More Info On TrunkingBack to Pro Radio Scanner Home
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.
Thanks
to:
http://www.ppdonline.org/ppd2_tech_800mhz.htm
Back to Pro Radio Scanner Home