Field Guide · term

Also known as: multisite trunking, wide-area trunking, networked trunking

Multisite trunking is the topology in which many trunking sites are linked into a single wide-area system so that users stay on one logical network as they move across a region.1 Each site still runs its own control channel and voice pool, but a central network controller ties them together: when a call is placed, the system repeats it on every site where members of that talkgroup are currently affiliated, so a conversation can span an entire county or state.

network controller Site 1 Site 2 one call → repeated on every site with affiliated members
A wide-area system links sites through a controller; a talkgroup's call is keyed up on each site where its members are registered.

How it works

Radios affiliate with the strongest site’s control channel and register their talkgroup membership. The network tracks, per talkgroup, which sites have listeners. On a call the controller grants a voice channel on each of those sites and links their audio, while quiet sites are spared — saving spectrum. As a user drives out of one cell and into another, their radio reads the current site’s advertised neighbour list and hands off automatically, a process called roaming.

There are two ways sites can overlap. In multisite proper, adjacent sites use different frequencies and radios roam between them. In simulcast, a group of transmitters share one frequency in tight sync to act as a single large cell. Large networks mix both: several simulcast cells, each treated as one site, wired into a multisite whole.

In practice

  • Each site is numbered within its subsystem; P25 groups sites into an RF subsystem (RFSS), and RFSSs into a network identified by a WACN.
  • Only sites with affiliated members carry a given call, so what you can hear depends on where the talkgroup’s users physically are.
  • A monitor near a system edge often receives several sites at once and must choose which control channel to follow.

Relevance to SDR

A single software-defined radio tuned to one site hears only the calls that site is carrying. To follow a whole wide-area system you either move between control channels or run several receivers. GopherTrunk decodes one site’s control channel at a time — reading its identity, neighbour list, and grants — which is exactly the slice of a multisite network a scanner can observe from a fixed location. Modern P25 and DMR Tier III public-safety networks are almost always multisite.

Sources

  1. Trunked radio system — Wikipedia, on wide-area, networked multi-site trunking. ↩

See also