Field Guide · term

Also known as: CSBK, control signalling block, control signaling block

A CSBK (control signalling block) is the single-block control message of DMR. On a Tier III control channel, CSBKs carry call requests, channel grants, and system data — the DMR counterpart of P25’s TSBK.1

DMR Tier III control channel CSBKCSBK(grant)CSBKCSBK
DMR Tier III coordinates the system with CSBKs; a grant CSBK assigns a traffic channel and timeslot.

How it works

A CSBK is a single 96-bit DMR burst payload identified by a CSBKO (CSBK opcode) and a feature-ID field, followed by the message’s arguments. Like all DMR signalling, it is protected by BPTC coding and a CRC so a receiver can correct or reject bursts damaged in transit. The opcode names the message — a grant CSBK carries the target talkgroup or radio ID plus the assigned traffic channel and timeslot; other CSBKs handle registration, acknowledgements, and system status. Following the grant CSBKs is how a decoder tracks a trunked DMR system, just as TSBKs are followed on P25.

Because DMR is two-slot TDMA, a grant CSBK must specify both the RF channel and which of its two slots the call will use. On a Tier III system with a dedicated control channel this is straightforward; on lighter modes the control function itself moves, which changes where the CSBKs live.

Variants

  • Tier III standard CSBKs — the ETSI-defined set for a dedicated-control trunked system.
  • Vendor / feature CSBKs — Motorola Capacity Plus and Connect Plus, and Hytera systems, extend the CSBK set with proprietary opcodes; Capacity Plus in particular rotates control onto a rest channel rather than dedicating one.
  • Preamble CSBK — a lead-in block that wakes and addresses radios before a data or call sequence.

In practice

The CSBK opcode set is the key to identifying and following a DMR trunked system: standard Tier III systems are well documented, while Capacity Plus and Connect Plus require handling their vendor-specific blocks and, for Capacity Plus, tracking the moving rest channel. As with P25, a monitor validates each block and reconstructs system state from the CSBKs that pass their CRC.

Relevance to SDR

GopherTrunk deframes the DMR control channel, decodes CSBKs through their BPTC/CRC protection, and dispatches on the opcode to follow grants onto the correct traffic channel and slot. Handling both standard Tier III and common vendor CSBK variants is what lets it track real-world trunked DMR deployments.

Sources

  1. Digital mobile radio — Wikipedia, on DMR control signalling and Tier III trunking. ↩

See also

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