TAD)
Thmouehput (S)
P(ts1)
pore
P( t23)
P(ts4)
EFE <p)
TABLE
6c
SLOTTED ALOHA
EU
0%
4.31
E
IO Y
"9999099
. 9999999
ooo
ISS Y
OSO OS
G=
095
TABLE
SLOTTED
Oye!
LO
-9979909
. 9999922
9999999
IIS
IIS IS
oy
G0, 2
Ui.
9 08
.164
- 9910019 99
700099999
SEL) CIS)
SUIS
SUDO
Sere GC» C)
6d
ALOHA
T81
"oye
9009349
-9979999
-9999999
-9999999
-9099999
199999257
99999099
9990999
¿TITS
TITO
8503
IAN
. 99920199
9999999
7992799
499909009
107- Distress
Phob of Delay (ROID
Average Delay (TAD)
Thsemshput (S)
P( t=)
PIED
pete)
PES
TEES
05 Daseumess
Prob of Delay (PCL)
Average Delay (TAD)
Throne hput (Ss)
P( teig)
pie
P(t¢3)
P(t£4)
pute»
TABLE 6e
G=0.1
0L.
O50
UNKNOWN
OO ODIO
G=
OF
28095
USE
IIED
III
" TIE
IO
Table 6f
Ó el
163
UNKNOWN
Seas © ©: ©
ET.
. 9999993
. IV
. PIES
. IPS
IO
98
SLOTTED ALGRASWITH CAPIUKE
pe
vm
099
UNKNOWN
OO WO O D
ESO
79999999
: 9999999
19999999
799907999
nou
SLOTTED ACONASVITH CAPTURE
G=0.4
Ir
196
UNKNOWN
Oe See
ee
ee IS,
so) DO
. 9999999
IO
. 99999919
(0:9
Que
148
UNKNOWN
OS Citi
TO
99090
09090
9999
999°
. 9990059
Gs
OF
242
UNKNOWN
O OOOI
9 0
999908
. 9999
9997
999
.0999] 99
Distress
Prob of Delay (PC1)
Average Delay (TAD)
Paeowehput (S)
Dp.)
Pee = 2 )
Du)
Pp(t<4)
O )
10% Distress
Prob of Delay (PC1)
Average Delay (TAD)
Throughput (S)
Pican)
PES 2a
ARS
DL y
=>)
TABLE 0g
CSMA NONPERSISTENT
G=
Oe
SL
BOD)
EO
1790900
CE
. 9999920
27091519 9.9
Sooo 2 @
Ome]
Ora
TABLE 6h
EU
212111130
EU
EAE
.9999999.
AI 909
ComAyNONPER SISTENT
G=
ssl
.249
-999 9091
. 9990099
DU OE
4099090
MODI
99
G=0.4
os
286
32.40
CO OO ORE
. 286
. 9999964
O O
y
II O
OS
42090 EI
9 OIDIMIO D
. 999899
OMEN)
O
G=0.5
OF
2333
40.50
SOGO OO
2903
29/9 BST
. 99900999
oe
. 99901099
DOSE
O7 Distrëss
Prob of Delay (PGE)
Average Delay (TAD)
ThremeNPut (5)
Pt SUN
P( c22)
P( t23)
PRU
PEDE)
10/5. DINI OS
Prob of Delay (PC1)
Average Delay (TAD)
Thromeaput (5)
Ie C TES LO
PEE
DASS
P(té4)
P(ts5)
TABLE
CSMA. 1-PERS E TENT
G=
OF
DE
099
9999999
. SOO
999090 oe
- 99 oe
.999 995
COBGSNCS CC)
e
009
TABLE 6j
G=0.2
0.036
Hee oO O
Du
MD
-9999999
AS
19999999
9999999
PTS
CSMA 1-PERST o TENH
G=
09M
G=0,4
er S
0.05
OO. COUCO CO (ED
6.54
TO
. O9'O SIME
.O90 0E
. 997
9999
99D
GUNS
USO
E
Dp
Ll. oe
0. 172
Ld DNE
Sea O
D.
. 909290
C090
- II UU
. II SUN
. 099991
100
OSO CE
ore
PS
TO
-999909099
¿DAI Y
IIS
O QOQ QOQ
.411
. III
990007
. 99992
. 99 9m
. 99 710
In order to determine the effect of general calling on a
single DSC channel, the number of general calls, navigational
calls, and distress calls must first be determined from the
given channel loading figure. The number of navigational
calls is assumed to be 904 of total general calls. This is
derived from the calling statistics given in the IWP paper.
Distress calling is assumed to be 14 of the commercial calls
(general + navigational calls). Based on these assumptions,
the total channel loading can be broken down into the
following:
Load Total (LT) = Load (lgen) + Load (lnav) + Load (ldist)
LT
(CAI (Neat) )(0.57) + (Dcal11)(10)(0.38)
3600 3600 3600
uF Gea 20006333) F Ncall(.0006333) + Dcall(.0010555) `
Ncall (0.9)Gcall
Dea Aca TUE SN call) = (0.0l1)(Gcall + (0.9)(Gcall))
ot Geasll + (0.009)(Gcall)
MD) (9) (EGcal(.0006333)+(.01)Gca1+(.009)Gcal
CCAA OOO 12234
Gcall DON = 981 General calls = 324 DSC seq
Ncall
Eu 72 Nav calls = 288 DSC seq
TROM
Deall = (0.01)(Gcall + Neall) = I Distress all wi DSta
Assuming a 2 to l ratio for ship to shore radio traffic,
the total calling is broken down into the following:
Total Gcal = cu
Shorea E /
Sign, ¿ds 54
Tota lMlWe a L1%= .72
shore Neal y= a4
uS MT 48
Inc au E ]
If a calling channel had only general calling allowed
it, the probability that a general call DSC sequence will
cause a collision with another general call DSC sequence is:
PS10 = S / Lgen where S = throughput due to general calls
Lgen = load due to general calls
=O 04 bay) 07051
Poe EIE
POLOS D
Disciplined access will take place for only the shore
station calls plus 30% of the ship station calls [Ref < mE
KOS The total disciplined calls will, therefore, be 43.
O2
This means that 38 general calls enter the calling system in
a random manner.' The probability that a general DSC sequence
is destroyed by a randomly entering general call DSC sequence
is:
PS11 « S / Lgen-r where S = throughput due to random calls
Lgen-r = load due to random calls
= 0.023 / 0.024
- cS S
Em = 1 - PS1l = 0.047
Pico ababality of losing a general call DSC
sequence is then:
EN PCIO + €¢ 1 —- PC1O )(PC11) = 0.139
If the calling channel is also used for navigational
Gumame, 70% of both shore and ship navigational calls (50)
are regarded as entering the system randomly as these calls
are generated by ship stations or low range shore-based
B e Therefore, a total of 88 general and navigational
calls will be accessing the calling channel randomly. The
probability that a randomly entering commercial (general and
navigational) call DSC sequence will destroy another
commercial call DSC sequence is:
5 eomm where S = throughput due commercial calls
Lcomm = load due commercial calls
Pos
OCs 0097
103
RSLS 0.824
PETS 1 -"PSI = 0. TE
The probability that this commercial call DSC sequence is
destroyed by another commercial call DSC sequence that has
entered the system randomly is:
PS14 = S / Lcomm-r
= 0.050 / 0.056
= 0,895
P6614 = 1 -S l4 = 02 65
The total probability of losing a commercial calla
sequence is calculated to be:
PC15 = PC13 + ( 1 =- PCD QBUS UM IOS
If a calling channel was designed for both commercial and
distress calling, the effect of combining all calls onto one
channel should be determined. The probability tho
commercial call is in progress when a distress call is
initiated is the same calculations.) ele.
PC16 = 0.176
The probability that another commercial call DSC sequence
will randomly enter the system is the same as PC14:
POT? OS
104
M cubInedMprobsbuasrty that a distress call will be
destroyed by any type of commercial call DSC sequence is the
same as PC15:
Peis = 0,263
Because a complete distress call attempt consists of ten
DSC sequences, the probability of collision for the whole
distress call attempt by a commercial DSC sequence is:
10
roomed = (PCIS) eee OU0UTS3
If only distress calls are allowed on the channel, the
probability that any two distress call DSC sequences’ would
destroy each other is:
PS20 = S / Ldis
Oeo 0.00211
= 0" 9916
pL IPS»0 = 0.004
Finally, in the case of a combined commercial calling and
distress channel, the additional or marginal loss of distress
call attempts because of commercial calling on the channel
is:
A 020 PCI9S - PC20 = 0.00000158
roS
VI. ‘CONCLUSIONS AND RECOMMENDATIONS
A. a CONCE USTEONG
This thesis has presented a model of a VHF-FM Digital
Selective Calling (DSC) system using grade of service as a
criterion to ascertain if a single DSC channel coud
accommodate both distress and commercial calling. Although
the Interim Working Party (IWP) 8/10 model does use one grade
of service factor, probability of delay, the author’ Siem
takes into account several grade of service parameters, which
lend a greater credibility to its conclusions. The author's
proposed model calculates the probability of a call being
delayed, the ‘vee delay of a call, the probability OMS
call being answered within a certain time frame, and the
throughput for the random access calling systems of ALOHA,
Slotted ALOHA, Slotted ALOHA with Capture, Nonpersistent
CSMA, and l-Persistent CSMA. Based on the results these
calculations, as presented in Chapter V, one DSC chan EE
capable of accommodating all distress and commercial calling.
In addition, l-Persistent CSMA was found to be superior to
all the other random access calling systems evaluated. These
results concur with and strengthen the IWP model.
There are three factors that support the author's
conclusions. The first is, for channel loading conditions
ranging from 0.1 to 075. Ec m 1-Persistent Core
106
consistently has the lowest probability of delay out of all
the different calling systems. The maximum probability of
delay encountered for 1-Persistent CSMA was 0.178 at a
mice! loading of 0.5 Erlangs. Bue CCIR Interim Yaorking
Party (IWP) 8/10 model regarded a probability of delay of
Selo as reasonable [Ref. 4:p. 20]. As such, in the author's
Prob of Delay
0.000 0.200 0.400 0.800
Total Channel Looding
AAN
D BA + SA o Sc Lp X C 1-2?
Figure 14, Probability of Delay Versus Channel Loading
opinion, the probability of delay in the l-Persistent CSMA
system is acceptable for a combined distress and commercial
calling channel. The probability of delay versus channel
loading for the random access calling systems is plotted in
Figure 14, The following abbreviations will be used in
A A San AO: BA (Basic ALOHA), SA (Slotted
1:009
ALOHA), SC (Slotted ALOHA with Capture), C N-P (Nonpersisteme
CSMA), and C 1-P (l-Persistent CSMA).
The second factor is that the average delay of a call for
1-Persistent CSMA, as channel loading varies from 0.1 to 0.5
Erlangs, is also the lowest of all the systems examined.
Figure 15 shows a plot of the average delay as a function of
channel loading.
mm.
Q
4
"uf
8
Ù
à
o
Q
;
<
0.000 0.200 0.400 i 0.800
Total Chonnel Losding '
a BA + SA o Sc 4 C N-P. X Cc 1-2?
Figure 15. Average Delay Versus Channel Loading
The maximum channel loading limitation of 0.1 Erłlangs,
imposed by the International Maritime Organization (IMO) and
used as the basis for the IWP model, should not be the
primary criterion used to dictate the required number of
calling channels. Figures 14 and 15 indicate that a channel
108
can be loaded to 0.5 Erlangs and still remain within
acceptable levels for probability of delay and average delay
of a l-Persistent CSMA random access calling system.
>
È
o
2j
E
=
0.000 0.200 0.400 0.300
Tota! Channel! Loading
0 BA SS o SC A C N-P x C1=P
Figure 16. Throughput Versus Channel Loading
Thirdly, the l-Persistent CSMA random access calling
System has the highest throughput, for channel loading
between 0.1 and 0.5 Erlangs, of any of the systems studied.
As shown in Figure 16, throughput for 1-Persistent CSMA
continually betters the other random access calling systems.
In the author's opinion, there are two criteria that do
not affect the selection of a random access calling system.
First, the probability of a distress call being answered
within the 1 to 5 minute time frame does not appreciably
109
change among the systems analyzed. Nor does this probability
change as channel loading is varied from 0.1 to 0.5 Erlangs.
The calculations of Chapter V show the probabilities that a
call will be answered within 1 minute of the initial
transmission range from 0.9996 to 0.9999, The other
criterion that does not affect the random access calling
system selection is the amount of distress traffic on the
channel. Percentages of distress traffic from 1 CoE
percent were analyzed and no changes in the delay or
throughput figures were noted.
No matter which grade of service factors are considered,
the l-Persistent CSMA random access calling system is
consistently super norm Its low probability of a cal miea
delayed, low average delay of a call, and relatively AN
throughput show, in the author's opinion, that MAMMA
reasonable to use 1-Persistent CSMA and combine distress
and commercial calls onto a single VHF-FM Digital Selective
Calling (DSC) System channel.
B. RECOMMENDATIONS
Based on the conclusions presented above, it is
recommended that a single VHF-FM DSC channel be used for both
distress and commercial calling. In addition, l-Persistent
Carrier Sense Multiple Access should be chosen as the radio
communications method used for transmitting the DSC
sequences. The International Radio Consultative Committee
110
au" rhe international Maritime Organization (IMO)
must designate appropriate grade of service levels as these
levels could be instrumental in determining the required
mer of DSC calling channels. For example, the CCIR or IMO
could have a difference in opinion from that of the author's
and determine that the maximum average delay for a call
should be less than five seconds. This fact would cause more
than one channel to be designated as distress and calling
channels because the Erlang channel loading on a single
channel would be a limited. It is also recommended that the
actual percentage of distress calling to total calling be
determined in order to validate the model.
Finally, a cost-benefit evaluation should be conducted
for each of the random access calling methods. Although the
l-Persistent CSMA technique theoretically indicates
superiority to the other methods, it may not be economically
feasible. An approach to the study would be to look at the
marginal value a marginal cost relationship among the
different systems. For example, at the projected worst-case
channel loading of 0.331 Erlangs, the throughput of the
OA «with Capture system is 27.72 of the total
offered traffic. The throughput for l-Persistent CSMA at the
same channel loading is 30.25 of the total offered traffic.
If the cost was significantly higher to obtain the l-
Hm ecsbnolosy, the additional 2.35 in throughput
may not be justified. The cost-performance trade-offs must
be examined, not only for each random access calling systems
as a whole, but for each of the desired levels of grade of
service. Even though the throughput of the Slotted ALOHA
with Capture example above was only 2.3% below l-Persistent
CSMA, the accompanying probability of delay was almost
doubled.
The maritime mobile service will benefit greatly from the
Digital Selective Calling (DSC) system when it is in place
and operating. This thesis has presented a means to help
ensure that the DSC system is properly designed.
DRST F REFERENCES
Sommeon, E., Kump, C., and Turner, E., Maritime Mobile
Pal Ateo Calling: Equipment Development and
Deere Prepared by GTE Sylvania, Inc.
Mountain View: GTE Sylvania, Inc., 1974
International Radio Consultative Committee (CCIR) Report
SUS (DRAFT) anel Requirements For A Digital Selective
Calling System, 1 June 1984.
oni cat al SBECA EL: Calling and Emergency
System for Short-Wave Radio," Telektronikk (Norway), Vol.
ee No. leneloo., Written in Norwegian.
International Radio Consultative Committee (CCIR), Report
Of The Third Meeting of Interim Working Party 8/10. 12"
November 1984.
ECT M n AutsnEa Pedestals For Shipboard
Satellite Terminals," Symposium Papers RTCM Assembly
Meeting, Vol. 1, Satellite Communications/Automated Sys-
tems., Washington, D.C.: Radio Technical Commission for
Marine Semvices, 1961, part IE.
Parker, J.D., "Development of Automatic Maritime Radio
Telephone Systems in Europe," Symposium Papers RTCM
Assembly Meeting, Vol. 1, Satellite Communications/Auto-
mated Systems. Washington, D.C.: Radio Technical
Commission for Marine Services, 1981, part 1G pp. 1-14.
Eun 6G-. Preparing for Data Communications on
Marine Satellite/VHF Radio/SITOR," Symposium Papers RTCM
Assembly Meeting, Vol. 1, Satellite Communications/Auto-
mated Systems. Washington, D.C.: Radio Technical
Em -uon tor Marine Services, 1981, part 1B, p. 7.
Lundberg, O., "The International Maritime Satellite
Organization (INMARSAT)," Symposium Papers RTCM Assembly
Meeting, Vol. 1, Satellite Communications/Automated Sys-
Lom wWsshaungbon, D.C.: Radio Technical Commission for
Marime Services, 1981, part LA pp. 1-11.
LO
jm
T23
lieu
14.
loss
Lor.
Ie
Raye
Finis, L., An Analysis to Determine the Feasibility
QUÉ COMSAT Corporation Subsidy to Assist in Improving
Communications Interoperability Between U.S. Navy/U.S.
Flag Merchant Ships, M.S. Vilt@sas, Naval Fos eee agua
School, Monterey, California, Ih 19958
Maritime Administration, U.S. Department of Commerce
Report MTR-7217, Rev. 2, Maritime Digital Selective Call-
ing System Technical Description, by T. D. Haas and eee
Fee, October 1978,
Federal Communications Commission, "Digital Selective
Calling System in the Maritime Mobile Service," Further
Notice of Proposed Rule Making: PR Docket No. 84-499,
Government Printing Office, Washington, D.C., 26 Octobe
1984.
Hempton, G. F., "The Future Global Maritime Distress and
Safety System", Symposium Papers RTCM Assembly Meeting,
Vol. 1, Satellite Communications/Automated Systems.
Washington, D.C.: Radio Technical Commission for
Marine Services, 1981, part ED I
International Radio Consultative Commgttee (OOGTROS
Interim Working Party 8/10 Digital Selective Calling
Coordinations of Trials Report Bo Study Croup es MEME
February 1984.
International Radio Consultative Committee (CCIR),
"Digital Selective-Calling System For Use In The Maritime
Mobile Service," Recommendation 493-2 (DRAFT), [ime
1984.
International Radio Consultative Committee (CIRIE
"Operational Procedures For The Use Of Digital Selective-
Calling (DSC) Equipment In The Maritime Mobile Service
Recommendation 541-1 (MOD I) (DRAFT), modifications made
May 1984, pow20»5.
Tanenbaum, A. S. Computer Networks. Englewood Cliffs:
Pmerntwicecuet. Inc. =e
Abramson, N., "The ALOHA System - Another Alternative
for Computer Communications. MO pp. 281305»
1970.
Abramson, N., "The Throughput of Packet Broadcasting
Channels,” IEEE Transactions On Communications, Vol. COME
25, pp. 117- e
1.
Ze),
Zale.
Oe.
D.
24.
homer, KR. Do, Packet Switching. Belmont: Lifetime
earning Publications, 1982,
stallinegs, W., Data and Computer Communications.
New York: Macmillan Publishing Company, 1985.
Robemes, bo.) Eetensions of Packet Communication
Hecimobroevwro a Hand Held Personal Terminal," Proc. SJCC,
pp. 295-298, 1972. —
Tobagi, Fouad A. "Multiaccess Protocols in Packet
Sonmmunieatvon Systems, IEEE Transactions On
Communications, Vol. COM-28, No. 4., 1980, pp. 468-488.
Nro sand Tobaei, EF. A., "Packet Switching in
Radio Channels: Part 1] - Carrier Sense Multiple-Access
Modes and Their Throughput-Delay Characteristics," IEEE
Mu cmMHUsEE5B:sou Conmubnrcations, v. COM-23, No. 12,
December 1975..
Melee 5ystems Volume I: Theory.
New York: John Wiley & Sons, 1975.
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