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`DEPARTMENT OF THE AIR FORCE
`AIR UNIVERSITY
`AIR FORCE INSTITUTE OF TECHNOLOGY
`
`Wright-Patterson Air Force Base, Ohio
`
`'nm9
`'..,3
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`1 17 098
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`ARRIS GROUP, INC.
`IPR2015-00635 , p. 1
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`9 J AN 199U
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`DEPARTMENT OF THE AIR FORCE
`AIR UNIVERSITY
`AIR FORCE INSTITUTE OF TECHNOLOGY
`
`Wright-Patterson Air Force Base, Ohio
`
`'nm9
`'..,3
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`1 17 098
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 1
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`
`
`AFIT/GE/ENG/88D-II
`
`A COMPUTM SIMULATION ANALYSIS OF CONVENTIONAL
`AND TRUNKED LAND MOBILE RADIO SYSTEMS AT
`WRIGHT PATTERSON AIR FORCE BASE
`
`THESIS
`
`Thomas C Farrell
`Captain, USAF
`
`AFIT/GE/ENG/88D-11
`
`AfbDTIC1
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`"NLECTEn
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`E 0
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`Approved for public release; distribution unlimited
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 2
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`
`
`AFIT/GE/ENG/88D-11
`
`A COMPUTER SIMULATION ANALYSIS OF CONVENTIONAL AND TRUNKED
`
`LAND MOBILE RADIO SYSTEMS AT WRIGHT PATTERSON AIR FORCE BASE
`
`THESIS
`
`Presented to the Faculty of the School of Engineering
`
`of the Air Force Institute of Technology
`
`Air University
`
`In Partial Fulfillment of the
`
`Requirements for the Degree of
`
`ACOession For
`
`Master of Science in Electrical Engineering DTIC TAB
`Unannounced
`Justificatio
`B
`
`I
`
`Distribution/
`Availability Codes
`a
`Ava1Y and/or
`Special
`
`L
`Dist
`
`Thomas C Farrell, B.S.
`
`Captain, USAF
`
`November 1988
`
`Approved for public release; distribution unlimited
`
`DI c
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`i py
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`ARRIS GROUP, INC.
`IPR2015-00635 , p. 3
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`
`
`Preface
`
`My interest in land mobile radio (LMR) began in Europe when, as an
`
`additional duty, I became our unit's Site Security OIC. Subsequent
`
`exercises and real world events demonstrated the need for reliable
`
`intra-base communications, and how easily the communication systems
`
`(public telephone, field phone, and radio) could become saturated with
`
`calls in an emergency.
`
`Hybrid trunked LMR should go a long way to solving these problems.
`
`Although this thesis explores the effects of some increases in loading
`
`on fleets of a trunked system, more research on LMR loads during
`
`exercises would be profitable. Of particular interest would be the
`
`probability distributions and statistics (described in Chapter V) of
`
`various LMR nets currently in use at Air Force bases during exercises.
`
`In conducting this research I have been helped by many people. In
`
`particular, I would like to express gratitude to my sponsor,
`
`Mr Gardner, who provided much of the background information about LMR
`
`systems and answered many questions, and to my committee, Maj Prescott,
`
`Maj Norman, and CPT Shaw. CPT Shaw deserves special thanks for the
`
`time he spent and advice he gave, both on the queueing aspects of this
`
`thesis, and on good engineering practices in general. I would also
`
`like to thank my parents who, through example, demonstrated the
`
`benefits of academic discipline and self motivation. Finally, I would
`
`like to thank the technical people I have known, and learned from, who
`
`are serving in the United States armed forces around the world.
`
`Thomas C Farrell
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`Preface...................... . .. .
`
`
`
`
`
`
`
`
`
`..... . .. .. .. .. . .....
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`Page
`
`v
`
`List of Figures .............................
`
`List of Tables............................vii
`
`Abstract ..............................
`
`ix
`
`I. Introduction............................
`
`Background ...........................
`3
`Problem and Scope. ....................
`Approach..........................3
`Assumptions.........................6
`Equipment..........................7
`
`Ii.
`
`Literature Review. .......................
`
`Trunking Schemes .. ....................
`Air Force Requirements .. .................
`Description of the Hybrid Trunked System .....
`Load Analyses .......................
`
`III.
`
`Conventional Model. ......................
`
`Introduction. .......................
`Description of the Computer Model .............
`Discussion of the Model ..................
`Mathematical Verification of the Model ......
`
`IV.
`
`Trunked Model .........................
`
`Introduction. .......................
`Description of the Computer Model .............
`Discussion of the Model ..................
`Mathematical Verification of the Model ......
`
`V.
`
`Analysis of Data Collected Via Monitoring ......
`
`Objectives. ........................
`Procedure Used to Collect Data .. ............
`Monitoring: Phase I. ...................
`Monitoring: Phase II ...................
`Results ..........................
`
`8
`9
`10
`12
`
`16
`
`16
`17
`19
`21
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`25
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`25
`25
`30
`33
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`39
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`39
`39
`40
`41
`43
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`VI.
`
`Normal Configuration Runs ....
`
`..............
`
`Overview .......
`..................... ..
`Comparison of Conventional
`and Trunked Systems ....
`............... ..
`Interpretation of Results .. ............ .49
`Sub-fleets .......
`.................... ..
`Priority .......
`..................... ..
`Sensitivity ......
`................... ..
`Optimum RU .......
`.................... ..
`
`VII.
`
`Contingency Model Runs .....
`
`................ ..
`
`Overview .......
`..................... ..
`Increase in Load; No Increase
`in Sub-fleets ......
`.................. ..
`Creation of New Sub-fleets .............. .69
`Failure of Parts of the
`Trunked System ......
`
`.................. ..
`
`VIII. Conclusions and Recommendations .. ........... .73
`
`..................... ..
`Summary .......
`Conclusions ......
`...................
`Recommendations For Further Work ........... .75
`
`Appendix A: SLAM Code For the Conventional
`................ ..
`Simulation Model .....
`
`Appendix B: SLAM Code For the Trunked
`Simulation Model .....
`
`................ ..
`
`Appendix C: Frequencies of Number of Transmissions
`Per Message For Each Channel Monitored .....
`
`Appendix D: SLAM Output From the Conventional Model . .
`
`. .i1
`
`Appendix E: SLAM Output From the Trunked Model ....... .125
`
`Bibliography ......... ........................ ..
`
`Vita ...........
`
`............................ ..
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`Page
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`47
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`47
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`47
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`56
`59
`60
`63
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`65
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`65
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`65
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`71
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`73
`73
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`76
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`92
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`107
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`139
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`141
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`Lit-_ 2L Figures
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`Figure
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`1. Conventional Land Mobile Radio Model .
`
`......... .
`
`2. Predicted and Measured Wait Time
`(Conventional Model) .....
`
`................. ..
`
`3. Trunked System Model (Part 1) ....... .............
`
`4. Trunked System Model (Part 2) ...
`
`............. ..
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`Page
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`18
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`24
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`26
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`28
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`5. Flow of Entities in the Modified
`Trunked System Model .....
`
`................. ..
`
`33
`
`6. Predicted and Measured Channel Queue Length
`(Modified Trunked System Model) .. ............ .
`
`7. Predicted and Measured Channel Queue Length
`as the Number of Messages Per Transmission
`is Varied (Modified Trunked System Model) ....... .
`
`8. Percent of Callers Obtaining a Channel
`Within I Second ......
`.................... .
`
`9. Wait Time Until 80% of Callers Obtain
`a Channel ........ ....................... .
`
`10. Wait Time Until 90% of Callers Obtain
`a Channel ........ ....................... .
`
`11. Wait Time Until 98% of Callers Obtain
`a Channel ........ ....................... ..
`
`12. Delay in Obtaining a Channel For Trans-
`missions Other Than the First One in a
`Message as a Function of Parameter RU . ......... .
`
`13. Frequency of Messages By Number of Trans-
`missions For the Security Police Net .. .......... .
`
`14. Frequency of Messages By Number of Trans-
`missions For the Motorpool Net ...
`............. ...
`
`15. Frequency of Messages By Number of Trans-
`missions For the Base Supply & Distribution
`C Net ......... ......................... .
`
`37
`
`38
`
`50
`
`51
`
`52
`
`53
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`64
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`107
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`108
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`108
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`16. Frequency of Messages By Number of Trans-
`............ .
`missions For the Fire/Crash Net ...
`
`17. Frequency of Messages By Number of Trans-
`missions For the Civil Engineers Channel
`i Net ......... ......................... .
`
`19. Frequency of Messages By Number of Trans-
`missions For the Civil Engineers Channel
`2 Net ......... ......................... ..
`
`13. Frequency of Messages By Number of Transmis-
`sions For the Specialist Dispatch/POL/Base
`Operations Net .......... .....................
`
`Page
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`109
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`109
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`110
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`110
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`Table
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`Page
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`List 2f Tables
`
`I. Predicted and Measured Wait Time
`.................. ...
`(Conventional Model) .....
`
`II. Predicted and Measured Channel Queue Length
`............ .
`(Modified Trunked System Model) ...
`
`III. Predicted and Measured Channel Queue Length
`as the Number of Transmissions Per Message
`is Varied (Modified Trunked System Model) ....... .
`
`IV. Number of Messages Noted During Various Times
`of the Day (Measured During Weekdays For
`3107.5 Seconds or the Hour) ....
`.............. .
`
`V. Measured Characteristics of Transmission
`Length and Time Between Transmissions
`(Within a Message) ......
`................... ...
`
`VI. Measured Characteristics of the Number of
`Transmissions Per Message ....
`............... .
`
`VII. Channel Load Used in the Computer Models
`to Simulate Normal Conditions ...
`............. .
`
`VIII. Fleet and Net Inputs Used in the Computer
`Simulation Models to Compare the
`Conventional and Trunked Systems .. ............ ...
`
`IX. Comparison of 7 Channel/7 Fleet Trunked
`Model (With MD Set to 0) and the
`Corresponding Conventional Model .. ............ ...
`
`X. Comparison of the Effects of Division
`Into Sub-fleets of the Original Seven
`Fleet Trunked System (Time For 98% of
`Callers to Obtain a Channel) ...
`.............. ...
`
`XI. Comparison Between a Prioritized Trunked
`System and a Similar System With Priorities
`Set to the Same Value .....
`................. .
`
`XII. Effects of a Change in Mean Transmission
`Length of the Security Police Fleet on
`the Conventional and 4 Channel Trunked
`Models (Time For 98% of Callers to Obtain
`a Channel) ........
`....................... ...
`
`23
`
`36
`
`38
`
`42
`
`44
`
`45
`
`45
`
`48
`
`55
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`58
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`60
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`61
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`XIII. Effects of a Change in Mean Messages/Hour
`of the Security Police Fleet on the Con-
`ventional and 4 Channel Trunked Models
`(Time For 98% of Callers to Obtain a Channel) .....
`
`XIV. Effects of a Change in Mean Transmis-
`sions/Message of the Security Police Fleet
`on the Conventional and 4 Channel Trunked
`Models (Time For 98% of Callers to Obtain
`...................... ..
`a Channel) ........
`
`XV. Effects of a Change in Standard Deviation
`of Transmissions/Message of the Security
`Police Fleet on the Conventional and 4
`Channel Trunked Models (Time For 98% of
`.............. .
`Callers to Obtain a Channel) ...
`
`XVI. Results of an Increased Load on the
`................ ..
`Security Police Fleet ....
`
`XVII. Results of an Increased Load on the
`................... ...
`Fire/Crash Fleet ......
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`Page
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`61
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`62
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`62
`
`67
`
`67
`
`XVIII. Results of an Increased Load on the
`Security Police and Fire/Crash Fleets . ..........
`
`68.-..
`
`XIX. Overall Message Delay on the Trunked
`System With an Extra Security Police
`Fleet Added, Compared With the Normal
`7 Fleet System (Time For 90% of Callers
`................ ...
`to Obtain a Channel) .....
`
`XX. Overall Message Delay on the Trunked
`System With an Extra Fire/Crash Fleet
`Added, Compared With the Normal 7
`Fleet System (Time For 90% of Callers
`................. ..
`to Obtain a Channel) .....
`
`XXI. Overall Message Delay on the Trunked
`System With Extra Security Police and
`Fire/Crash Fleets Added, Compared With
`the Normal 7 Fleet System (Time For
`90% of Callers to Obtain a Channel) . .......... .
`
`70
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`70
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`71
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`(
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`AFIT/GE/ENG/89D-il
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`'/
`
`Abstract
`
`Trunked land mobile radio systems, currently being developed by
`
`/ ,
`
`several companies, allow many groups of land mobile radio (LMR) users
`
`to share a set of channels dynamically, reducing the total number of
`
`channels needed to support these groups. These systems also support
`
`"'dynamic regroupinG", reassigning individual users to different groups
`
`through software in the controlling computer. Hybrid trunked systems
`
`(HTSs) have the added advantage of being able, in the event of
`
`controlling system failure, to default to certain channels, adding a
`
`degree of robustness to the system. HTSs seem to be an answer to many
`
`of the Air Force's intra-base communications needs. These needs
`
`include the ability to support an ever increasing number of users with
`
`a minimal increase in allocated channels, a very high level of system
`
`reliability under extremely adverse conditions, and an ability to
`
`manage users under a variety of contingencies (base attack, aircraft
`
`crash, etc.) In order to determine the number of channels a HTS will
`
`require for a specific facility, information about traffic loading, and
`
`how the system reacts to it, is needed.
`
`This paper discusses a computer model of existing LMR networks on
`
`Wright Patterson Air Force Base (WPAFB), and a model of a possible
`
`trunked system for the base. Data was collected from off the air
`
`monitoring of LMR nets, and was used to dteLrmine numerical values for
`
`various parameters. These values were input to the computer models to
`
`determine the time required for a user to obtain a channel while
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`traffic load and (for the trunked model) user grouping were varied to
`
`simulate various conditions.
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`A 5 (1 data, 4 voice) channel HTS was found to adequately support
`
`WPAFB, even with a loss of one repeater and an increase in LMR traffic.
`
`With proper user grouping, trunked system performance is shown to be
`
`superior to the existing conventional system while using fewer
`
`channels.
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`A COMPUTER SIMULATION ANALYSIS OF CONVENTIONAL AND TRUNKED
`
`LAND MOBILE RADIO SYSTEMS AT WRIGHT PATTERSON AIR FORCE BASE
`
`L. Introduction
`
`Bakground
`
`Land Mobile Radios (LMRs) (also called "walkie-talkies" or
`
`"bricks") are small, hand held radios used by police, fire departments,
`
`and other organizations desiring portable, rapid communications.
`
`Because of the LMR's decreasing cost and increasing availability, many
`
`organizations on Air Force bases now have, or want, their own LMR
`
`network (net). Because of this, the Air Force now faces the problem of
`
`obtaining allocation of a larger number of channels from the Federal
`
`Communications Commission (FCC) and host nations.
`
`Trunked LMR systems reduce this problem by allowing users to share
`
`a set of channels dynamically. In one type of trunked system, all of
`
`the radios are originally tuned to a digital channel monitored by a
`
`computer driven central controller. If a user, a fireman for example,
`
`wants to talk with his department, he keys the radio, which sends a
`
`digital signal to the central controller. The controller examines the
`
`set of allocated voice channels and, if it finds one not currently in
`
`use, it sends a digital signal to every radio on the fireman's net
`
`(called "fleet" in trunked systems) re-tuning them to the channel.
`
`When the fireman de-keys his radio all the radios in the fleet re-tune
`
`back to the digital channel. Normally this whole procedure occurs so
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`quickly the user doesn't notice any difference from a conventional
`
`system. However, if all of the voice channels are in use, other users
`
`trying to get a channel are queued on a priority basis by the con-
`
`troller.
`
`Trunked systems have several advantages over conventional systems:
`
`1. As mentioned above, the primary advantage is in requiring
`
`fewer channels to satisfy more users. This is based on the observation
`
`that transmissions usually take place on a conventional net for only a
`
`small percentage of time.
`
`2. Individual radios in a trunked system can be reallocated to
`
`different fleets based on programs stored in the central controller.
`
`This has great advantages on an Air Force base, particularly during
`
`contingencies when individuals are performing different missions,
`
`reporting chains are changed, and some conventional LMR nets would
`
`become saturated.
`
`3. Assuming compatibility between Air Force trunked systems,
`
`deployed units can communicate with other units at their new location.
`
`For example: national guard units deployed overseas can integrate
`
`their LMR system with that of their host base.
`
`4. Individual radios can be "turned off" of a system. This is an
`
`advantage in situations such as a hostage scenario where the hostage's
`
`captured radio can be taken off of the fleets used by the rescue force
`
`and, if desired, assigned to its own fleet for use by the negotiating
`
`team.
`
`Hybrid trunked systems are trunked LMR systems with the added
`
`advantage that, if the central controller goes down, radios automati-
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`cally re-tune to preallocated channels. This is vital in the military
`
`environment, where loss of one element of the system shouldn't com-
`
`pletely eliminate communications.
`
`The 1842 Electronics Engineering group, Scott AFB, Il is develop-
`
`ing Air Force requirements for the hybrid trunked LMR systems described
`
`above and needs data to determine the number of channels necessary to
`
`provide reliable communications in a contingency situation. They would
`
`like to have a computer model developed which will simulate a trunked
`
`system and determine its performance characteristics during various
`
`contingencies.
`
`Problem and ScoRe
`
`The objective of this thesis is to design and build a computer
`
`simulation model of a trtnked system for a specific Air Force base,
`
`determine appropriate values for input parameters for both day to day
`
`and contingency operations, and use the model to determine the number
`
`of channels needed to provide the base LMR users with a reasonable time
`
`to access a channel.
`
`Approach
`
`Computer Models. A computer model of a conventional LMR system
`
`was built as a baseline for measuring performance differences between
`
`it and the trunked model. In a conventional system there are two
`
`possible reasons a user would have to wait for a channel: 1) someone
`
`else on the user's net is already talking, or 2) someone on another net
`
`(sharing the channel) is talking. The computer model measures these
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`conditions for a given load and presents curves of the percent of
`
`transmissions delayed vs. the amount of time they are delayed.
`
`On a trunked system, delays in granting a user a channel can be
`
`due to somebody else talking on the same fleet, all of the voice
`
`channels being in use, and mechanical delay in the system (which
`
`includes delay in accessing the controller on the digital channel and
`
`delay in the controller itself). The computer model of the trunked
`
`system assumes a constant mechanical delay and measures the other two
`
`delay conditions for a given load. Like the conventional model, the
`
`results are plotted as the percent of transmissions delayed vs. the
`
`amount of time they are delayed.
`
`Both computer models were built using SLAM II, a FORTRAN based
`
`simulation tool (7:vii). The models were verified by setting the input
`
`parameters to match simple mathematical models and comparing results.
`
`C
`
`cion 2 DaLa. Data was collected from off the air monitor-
`
`ing of nets in use at Wright-Patterson Air Force Base (WPAFB). The
`
`data was used to determine, for each net, the number of messages per
`
`hour, the mean transmission length, the mean time between transmissions
`
`(within a message), and the mean number of transmissions per message.
`
`(Usually a conversation over LMRs consists of several transmissions
`
`making up a message. For example, a dispatcher asks for a police
`
`officer's location, the officer tells him, and the dispatcher responds.
`
`This is considered one message and consists of three transmissions:
`
`one by the police officer and two by the dispatcher.) The data was
`
`also used to verify the legitimacy of the various distributions used in
`
`the computer models.
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`Normal Cfiguraion
`
`s. The data collected by off the air
`
`monitoring was put into the computer models and they were set up to
`
`simulate the existing conventional system, and a hypothetical trunked
`
`system, at WPAFB. The models were run for various loads, and for
`
`different numbers of channels in the trunked model. The curves
`
`obtained were then compared to determine how many channels a trunked
`
`system would need to provide performance comparable to the existing
`
`system.
`
`CRnuneen
`
`&ns. Various contingencies were also examined.
`
`Contingencies can affect an LMR system in at least three ways:
`
`1. In certain circumstances, load might increase dispropor-
`
`tionately for a few nets (or fleets). For example, an automatic fire
`
`alarm going off in a hospital storeroom might cause increased activity
`
`on the fire net, the hospital net, and the security police net, but
`
`would not affect the load on other nets at all.
`
`2. On a computer controlled trunked system, fleets might be
`
`reallocated during certain contingencies. Most notably, if the base is
`
`located in an area that could become a war zone, contingency plans
`
`probably call for reallocating resources (manpower and equipment) from
`
`non-essential functions to areas vital to the base's wartime mission.
`
`3. Certain contingencies might affect the LMR system itself. For
`
`example, a fire in the room housing a repeater would not only increase
`
`traffic load, but might take the repeater off the air.
`
`These situations were examined with the trunked model.
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`Assuutin
`
`n.
`
`There appears to be no published data on call inter-arrival dis-
`
`tribution and call length distribution specifically taken from Air
`
`Force LMR nets. The assumption was made that these distributions, in
`
`general, are similar to commercial nets as described in the literature
`
`review. This assumption was checked to some extent through off the air
`
`monitoring of WPAFB nets (see Chapter V).
`
`In off the air monitoring of WPAFB nets to determine mean call
`
`inter-arrival times and mean call lengths, the statistical fluctuation
`
`over periods of time greater than several days was assumed to be
`
`negligible. This was necessary due to the time constraints of the
`
`research.
`
`The nature of the LMR users on WPAFB led to an assumption that
`
`traffic intensity is fairly constant throughout the day, and equal or
`
`heavier (depending on the specific user) during daytime than at night.
`
`This assumption was checked through off the air monitoring (see
`
`Chapter V).
`
`The Air Force will require an adjustable 0 to 6 second "drop out"
`
`time for its hybrid trunked systems (16). Drop out time is an inten-
`
`tional delay in releasing a channel after a user de-keys, and allows a
`
`user to complete a transmission if he inadvertently de-keys for a
`
`moment. This is not modelled in the simulation and the effects on the
`
`measured results are assumed to be negligible. (Actually, the simula-
`
`tion models a trunked system with a drop out time set to 0 seconds.
`
`Any other drop out time would require modifying the trunked computer
`
`model.)
`
`6
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 18
`
`
`
`Eq~uipment
`
`A VAX/VMS computer system owned by the Air Force Institute of
`
`Technology (AFIT) was used to run the simulation models. Data was
`
`collected using a Realistic PRO-2004 programmable scanning receiver and
`
`recorded on a Realistic VSC-2000 variable speed cassette tape recorder,
`
`both owned by the researcher.
`
`7
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 19
`
`
`
`i
`
`, .
`
`,j
`
`,
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`i
`
`,.-
`
`, ..
`
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`
`,
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`•
`
`Inking Schemes
`
`Reeves (8:3) discusses several trunking schemes. One of these,
`
`the simplest in terms of hardware required, includes a repeater for
`
`each channel and a number of mobile (or portable) radios, assigned to
`
`specific nets. Each radio automatically scans through the channels,
`
`stopping when it finds a signal indicating a call is about to start on
`
`the channel for that radio's net. A radio making a call finds an idle
`
`channel and sends a signal indicating which net the radio belongs to
`
`and telling other radios on the net to monitor that channel.
`
`Another technique (8:3) involves connecting a computer driven
`
`controller to the repeaters and broadcasting an idle tone on an unused
`
`channel. Each mobile radio scans the channels until it finds the tone.
`
`When a call is made, the controller has the channel's repeater send a
`
`signal indicating which net is involved. Radios not on that net then
`
`continue scanning until they find the idle tone again, which the
`
`central controller has moved to another idle channel.
`
`A third technique discussed by Reeves, and described by Thro
`
`(11:302), uses a computer to control the repeaters, as with the system
`
`previously discussed, but uses one of the channels exclusively for
`
`signalling. When radios are idle, they monitor the signalling channel.
`
`When a call is made, the calling radio sends a digital signal to the
`
`central controller, indicating which fleet the radio is on. The
`
`central controller then sends a digital signal over the signalling
`
`channel telling each radio in the fleet to tune to an idle ch:-=d.
`
`8
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 20
`
`
`
`When the call is over, each radio re-tunes back to the signalling
`
`channel and continues monitoring. This technique gives the system fast
`
`access time and good reliability.
`
`Ai Force Ru irement
`
`As in the civilian sector, the Air Force faces an increasing
`
`number of LMR users (about 30 nets on one base, for example) (1:K-2-1)
`
`and a limited number of channels available for their use. In addition,
`
`the Air Force requires a robust system capable of withstanding harsh
`
`conditions while performing reliably. The ability to inter-net
`
`(transfer a radio from one net or fleet to another) is also highly
`
`desirable, as is the ability to deploy radios from one location to
`
`another and use them with an existing system at the new location. An
`
`Air Force Communications Command (AFCC) technical report (12:7)
`
`examined several conventional and trunked LMR systems based on these
`
`requirements and concluded a hybrid trunked system would best meet Air
`
`Force needs.
`
`As explained in the report, the hybrid trunked system operates
`
`like the trunked system with a central controller and dedicated
`
`signalling channel as described above, with the added advantage of
`
`allowing each radio to operate in a conventional mode if the central
`
`controller is disabled.
`
`Air Force specifications for hybrid trunked portable radio
`
`transceivers (15), hybrid trunked mobile transceivers (14), hybrid
`
`trunked control station transceivers (13), and trunked system central
`
`"ontroller equipment (16) are currently being written.
`
`9
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 21
`
`
`
`Descriptio
`
`f
`
`U Hybri
`
`Trunked Sysl
`
`Zdunek describes an existing hybrid trunked system built by
`
`Motorola Inc. for use in the United States (17) and a similar proposed
`
`system for use in the United Kingdom (18). Both of these systems can
`
`support between 5 and 20 channels and any of the four highest in
`
`frequency can be used as the data channel. Since the radios automati-
`
`cally scan until they find the data channel, there is protection
`
`against system failure should the data channel's repeater fail: the
`
`controller simply picks another channel and the radios quickly find it.
`
`Each channel consists of two frequencies, one used as an inbound link
`
`from the broadcasting radio to the repeater, and the other used as the
`
`outbound link from the repeater to the radios in the fleet. These are
`
`often referred to as the "inbound channel" and "outbound channel" in
`
`the literature, even though both make up the channel.
`
`Motorola's trunked system can operate so either the whole message
`
`is assigned a channel, or each transmission is assigned a channel,
`
`which may, or may not, be the same channel used in the last transmis-
`
`sion. Zdunek shows better performance is realized with the transmis-
`
`sion trunked mode (17:195).
`
`The transmission trunked mode is easy to implement, because a
`
`transmission is indicated to the central controller through the push to
`
`talk (PTT) switch on the transmitting radio. A transmission starts
`
`when the radio's user keys the PTT switch and ends when the PTT switch
`
`is de-keyed. A desirable modification to this scheme is to allow a
`
`small amount of "drop out" time after de-keying. This gives the
`
`broadcasting radio's user a chance to complete a transmission if he
`
`10
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 22
`
`
`
`inadvertently de-keys for a moment. The Air Force will require a drop
`
`out time of 0 to 6 seconds (adjustable through the central controller)
`
`(16). On a busy system channels might not always be immediately
`
`available, and this might cause a delay in the middle of a message on a
`
`transmission trunked system. This condition is very undesirable, and
`
`is taken care of with a "recent user" queue which gives fleets complet-
`
`ing a transmission recently first priority in obtaining a newly
`
`available channel. The Air Force will require a queue allowing recent
`
`users to remain in it for between 0 and 90 seconds (adjustable through
`
`the central controller) and operating on a last-in-first-out discipline
`
`(16).
`
`In the Motorola system, when the user keys the PTT switch on his
`
`radio, the radio sends a 78 bit digital signal to the central con-
`
`troller via the 3600 BPS inbound signalling channel (17:198). The
`
`radio coordinates these signals in time with received signals from the
`
`central controller, so the 78 bit signal always begins at the start of
`
`a fixed length time slot (18:14). There is a chance two or more radios
`
`may try to send signals at the same time, and, because these signals
`
`are synchronized in time with the signals coming from the outbound
`
`signalling channel (the scheme is a modification of slotted ALOHA) the
`
`usable capacity of the inbound channel is about 1/(3e) = 0.123 of the
`
`total capacity on a fully loaded system (where e is the base of the
`
`natural logarithm) (17:197). A fully loaded system, in this case, is a
`
`20 channel system with 3000 radios making an average of one call each
`
`an hour. On a fully loaded system, taking into account the usable
`
`11
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 23
`
`
`
`capacity/total capacity ratio, a total capacity of 34 slots/second is
`
`required for the inbound channel (17:197).
`
`When the central controller receives a request for a voice
`
`channel, it checks and, if a channel is available, a digital signal is
`
`sent over the outbound signalling channel telling all of the radios on
`
`the requesting radio's fleet (including the requesting radio itself) to
`
`re-tune to the available channel. In the Motorola system, a 3600 BPS
`
`handshaking signal is sent over the outbound voice channel until the
`
`requesting radio re-tunes, recognizes the signal, and responds over the
`
`inbound voice channel with an 1800 Hz tone. Both the radio and the
`
`controller continue to send sub-audible signals over the voice channel
`
`for the duration of the transmission (digital data from the central
`
`controller and a constant tone from the radio) (18:14-15). On the
`
`proposed United Kingdom trunked system, access time, the time between
`
`the channel request and achieving the voice channel, is estimated to
`
`take about 460 msec when a channel is available (18:13). For the Air
`
`Force system, a 350 msec access time will be required (16).
`
`When the user finishes a transmission, he de-keys the PTT switch,
`
`and, after the appropriate drop out interval, his radio re-tunes to the
`
`signalling channel. The other radios on the fleet detect the transmis-
`
`sion is over and also re-tune to the signalling channel. The central
`
`controllcr detects the transmission is over and assigns the channel to
`
`another user as necessary.
`
`Load Analyses
`
`The obvious drawback to trunked systems is that a channel may not
`
`always be available when needed. If nineteen users, from nineteen
`
`12
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 24
`
`
`
`different fleets, are using a twenty channel system (nineteen voice
`
`channels and one signalling channel) at a given time, other users will
`
`have to wait to obtain a channel. (When they attempt to make a call,
`
`they are said to be "blocked".) It is important for trunked system
`
`designers to be able to predict, for a specific system with a certain
`
`number of channels, what the probability of this occurring will be.
`
`Also of interest is the average wait time for a blocked user, and the
`
`wait time cumulative distribution function (CDF).
`
`Another issue is whether users tend to talk longer on trunked
`
`systems than on conventional shared repeater systems (systems in which
`
`two or more distinct user groups share a common frequency). The
`
`concern is, where users on a conventional system can hear each other
`
`and may have a natural channel discipline (short, concise,
`
`transmissions), trunked users, not being able to hear other fleets, may
`
`tend to transmit
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