Also known as: trunked radio, trunking
Trunked radio is a system architecture in which many user groups share a small pool of frequencies, with a computer assigning a free channel to each call for its duration and reclaiming it afterward.1 A control channel coordinates the whole system, so a handful of radio channels can serve hundreds of talkgroups that would each need a permanent frequency under conventional radio.
How it works
When a user keys up, their radio sends a call request on the control channel. The site controller picks a free voice channel from the pool and broadcasts a channel grant that points the target talkgroup to it; every radio affiliated to that group — plus any monitor following the system — retunes to the named channel and slot. When the talker unkeys and a hang-timer expires, the channel is released back to the pool, ready for the next call.
The efficiency comes from statistics. Real voice traffic is bursty: any one group transmits only a small fraction of the time, and groups rarely all key up at once. Trunking exploits this by pooling demand, a form of the same queuing theory (measured in erlangs of offered load) that lets a telephone exchange serve many subscribers with fewer trunk lines than customers.2 A well-engineered system sizes its channel count so that the probability of a call finding every channel busy — the grade of service — stays low. When it is exceeded, the system either queues the request or returns a busy tone.
Variants
- FDMA vs. TDMA. FDMA systems (P25 Phase 1, NXDN) give each call its own frequency; TDMA systems (P25 Phase 2, DMR Tier III, TETRA) divide each frequency into timeslots so one RF channel carries several calls at once.
- Dedicated vs. distributed control. Most systems dedicate one frequency as the control channel; lighter DMR modes rotate control around the pool using a rest channel.
- Single- vs. multi-site. A lone trunking site covers one cell; multisite trunking links many sites into a wide-area network, sometimes with simulcast transmitters and automatic roaming between neighbor sites.
In practice
Public-safety and commercial fleets are the dominant users: police, fire, transit, utilities, and campuses. The largest deployments — statewide P25 systems, TETRA networks in Europe — carry tens of thousands of subscribers across hundreds of sites, tied together by registration and group-call signalling. When the network backbone fails, sites can drop into failsoft, reverting affected channels to conventional repeater operation so local traffic survives.
Relevance to SDR
To monitor a trunked system you decode the control channel first, then follow the grants it issues — exactly what GopherTrunk does for P25, DMR Tier III, and others. Because a monitor only has to keep up with one data channel plus whatever calls are active, a single wideband SDR capture can reconstruct dozens of concurrent conversations, tagged by talkgroup and radio ID. This is the core reason trunk-following is worth the extra decoding complexity over simply scanning fixed frequencies.
Sources
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Trunked radio system — Wikipedia, on trunking architecture and control-channel coordination. ↩
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Erlang (unit) — Wikipedia, on the traffic-engineering measure that underlies channel-pool sizing. ↩