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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.