Subject: [NEXTEL-1] Philadelphia Police Technology ::: 800MHz Trunked Radio System

From: Marcel <[email-masked]>
Date: Sat, 25 Jan 2003 15:16:08 -0500

http://www.ppdonline.org/ppd2_tech_800mhz.htm
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Philadelphia
Police Technology

800MHz Trunked Radio System

The words "800 MHz" have become the buzzword of police departments
lately. Trunked 800 MHz radio systems are slowly shaping
the way all public safety agencies are organized, dispatched
and operated. The Federal Communications Commission (FCC)
is determined that trunking will be the radio standard into
the 21st century because it uses the radio spectrum more
efficiently and allows more users onto the airwaves.

The Advantages

Besides making greater use of an increasingly limited radio
spectrum, trunked radio systems generally permit a larger
number of users than a conventional system. For some police
departments, this alone is justification enough to spend
the money on trunked radio.

The key to this capability lies in the ability of a trunked
radio system to use a pool of frequencies for any of the
system's users. When an officer wants to transmit, a trunked
system simply looks at which frequency is unused and quickly
assigns it to the officer for the duration of the transmission.
Since field units infrequently transmit at the same time
and most police transmissions are relatively short, frequency
sharing can create more on-air time.

Another advantage of trunked systems is their flexibility.
They're controlled entirely by computer, so it's easy
to set up complex channel assignments, and to reconfigure
the system as needed without the services of a technician.
Police departments can quickly "create" an additional
channel to handle a hostage situation or change channel
assignments to allow public works or other city agencies
to communicate on the police channel during natural disasters.

In contrast, a conventional system limits each user to
just their assigned frequency. If the police channel is
busy, officers just have to wait their turn, even though
the fire or public works channel is going unused at the
moment. And in most systems, channel assignments can't
be easily reconfigured or shared with other city agencies.

Technically Speaking

Trunked radio is synonymous with the 800 MHz band, although
the technology can be applied to frequencies in any radio
band. Only FCC rules and channel availability currently
make the 800 MHz UHF band the most efficient for trunking.
In the future, trunking may become the standard in all radio
bands.

Like most other UHF systems, 800 MHz trunked systems
are designed to be repeated and to use satellite receivers
to provide complete coverage. Each trunked channel consists
of a pair of frequencies--one base station frequency and
one mobile frequency spaced 45 MHz apart.

After the FCC allocated a portion of the 800 MHz band to
public safety, they mandated a complex planning process
for each region of the country which wanted to use the band.
They established 240 channel pairs for police, fire and
related agencies with 12.5 KHz spacing in the 821-823 band.
They also established five specific channels that all agencies
must have for mutual aid and coordination.

How Does It Work?

Despite its sophistication, trunked 800 MHz is designed
to shield the user from the all the technology. A radio
operator needs to know nothing about how the system works
or why, but simply presses the button to receive the benefits
of years of research.

Basically, trunked radio is method of making a group
of radio channels available to several different groups
through an electronic switching system that is controlled
by a computer.

For the end user, trunked radio systems don't have traditional
channels. Instead, each mobile radio in the system is
assigned a "talk group," which is simply a programmable,
internal identification that is transmitted each time
the radio begins a broadcast. The ID is not heard by the
dispatcher, but it's recognized by the computer controlling
the radio system.

For the radio techs, trunked systems really do have channels,
which are typically provided in groups of four or five.
One channel is arbitrarily designated as the control channel
and the other four are available for voice communications.

In a conventional, one-channel radio system all mobile
radios listen for a transmission the single channel. Everyone
checks for a clear channel before broadcasting. When someone
pushes the XMIT button, all other radios on the channel
hear the transmission. Another user pushes the XMIT button
to respond and everyone hears that transmission, too.

In a trunked radio system, every radio is tuned to a
control channel. When someone pushes the XMIT button,
the mobile radio transmits its individual ID and assigned
talk group ID on the control channel to the base station.
It's heard only by the trunked radio system computer and
the data is decoded. The computer does some "thinking",
and instantly transmits a digital message back out on
the control channel to all the system's radios.

The digital message says, "Every radio on talk group
XX, switch to channel 1 (or 2 or 3 or 4). Again instantly,
every mobile with the same talk group code as the transmitting
mobile switches to the assigned channel and they hear
the calling mobile unit's voice transmission. Radios assigned
to other talk groups continue to monitor the control channel
and do not hear the voice transmission.

When the transmitting mobile releases the XMIT button,
all mobiles resume listening to the control channel --
and the process starts all over again if another unit
transmits. A transmission can occur on any of the assigned
voice channels. A user never knows exactly what frequency
is being assigned--and doesn't have to know, either.

This complex channel-switching process happens in just
milliseconds--so short a delay that users don't notice
that it's happening under most circumstances.

Fail Safe

As a cautious police officer, you might feel that putting
an entire city's public safety agencies on one system would
create a tremendous opportunity for disaster if the system
fails. But trunked radio systems are designed so that a
failure of one or more components doesn't affect communications.

Back-up power supplies, such as batteries or a generator,
insure that the system is always powered. Redundant radio
components for each radio channel insure that a failed
circuit board or electronic part doesn't cause that channel
to go off the air.

And since a trunked system is controlled by computer software,
when failures do occur they can be handled automatically
in an "intelligent" manner. For example, if the control
channel fails, its duties can be shifted in seconds to one
of the remaining communications channels. Channel availability
is decreased somewhat, but no one suffers from an outage.

Conclusion

The Philadelphia 800MHz system is now well into it's
development period. Once installed, it is expected that
it will result in more efficient radio communications. That
translates directly into an increase to officer safety and
their ability to quickly respond to your needs.

Source: Adapted
from article that appeared in "Dispatch Monthly Magazine."

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