FOCUS Adapting to P25 Digital Radio
An example of trunking operations.
mulcast, and suitable for use across both isolated areas and high-density city areas with lots of radio users. Other major ben- efi ts include signifi cantly improved audio quality, enhanced user features and secure end-to-end encryption, all without loss of audio clarity.
P25 was developed in two stages: Phase 1 and Phase 2. Phase 1 uses Frequency Di- vision Multiple Access technology where channels are divided according to fre- quency. The channel bandwidths for voice and traffi c are 12.5 kHz and it is backward compatible with analog. This backward compatibility is a feature of the P25 stan- dards, requiring equipment to also support an analog mode of operation.
Phase 2, only available for trunked net- works, uses Time Division Multiple Access technology, which divides the 12.5kHz into two time slots, effectively giving a 6.25kHz- channel bandwidth for greater spectrum effi ciency. It is also backward compatible with Phase 1. Phase 1 is designed for direct replacement of narrowband analog radio channels while Phase 2 is designed to in- crease the capacity of trunk-to-radio sys- tems and means you can have two separate
34 LAW and ORDER I March 2016
conversations on the same repeater. The fi nite radio spectrum is an issue in the U.S., so in a city with a large number of users you still need enough capacity and channels to handle all the calls that will be on that site at the same time. Phase 2 is de- signed to increase the capacity of trunked radio systems and means you can have two separate conversations on the same repeater. You can double the capacity of your system in Phase 2 without putting extra repeaters in.
Phase 2 is only standardized for traf-
fi c channels on a trunked radio system, but for radio-to-radio calls you still use a Phase 1 mode. A major advantage of trunking is the ability to use channels more effectively. Networks are designed with understanding of the likely call vol- ume and the number of teams or groups that need their own channel. But not all teams will use them at the same time, so the network will often be underused. With a conventional system, the user manually selects the channel he/she wants to communicate on. These networks work well when there is a low number of users over a wide area, and as the number of
users increases, additional channels may be needed for each team. But a conven- tional network will often be underused, since not all teams will use their channels at the same time.
With a trunked system, you put in a pool of available channels, and since teams aren’t all talking at the same time, you can have more teams using the network than there are channels available. Intelligence is built into the radios and the network, so the user simply presses the push-to-talk button on the radio and is automatically switched to an available channel to make the call. Ease of migration from conventional sys- tems to a P25 system is another major ad- vantage. The backward compatibility and compatibility with different manufacturers means Phase 1, Phase 2 and analog radio equipment can work together, enabling a gradual migration of your fl eet. You can have a mixed fl eet and still be interoper- able with other agencies not yet using P25. For instance, you can still have car-to-car and radio-to-radio channels, and P25 sup- ports all of them. A key factor for fl eet managers is the ease of radio confi guration when migrating to P25.
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