Public Safety Land Mobile Radio Systems and Architectures
What this domain tests
Domain 2 covers how a public safety call is actually delivered: P25 standards, conventional versus trunked operation, simulcast, degraded modes and the capacity maths that decides whether a busy hour produces bonks.
Core rules
- P25 is the TIA-102 family. Phase 1 is FDMA in 12.5 kHz channels at 9600 bit/s using C4FM. Phase 2 places two TDMA slots in a 12.5 kHz channel (6.25 kHz equivalent) using H-DQPSK.
- A trunked site has one control channel and N traffic channels. The control channel broadcasts site identity and channel grants; the radio requests a channel, the site grants one, and the radio moves to voice.
- Hierarchy: subscriber, site, RF subsystem (RFSS), system (WACN plus System ID). Talkgroup and unit IDs determine who hears whom.
- Simulcast: several sites transmit the same channel on the same frequency at the same instant. Timing must be aligned to microseconds, and LSM/CQPSK modulation tolerates the resulting delay spread.
- Interfaces: ISSI links systems for roaming; CSSI and DFSI link consoles; consoles can patch talkgroups.
- Degraded modes: site trunking (site loses the core and serves local users only), failsoft (control channel lost, radios revert to programmed conventional channels), and bonk when no channel is free.
- Capacity: use Erlang B with a busy-hour load and a target grade of service. Per-user load depends on PTT habits, not call duration alone.
Worked example
A site has 4 traffic channels at a 1% blocking target. Erlang B gives 1.52 Erlangs of offered load. If each officer generates 0.02 Erlang in the busy hour, the site supports about 76 users (1.52 / 0.02). Doubling talkgroup traffic without adding channels raises blocking steeply, because blocking is non-linear.
Traps
Confusing site trunking with failsoft, assuming Phase 2 doubles RF coverage (it doubles voice capacity per channel), and treating a talkgroup as if it were a frequency.