textfiles/hamradio/800trunk.txt

John W. Reed
Coats
English 112
30 May 1990
                 Public Safety Trunking Systems

Thesis: The use of trunked communications systems by police      
        departments has grown since its invention several years  
        ago. Each year, more and more public safety agencies are
        changing out their conventional communications systems   
        with trunked communications systems.  They do this       
        because the use of trunking provides the agency with a   
        system that is in that trunking is reliable, efficient,  
        and versatile.

I.    Trunking defined
      A.  Band location
      B.  Who invented it
II.   How trunking works
       A.  Motorola
           1. Smartnet
           2. Smartnet II
        B.  General Electric
            1.  Mastr II
        C.  E.F. Johnson
III.  Why trunking is reliable
      A.  Multiple repeater
      B.  Fail soft systems
IV.   Efficiency of trunking
      A.  Channel saving
      B.  More people on a trunked system
V.    Versatility
      A.  Phone interconnects
      B.  Security
      C.  County-wide systems
      D.  It meets the needs of the user
      E.  Computer programmability
VI.   Widespread use by police agencies
      A.  Dekalb
      B.  Gwinett












John W. Reed
Coats
English 112
30 May 1990

                  Public Safety Trunking Systems

	Having a reliable radio communications structure in any

public service agency is vital to its operation.  The use of

trunked communications systems by police departments has grown

since its invention several years ago.  Each year, more and more

public safety agencies are changing out their conventional

communications systems with trunked communications.  They do this

because the use of trunking systems provides the agency with a

system that is reliable, efficient, and versatile.

	Land mobile radio is the fastest growing telecommunications

industry (Dordick 223).  In 1980, there were nearly 760,000

licensed stations.  As of April 1989, there were over 1.3 million

licensed stations (Popular Communications).  In common land

mobile radio communications, there are 4 major bands.  These

consist of Low Band (25-50 MHz), High Band (150-174 MHz), UHF Lo

(450-470 MHz), UHF T (470-512 Mhz) and the UHF HI or 800 band

(806-942 Mhz)  (Hughes 3). Depending on the agency's need,

different bands are selected.  For a rural fire department, Low

Band might be used, as Low Band provides more range and less

electrical noise.  For a security agency or other personnel that

spends a lot of time indoors, UHF would be used, as UHF signals

travel through walls unlike those of Low Band which bounce off   

them.  

	Today, the lower bands, UHF, High, and Low, are extremely

congested.  No longer can one license a repeater, a device that

automatically retransmits the signal of a mobile unit, in turn

giving the mobile a greater span of communications, on those

bands.  The only way to go is up!  Mr. Inglis adds that "the

rapid growth of land mobile sources has placed continuing

pressure on regulatory authorities to provide additional RF

spectrum to accommodate new land mobile system users" (21.18). So

why 800 MHz?  800 is the least full of all the bands.  This is

also the band where trunking systems exist.  Trunked

communications systems are licensed for five to twenty channels

in the 806-942 MHz band (Hughes 7). 

     According to Edward Loughlin,  trunking is derived from the

telephone system.  Suppose the phone company was going to provide

service to Podunk.  50 people in Podunk subscribe to the phone. 

Never would all 50 people be talking on the phone at the same

time, and the phone company takes advantage of this.  Instead of

running 50 lines to each of the 50 people in Podunk, the phone

company runs 12 lines. And, at any given time, 1 of those 12

lines would not be occupied.  Thus, more people can be

accommodated with less number of lines.  The telephone company

uses this same principle for routing long distance calls. 

Suppose a caller in Atlanta places a call to Memphis.  The call

goes to a long distance exchange in Atlanta, where there may be X

number of lines to Memphis.  The switch searches for an unused   

line and connects the call to that line, while the exchange in

Memphis connects the line to the party being called.  Trunking

works all the same way.

	In a trunked system, five or more repeaters work together as

a group (Hughes 7).  According to a service manual for a General

Electric Trunked System, the channels are "uniformly spaced 25

KHz apart with the repeater transmitting 45 MHz higher than the

mobile."  Suppose a trunked system has 20 repeaters.  All of the

mobiles and base units are programmed to operate on any of the

twenty channels.  Similar to cellular phones, the programming and

switching of frequencies is accomplished with the aid of

computers.   Unlike cellular phones, a trunked system does not

switch to another channel in the middle of a conversation.

	There are three major types of trunking systems: Motorola,

General Electric, and E. F. Johnson (Hughes 7).  More or less,

they all work the same way.  When a mobile user presses the

microphone button, the radio automatically tunes to an empty

channel.  At the repeater site a computer then tunes all the

other radios in the system to the same channel.  If all the

channels are busy, a flashing "busy" light will be activated on

the mobile radio (Kay 10).  According to Edward Loughlin, at any

given time one channel should be available.

	There are distinct differences, however.  In the General

Electric system, each radio constantly scans all available

channels.  When the microphone button is pressed, the scanning

stops on a vacant channel and the radio transmits "a fleet-      

specific code" (Hughes 7).  All radios with the same code switch

to that particular channel.  When the microphone button is

released, the repeater holds a few seconds to allow a unit to

reply before scanning starts again.  Each repeater in the system

stands alone, and its failure would not be catastrophic to the

rest of the system.

	The E.F. Johnson system is quite different.  Each group of

radios is assigned a channel in the system and it is designated

the "home" channel.  All the repeaters are linked together via

phone or other high speed transmission lines.  If the home

channel is busy, a radio originating the call will switch to an

unused channel, and simultaneously, send a burst of data that

will connect the other units to the new channel.  Each time the

microphone button is released, the repeater deactivates and

returns all units to the original home channel.  Of course, in

the Johnson system, there is a possibility of the system becoming

full. 

	The Motorola system, in my opinion, is the best of all

trunking systems.  On a twenty channel trunking system, one of

the channels is designated the data channel.  All radios in the

system are tuned to the data channel.  When the microphone button

is pressed, a burst of information is sent to the repeater site

controller, a computer.  The controller then switches all radios

in that particular person's group to an empty channel.  If all

channels are busy, the radio group goes on a computerized waiting

list and is literally called back when a channel becomes free.   

If the controlling computer happens to fail, each radio in a

group, and there can be many groups to a system, reverts to a

predetermined base channel.  The system then functions as a

conventional repeater system.  

      Trunking is far more reliable than a conventional repeater

system.  In a conventional system, a service may have one or more

separate radio channels. Sometimes, in a shared conventional

system ( a system with more than one user ), a user would have to

wait his turn.  If all the stations on the system try to talk at

once, the result is more madness! (Hughes 7)  The trunked system,

however, allows for all users in the system.  When a group in the

trunked system is allocated a channel by the system, all other

users on the system are locked out of that particular channel. 

This prevents eavesdropping by the other users and also prevents

interference from the other groups, like being covered up or

"stepped on."

	A trunking system has an incredibly high level of

reliability.  A trunked user would, normally, not be aware of a

failure of any of the repeaters in the system.  If the system

controller recognizes a failure on one of the channels, it locks

that channel out of the system.  The system then redistributes

the remaining channels.  Should the control repeater fail, the

controller assigns one of the other repeaters as the control

repeater.

	In a conventional system, if the repeater malfunctions and

the users do not have a backup repeater, the users are cut off.  

This could be disastrous in an Ambulance or Police communications

system.  In a trunked system, likewise, trouble would occur if

the system computer malfunctioned.  In a public safety trunking

system, there is usually a backup computer, or a system that

"distributes the control logic in such a way that failure does

not disable the system" (Hughes 8).  In a trunking system, if the

system computer and its backup fails, the system goes into Fail

Soft mode.  Each group on the system is preassigned to one of the

channels which then functions as a conventional repeater.  The

Motorola system takes advantage of this wholeheartedly.

    Trunking is, by far, the most efficient means of land mobile

communication.  Theoretically, if there are five channels, each

channel is not busy one fifth of the time.  There should always

be one unused channel.  Trunking makes use of the channels by

distributing the load resulting in efficiency (Hughes 7).

A non trunked single channel repeater could accommodate fourteen

mobile units with fourteen units per channel.  On a five channel

trunking system with one of the channels as a data or control

channel, two hundred sixty mobiles could be accommodated with

fifty-two units per channel.  In a twenty channel trunking system

with one channel as the data channel, one thousand nine hundred

mobile units could be accommodated at ninety-five units per

channel (Inglis 21.22).

	Trunked systems are more versatile than a conventional

system.  True, a conventional system can be fitted with a phone

interconnect, but the trunking phone interconnect is full duplex,

it receives and transmits at the same time like a normal phone

(Fowler).  Plus, "the addition of computer like functions and

control signaling techniques have greatly expanded the capacity

of two-way radio systems"  (Dordick 223).  Unlike a conventional

system, a trunked system has the ability to reprogram all of the

radios in the field at the same time and over the air.  For

example, in a police trunking system there is a group for traffic

enforcement and a separate group for homicide.  There is a high

speed chase which results in a murder.  The base can reprogram

the detectives' radios to operate on the traffic enforcement

group while the detectives' are enroute to the scene.  For a

conventional system, the detectives would have to have the

traffic channel preprogrammed on a separate channel or they would

have to take the radios back to the base to be reprogrammed by a

computer.  Plus, according to Ben Ho, trunking equipment can be

interfaced with data equipment (55).  For example, a police car

could have a terminal in the vehicle to run license checks

without having to call the base.  Ho also says that the use of

trunking makes expandability cost effective in the long run (55). 

	Day by day, more and more agencies are deciding to move from

their old radio system to a trunked radio system.  Years ago,

Manual Maloof, Executive Officer for DeKalb County, opted to put

the entire county on a trunked system.  All county services are

now on their public safety trunking system.  This provides the

police the ability to talk to the sheriff which can then talk to

the fire department, all on the same system.  DeKalb has its     

system placed on top of Stone Mountain which provides the county

superior radio coverage.  Cobb County Water Department is also on

a trunked system.  It will not be too long before the entire town

goes trunking.  Gwinnett County also has a set of trunking

channels licensed to them.  It will only be a matter of time

before they go to trunking.  Fulton County is on trunking, as is

Clayton County.  

	Police cars were equipped with radios as early as 1926

(Dordick 223).  Little did they know that radio communication

would traverse so far.  Inglis remarks that it is obvious that

trunking is beneficial to the preservation of radio spectra. 

Figures prove that a 20 channel system (19 with one data channel)

can provide seven times the number of a single channel repeater

system with the same if not increased quality of service. 

(21.22)  With today's congested band space, it is vital to come

up with new efficient means of communications.  At the same time,

it is vital for public service agencies to have a competent

communications system.  Without, lives could be lost, and the

safety of the user could be impaired.  

  






                           Works Cited

Dordick, Herbert S., Understanding Modern Telecommunications.    
  New York: McGraw-Hill, 1986.

Fowler, Ralph W.  Personal Interview.  30 April 1990.

General Electric Company.  Training Manual for G.E. Marc V
     Trunking System.  Lynchburg, VA: General Elecrtic Company,
     1983.
Ho, Ben.  "Centralized Dispatching and Tracking Viz Mobile and
     Packet Radio."  Telecommunications August 1986 : 55.

Hughes, Gene, ed.  Police Call.  Los Angeles:  Hollins Radio Data
     1988.

Inglis, Andrew F, ed.  Electronic Communications Handbook. 
     New York:  McGraw-Hill, 1988.

Kay, Bob.  "Trunk Busting Basics for Scanner Listeners."
     Monitoring Times Sept 1988:  10-11.

Loughlin, Edward.  Personal Interview.  8 May 1990.

"More Frequencies Proposed for Trunked Technology in the Private
     Land Mobile Radio Services."  Popular Communications
     April 1989:  50.

Watts, Frank.  Personal Interview 21 May 1990.