Also known as: neighbor site, neighbour site, adjacent site
A neighbor site is an adjacent site of a multi-site trunked network, and the term also names the broadcast — an adjacent-site status message — by which a site advertises those neighbors on its control channel.1 Each such message names a nearby site and, crucially, its control-channel frequency, so a roaming radio knows exactly where to look when it needs to move to a stronger site instead of blindly searching the band.
How it works
A site periodically transmits adjacent-site status messages on its control channel. Each message identifies a neighboring trunking site — by site number and system identity — and gives the parameters a radio needs to acquire it, principally the neighbor’s control-channel frequency, plus flags describing the neighbor’s status (whether it is in normal operation, whether it accepts the radio’s system, and so on). Taken together, these broadcasts form a small routing table of the local network topology that every radio on the site receives for free.
A radio uses this table when roaming: rather than scanning the whole band to find another site when its current one fades, it jumps directly to a neighbor’s advertised control-channel frequency and evaluates the signal there. The neighbor list therefore turns site selection from a blind search into a directed one, which is what makes hand-offs across a wide-area system fast enough to be seamless.
In practice
Neighbor broadcasts are also how the extent and shape of a network becomes discoverable. Because every site advertises its neighbors, walking the graph — site 3 names sites 2, 4 and 7; site 7 names 3, 8 and 9; and so on — reconstructs the whole multi-site system and all of its control-channel frequencies. This is invaluable for anyone mapping a system, and it is exactly the data a mobile radio consumes to stay connected while moving.
The frequency of these broadcasts is a design balance. Advertise neighbors too rarely and a fast-moving unit can reach the edge of coverage before it has learned where to go next, producing dropouts; advertise them too often and the airtime spent on housekeeping crowds out grants and other useful signalling. Systems therefore interleave neighbor messages with the rest of the control-channel traffic at a modest rate, enough that a radio parking on a site accumulates the full neighbor picture within a short window. For a monitor this means a site’s neighbor list is not learned instantly on tuning; it fills in over the first several seconds as the individual adjacent-site messages come around.
There is a limit to how much of the network any one site reveals. A neighbor broadcast usually lists only the immediately adjacent sites, not the entire system, and a large network is advertised in fragments — each site knows its own neighborhood. Reconstructing the whole thing therefore means monitoring several sites and stitching their neighbor lists together, following the edges of the graph outward. The lists can also change: a site added, removed, or taken out of service updates what its neighbors advertise, so the topology a monitor builds is a living map rather than a fixed one. Some systems further qualify each neighbor entry with status flags — for example indicating that a neighbor is in a degraded mode or does not currently accept roamers — which a radio weighs before choosing to move there.
Relevance to SDR
For a monitor, neighbor-site broadcasts are one of the most useful control-channel messages, because a single site reveals the frequencies of the sites around it. GopherTrunk parses adjacent-site status messages to learn a multi-site system’s topology automatically: from one tuned control channel it can enumerate the neighboring sites and their control-channel frequencies, building a map without the user having to enter every site by hand. That map, combined with registration events, is what lets a scanner understand roaming and decide which sites to watch for a given talkgroup.
Real systems that carry neighbor/adjacent-site broadcasts include P25 (Adjacent Status Broadcast / RFSS Status Broadcast), Motorola SmartZone, and DMR Tier III connected systems. GopherTrunk reads these passively from the control channel — the neighbor list is information the system publishes for its own radios, and a receiver simply overhears it.
Sources
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Trunked radio system — Wikipedia, on multi-site systems advertising adjacent sites for roaming. ↩