Also known as: roaming, site roaming
Roaming is a radio moving between the sites of a multi-site trunked network and re-attaching to whichever site gives it the best signal, so wide-area coverage is seamless.1 As it moves, the radio drops the old site’s control channel, acquires a new one, and performs a fresh registration; the network updates where the unit is so it can still deliver the unit’s calls.
How it works
Each site of a wide-area system runs its own control channel. A radio continuously monitors the signal quality of its current site, and each site broadcasts a list of its neighbor sites — their control-channel frequencies and identities. When the current site’s signal degrades, the radio consults that neighbor list, evaluates the candidates, and switches to a stronger site. It then performs a registration on the new site, which propagates through the network so the system knows the unit’s new location. A call for the unit or its talkgroup is then delivered to the site the unit is registered on.
Roaming is what makes a multi-site system feel like one big system to the user: a mobile unit driving across a state can move through many sites and stay in contact, with the hand-offs happening automatically and, ideally, imperceptibly. The neighbor-site broadcasts are the map the radio uses to roam efficiently, sparing it from blindly searching for a new control channel each time.
In practice
The roaming decision is a trade-off between clinging to the current site (avoiding needless switching) and moving early enough to prevent dropouts. Systems tune parameters such as signal-quality thresholds and hysteresis so a unit near a coverage boundary does not “ping-pong” between two sites. During an active call, well-designed systems can hand a unit’s call over as it roams so the conversation is not dropped mid-transmission, though the exact capability depends on the system and its network backhaul.
Roaming policy also has to account for which sites a unit is even allowed to use. On shared or regional networks, a radio may be permitted on some sites and barred from others, and a site can decline to accept a roamer during congestion. So the neighbor a radio would prefer on signal strength is not always one it may register on, and the roaming algorithm weighs permission and site status alongside raw signal quality. From the outside, a unit that seems to “skip” the nearest site and register on a farther one is often obeying exactly these constraints rather than making a poor choice.
Roaming and registration are two halves of the same behaviour: roaming is the decision to change sites, and registration is the announcement that carries it out. The system’s picture of where every unit is stays correct only because each roam ends in a registration, and delivering a wide-area call depends on that picture — the controller sends the call to the site (or sites) where the target’s registration says it is. When a unit roams between two RF Subsystems of a very large network, the transfer may involve a hand-off between controllers as well, so that the unit’s “home” record is updated network-wide rather than just at the local site.
Relevance to SDR
For a monitor, roaming is visible as a stream of registration events: the same radio ID appearing on one site’s control channel, then on another’s, traces the unit’s movement across the network. GopherTrunk parses registrations and the neighbor-site lists a site broadcasts, so it can map the topology of a multi-site system and follow units as they roam between trunking sites. Practically, this also tells a scanner which sites are worth monitoring for a given unit or talkgroup.
Real systems with wide-area roaming include P25 (multi-site/networked systems), Motorola SmartZone, and DMR Tier III connected sites. GopherTrunk observes roaming from control-channel signalling; it does not roam itself — a receiver stays on whatever site’s control channel the user has it tuned to, though the neighbor lists tell it where the other sites are.
Sources
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Trunked radio system — Wikipedia, on multi-site operation and units moving between sites. ↩