Field Guide · term

Also known as: CSBK CRC, CSBK CRC-16, CRC mask 0x5A5A

The DMR CSBK CRC is the 16-bit cyclic redundancy check that closes every CSBK (Control Signalling Block).1 It is a plain CRC-CCITT (polynomial 0x1021, initial value 0x0000) computed over the block’s leading 80 bits, but with one twist that trips up naive implementations: before transmission a fixed 0x5A5A mask is XORed onto the 16-bit checksum, per ETSI TS 102 361-1 §B.3.11, Table B.21.2 A receiver recomputes the CRC over the recovered bits, applies the same mask, and accepts the block only on an exact match.

bits 0–79 · leader + opcode + FID + payload CRC-16 · bits 80–95 CRC-CCITT (0x1021, init 0x0000) stored = CRC XOR 0x5A5A match on both intact bits AND correct block type; a wrong mask rejects every real CSBK
The CRC covers the block's first 80 bits; XORing the fixed 0x5A5A mask onto the checksum before storing it ties acceptance to the block being a genuine CSBK, not merely error-free.

What the mask is for

A bare CRC answers one question: were these bits received unaltered? DMR carries several kinds of block on the same physical channel, all protected by the same CRC-CCITT, so a bare check cannot tell a corrupted CSBK apart from an intact block of some other type that happens to satisfy the polynomial. The mask solves this by giving each block type its own additive constant. A CSBK uses 0x5A5A; a receiver that computes the CRC and expects the CSBK mask will reject anything whose stored value was masked for a different type, even when the bits themselves are error-free. The check therefore verifies integrity and identity at once — “these bits are intact and this is a CSBK” — which is exactly what a control-channel decoder needs before it acts on an opcode. XOR-ing a fixed constant does not change the CRC’s error-detection strength; it only shifts which received value counts as valid.

Getting the convention right

The precise convention is load-bearing, and the wrong one fails in a way that hides during testing. GopherTrunk once used an “init 0xFFFF, store the bitwise complement” scheme; it passed every synthesized round-trip fixture — because the encoder and decoder made the same assumption — yet rejected every real off-air CSBK, since genuine transmitters use init 0x0000 with the 0x5A5A XOR. The current constants were pinned against real ETSI Tier III control-channel bursts (Aloha and Preamble blocks that decode cleanly through the BPTC layer first). The lesson mirrors the CRC-family rule elsewhere in the codebase: a self-consistent encode/decode pair proves nothing about on-air interoperability, so a CRC convention has to be validated against real captures, not just its own round trip.

Relevance to SDR

internal/radio/dmr/tier3/csbk.go implements the check. ParseCSBK takes the 96 information bits recovered from the BPTC decode, computes framing.CRCCCITTWithInit(info[:10], 0x0000) ^ 0x5A5A, compares it against the stored 16-bit trailer, and returns a CRCError (with the partially-parsed block preserved for diagnostics) on any mismatch:

const csbkCRCMask uint16 = 0x5A5A
storedCRC := binary.BigEndian.Uint16(info[10:12])
want := framing.CRCCCITTWithInit(info[:10], 0x0000) ^ csbkCRCMask

The CRC sits after BPTC error correction in the decode chain: BPTC repairs recoverable bit errors across the 196-bit burst, and this CRC is the final gate that decides whether the recovered 96-bit block is trustworthy enough to dispatch on. Because a control-channel decoder follows grants and tracks system state from these blocks, a block that fails the CRC is dropped rather than acted on — a wrong mask constant silently starves the whole trunking engine of input.

Sources

  1. Cyclic redundancy check — Wikipedia, on CRC computation and the role of an XOR output constant. ↩

  2. ETSI TS 102 361-1 — ETSI, the DMR air-interface standard defining the CSBK CRC mask in §B.3.11 / Table B.21. ↩

See also