Field Guide · term

Also known as: emergency call, emergency alarm, emergency bit

An emergency call is a group call that carries an emergency flag — a single bit or a dedicated opcode in the control-channel signalling — that marks it as the highest-priority traffic on the system and, on many systems, triggers a distinct alert at the dispatcher’s console.1 Pressing a radio’s orange emergency button sets this flag, so the request jumps the queue for a voice channel even when the system is busy.

radio 4567emergency btn EMERGENCY set control channel grant first jumps queue ahead of ordinary calls
An emergency-flagged request is granted ahead of ordinary queued calls and alerts the dispatcher.

How it works

Trunked systems queue channel requests when all voice channels are in use. An emergency call carries a flag that raises it to the top of that queue, and on most systems it also enables ruthless preemption: if no channel is free, the controller can drop a lower-priority call in progress to free one for the emergency. The flag rides in the service-request message a radio sends and is echoed in the resulting channel grant, so both the system and any monitor can see that the call is an emergency and which talkgroup and radio ID it belongs to.

Two related but separable events exist on most systems:

  • Emergency alarm — a signalling-only message that registers a unit’s emergency status at the console without necessarily opening voice. It is essentially a flagged status packet on the control channel.
  • Emergency call — the flagged voice call itself, carried on a granted voice channel. A radio in emergency mode typically transmits both.

The emergency condition usually persists at the console until a dispatcher clears it, so subsequent calls from the same unit may stay flagged for a while.

Variants

  • P25 signals emergency in the group-voice service options (an Emergency bit in the service-options byte of the grant and in the group affiliation/service request), alongside priority-level bits.
  • Motorola Type II / SmartNet uses distinct emergency command words and an “Emergency” talkgroup status derived from the last status bits of the ID.
  • DMR Tier III carries the emergency indication in the control-signalling block (CSBK) and pre-emption is a system option.

In practice

Emergency handling is one of the sharpest differences between a trunked system and a plain conventional channel. On a busy conventional channel an emergency simply has to wait for a gap; on a trunked system the emergency flag reorders the queue and, with ruthless preemption enabled, can seize a channel that is already carrying a lower-priority call — that call is dropped and its users hear their transmission cut off. Systems also route the emergency to a designated emergency talkgroup or console position, so the right dispatcher is alerted even if the unit’s normal talkgroup is monitored elsewhere.

A subtlety worth knowing is that the emergency indication is sticky. Once a unit declares an emergency, the condition typically remains asserted at the console — and may keep tagging that unit’s subsequent transmissions — until a dispatcher explicitly clears it, not merely until the unit stops talking. For a monitor this means an emergency can color a run of calls, not just one, and the clear event is itself a signalling message worth capturing. Accidental activations (“hot mic” or a bumped button) are common enough that the clear/acknowledge traffic is a normal part of the picture.

Relevance to SDR

Emergency traffic is often the most operationally significant traffic on a system, so a scanner that can recognize the flag can surface it to the user immediately. GopherTrunk parses the service-options and opcode fields in control-channel grants and requests, so it can label a call as an emergency and expose which talkgroup and unit raised it — useful for priority scan logic and for alerting. Because the emergency flag lives in unencrypted control-channel signalling, GopherTrunk can detect and log an emergency even when the associated voice is encrypted; only the audio content depends on the traffic being in the clear.

Real systems where this applies include P25 Phase 1/Phase 2 public-safety networks, Motorola trunking, and DMR Tier III. GopherTrunk treats the emergency indication as call metadata it recovers from the control channel, not as anything it can originate — it is a receiver only.

Sources

  1. Trunked radio system — Wikipedia, on priority and emergency handling in trunked systems. ↩

See also