Field Guide · term

Also known as: rest channel

A rest channel is the channel currently carrying control signalling in trunked systems that rotate the control function around the pool rather than dedicate one frequency to it.1 When a call is assigned to the current rest channel, control moves to another idle channel — the new rest channel — so a radio that wants to place a call must first find where control now lives.

ch 1rest (control) ch 2 ch 3 ch 4 control moves when this channel takes a call
With a rotating control channel, the "rest channel" is wherever control currently lives, and it moves as calls are assigned.

How it works

Instead of burning one frequency permanently as a control channel, a rest-channel system lets the idle channel that is not carrying a call act as control. It sends the CSBKs — grants, requests, status — until a call needs a channel. If the controller assigns the current rest channel to that call, it first tells listening radios which idle channel becomes the new rest channel, then hands the old one over to voice. Radios (and monitors) follow the pointer to the new rest channel and keep listening. The scheme trades a dedicated control frequency for one more usable voice channel — attractive on small systems with only a handful of frequencies.

The cost is monitoring complexity. There is no fixed frequency to camp on: a scanner must track the rest channel as it hops around the pool, using the “next rest channel” pointers embedded in the signalling, or it loses the thread of the system entirely.

Variants

  • Motorola Capacity Plus — Capacity Plus single-site uses a rotating rest channel across its channel pool.
  • Motorola Connect Plus — Connect Plus is multisite and uses a dedicated control channel per site instead, a useful contrast to the rest-channel model.
  • Other light-trunking modes — several small DMR and LTR-style systems rotate control similarly to conserve channels.

In practice

Rest-channel systems are common in commercial and business DMR where spectrum is scarce and every channel counts. Identifying one starts with recognising that the “control” carrier moves: a searcher sees signalling jump between frequencies as calls come and go. Correctly following the rest channel is essential, because a missed hop means missing every subsequent grant.

Relevance to SDR

Rotating control complicates monitoring: GopherTrunk must follow the rest channel as it hops rather than camping on one fixed frequency. It does this by reading the next-rest pointers in the CSBK stream and retasking its control decoder to the new frequency, keeping continuity of the system’s activity as control migrates around the pool.

Sources

  1. Digital mobile radio — Wikipedia, on DMR trunking modes that rotate the control (rest) channel. ↩

See also

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