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New Communications System - APCO P25

When the new Lewis County public safety radio communications system went on line, scanner listeners discovered they could no longer hear radio transmissions. It's not just because the frequencies changed, and it didn't happen just in Lewis County. Many counties have already switched, and the remainder will soon follow.

During the 9-11 attack and disasters such as Katrina, it became very obvious that interoperability was a big problem. Different agencies simply could not talk to each other on the radio. In some cases, different units of the same state or federal agency couldn't even talk to each other. In addition, in urban areas or during large scale incidents, there was not enough space available in the public service frequency bands to add more channels.

To solve these problems, the federal government mandated some changes.

The first change was narrowbanding. Channel bandwidth went from 25 kHz to 12.5 kHz or 6.25 kHz. This effectively doubled or quadrupled the number of available frequencies or 'channels.'

There's really no need for more channel space here in the north country, but in order to be interoperable with other agencies in case of mutual aid during a major emergency, everyone's equipment has to be the same.

Along with narrowbanding,other standards are required to ensure interoperability. Enter APCO P25.

APCO is the Association of Public-Safety Communications Officials. Project 25 is a standard for interoperable two-way wireless communications.

Along with narrowbanding, P25 specifies many other things. The most significant are standards for digital trunking.

In a conventional analog FM systems, radios have one or more channels (frequencies). All radios on the same frequency can hear each other, radios on other frequencies can't. Radios are preset to certain frequencies ( 'channels') and can't be easily changed. Channels are typically assigned to different functions or agencies.

A trunked radio system operates a little differently. Instead of being using 'channels' tied to a particular frequency, users are assigned to 'talkgroups.' Frequencies are dynamically assigned to talkgroups. This allows many different users or user groups to communicate using a relatively few frequencies. Radios are microprocessor-controlled, and a dedicated control frequency is used to coordinate the process.

Transmissions on the actual communication 'channels' can be analog FM or digital or a mixture of both.

Virtually all new P25 systems are digital. Audio is translated to and from analog to digital data by a vocoder, and the resulting data stream is sent over the air.

Advantages include better audio quality in most situations, and the ability to squeeze even more actual information in to the same bandwidth. For example, in some systems two separate talkgroups can use the same 12.5 kHz 'channel' at the same time. In addition, pure digital data (documents, pictures, etc.) can be sent as easily as digitized voice.

The bottom line for scanner listeners is that your old scanner won't work. You can program it to the frequencies used by the system, but all you'll hear is bursts of data.

Digital trunk tracking scanners are available but they aren't cheap, and programming them is not as simple as a conventional scanner. Alternatively, an inexpensive SDR (software defined radio) receiver and a computer can be used. It's a project I'm going to try shortly.


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