Also known as: TSBK, trunking signalling block, trunking signaling block
A TSBK (trunking signalling block) is the unit of control-channel signalling in P25.1 Each TSBK is a short, error-protected message on the control channel carrying one piece of system business — a channel grant, a registration, an affiliation, or a system parameter.
How it works
A TSBK is a fixed-length block: a manufacturer/feature flag, an opcode naming the message type, a payload of arguments, and a CRC so a receiver can reject blocks corrupted by noise. The opcode tells the decoder how to interpret the payload — a Group Voice Channel Grant carries a talkgroup and a channel number; a Unit Registration Response carries a source ID; system-service messages carry the WACN, system ID, and channel-plan parameters a radio needs to map channel numbers to frequencies. The control channel sends these blocks back-to-back at a steady rate, so a decoder simply reads block after block and acts on the opcodes it recognises.
A radio (or monitor) validates each block’s CRC, discards the bad ones, and reassembles the rest into a running picture of the system: which calls are active, which units are affiliated, and how the frequencies are laid out. Decoding TSBKs is exactly how a scanner follows a P25 system — read the grant TSBKs and retune to the assigned voice channel in step with the radios.
Variants
- Single-block TSBK — the common case; one message in one block.
- Multi-block PDU — longer control messages that span several blocks for larger payloads such as extended system data.
- P25 Phase 2 MAC PDU — Phase 2 replaces the TSBK with MAC messages that serve the same coordinating role on its TDMA control path.
- DMR CSBK — DMR’s CSBK is the direct counterpart on DMR Tier III.
In practice
Because TSBKs are self-contained and CRC-checked, a monitor can extract useful activity from a P25 control channel even under marginal signal — good blocks get through, bad ones are dropped, and the stream is redundant enough that missed grants are often re-announced. The opcode set is standardised, but manufacturers add proprietary opcodes (notably Motorola), so a complete decoder tracks both the TIA-standard and vendor messages.
Relevance to SDR
GopherTrunk demodulates the P25 control channel, deframes it into TSBKs, checks each CRC, and dispatches on the opcode to drive its trunk-following engine — following grants, recording affiliations, and learning the channel plan. Phase 2 systems are handled through the analogous MAC messages. This TSBK parsing is the core of GopherTrunk’s P25 support.
Sources
-
Project 25 — Wikipedia, on the P25 standard and its control-channel signalling. ↩