Field Guide · term

Also known as: extended colour code, TETRA scrambling code, ECC

The TETRA extended colour code is the 30-bit value that uniquely identifies a cell’s scrambling context and seeds the TETRA scrambler for every logical channel except the BSCH.1 It is built from three fields, MSB-first: the 10-bit mobile country code (MCC), the 14-bit mobile network code (MNC), and the 6-bit colour code that distinguishes neighbouring cells of the same network.2 Because the scrambler is seeded from it, a receiver cannot decode any traffic or signalling channel until it has learned the extended colour code — which it does by decoding the one channel that is not scrambled with it.

MCC · 10 MNC · 14 colour · 6 MSB LSB 30 bits total → seeds the scrambler LFSR e(1) is the MCC's high bit; e(30) is the colour code's low bit
The extended colour code packs MCC, MNC, and the 6-bit colour code into 30 bits; that packed value, with e(1) in the most-significant position, is what the scrambler's initialisation vector expects.

Cold-start learning

At cold start a TETRA receiver has no configured colour code, so it hunts the synchronisation burst, whose BSCH is scrambled with colour code zero — a value any receiver can undo. The BSCH carries a MAC-SYNC broadcast PDU whose 60 decoded bits contain the three components: the 6-bit colour code, plus (further along) the 10-bit MCC and 14-bit MNC. GopherTrunk’s ParseSyncPDU reads those fields, and ExtendedColourCode assembles them:

(MCC & 0x3FF) << 20 | (MNC & 0x3FFF) << 6 | (colour & 0x3F).

Once that value is in hand, every other channel — BNCH SYSINFO, SCH/HD, SCH/F, the AACH, and the traffic channels — becomes decodable without any operator configuration. This is why the BSCH is the linchpin of TETRA cold acquisition: it is the bootstrap that turns a locked-but-opaque carrier into a fully readable cell.

Seeding the scrambler

The packed value carries e(1) in bit 29 (the MCC’s most-significant bit) down to e(30) in bit 0 (the colour code’s least-significant bit). The scrambler, however, wants e(i+1) in state bit i, so the value’s low 30 bits are bit-reversed on the way into the LFSR — a detail that, when wrong, is invisible to any round-trip test because both encode and decode share the reversed seed (see TETRA scrambler). Keeping the packing order fixed here — MCC, then MNC, then colour, MSB-first — is what makes the reversed seed land the colour-code bits where §8.2.5.2’s initialisation equation expects them.

Relevance to SDR

internal/radio/tetra/sync_pdu.go implements the whole path: SyncPDU holds the parsed MAC-SYNC fields, ParseSyncPDU extracts them from the BSCH bit vector, and both the method form SyncPDU.ExtendedColourCode and the package function ExtendedColourCode(mcc, mnc, colour) produce the 30-bit seed. The scanner passes that seed into framing.NewScramblerTetra and the traffic extractor, so the learned identity flows straight into descrambling. An operator who already knows the cell’s MCC/MNC/colour can supply the three components directly, skipping cold-start learning — but on an unknown system the BSCH decode is what makes everything else possible.

Sources

  1. Terrestrial Trunked Radio — Wikipedia, on the TETRA standard and its cell-identity broadcast. ↩

  2. Mobile country code — Wikipedia, on the MCC/MNC pair that identifies a mobile network. ↩

See also