Field Guide · term

Also known as: CCW, EDACS CCW, control channel word

The EDACS Control Channel Word (CCW) is the 40-bit signalling block that rides every EDACS control-channel slot after sync detection.1 EDACS — the GE / Ericsson trunking system — sends a steady stream of these words on its outbound control channel, each announcing one piece of system business: a voice or data channel grant, a system-identity broadcast, an adjacent-site pointer, or an idle heartbeat. Every CCW is prefaced by a 24-bit sync sequence and protected by a BCH(40,28,2) code so it survives a noisy channel.2

sync · 24 Cmd 4 Sts 4 Address 16 LCN 5 Aux 11 the same 40 wire bits = BCH(40,28,2) codeword bits 39..12 information (28) · bits 11..0 parity (12)
Each EDACS control slot is a 24-bit sync followed by the 40-bit CCW. GopherTrunk reads the same 40 bits two ways: as five logical fields, and — with FEC enabled — as a BCH(40,28,2) codeword whose low 12 bits are parity.

Field layout

The CCW packs five fields MSB-first across its 40 bits:

Bits Width Field Meaning
39..36 4 Command Operation type — grant, system, idle, …
35..32 4 Status Per-command flag bits (bit 0 encryption, bit 1 emergency)
31..16 16 Address Talkgroup, radio ID, or site ID (per Command)
15..11 5 LCN Logical channel number into the operator band plan
10..0 11 Aux Command-specific auxiliary parameter

The 4-bit Command field is the opcode. GopherTrunk models the subset most useful for trunk-following:

Command Value Meaning
Idle 0x0 Control-channel idle / heartbeat
GroupVoiceGrant 0x1 Group voice channel grant
ProVoiceGrant 0x2 EDACS ProVoice (digital) grant
IndividualCall 0x3 Unit-to-unit call
DataGrant 0x4 Data channel grant
SystemID 0x5 System-identity broadcast
AdjacentSite 0x6 Neighbour-site announcement
Emergency 0x7 Emergency call
Affiliation 0x8 Unit affiliation
Encryption 0x9 Encryption signalling
Reserved 0xF Reserved

A voice grant carries the talkgroup in the Address field and an LCN that indexes an operator-supplied band plan; the receiving radio retunes to the physical channel that LCN names. The Status nibble layers flags onto a grant — encryption in bit 0, emergency in bit 1.

Forward error correction

Under GopherTrunk’s BCHOn mode the 40 on-wire bits are not raw field bits but a BCH(40,28,2) codeword: 28 information bits in the high positions (39..12) and a 12-bit parity field in the low positions (11..0). The code is a shortened form of the BCH(63,51,2) mother code over GF(2^6) with primitive polynomial x^6 + x + 1. Its generator is the product of the minimal polynomials of α and α^3:

  • m₁(x) = x^6 + x + 1
  • m₃(x) = x^6 + x^4 + x^2 + x + 1
  • g(x) = m₁(x) · m₃(x) = x^12 + x^10 + x^8 + x^5 + x^4 + x^3 + 1 = 0x1539

The code corrects up to t = 2 bit errors per codeword (designed minimum distance d = 5). GopherTrunk’s decoder computes the 12-bit syndrome, applies a single-bit correction from a per-position syndrome table, and — failing that — searches the 780 ordered position pairs for a matching double-bit error before re-deriving the fields. Per the canonical open reference (lwvmobile/edacs-fm), BCH is the only on-wire FEC on the Standard EDACS CCW; no outer interleaved or Reed-Solomon layer sits above it.

In practice

The 24-bit outbound sync is documented as 0x55D5AA across the public reference implementations GopherTrunk tracks; a sliding detector locks onto it (tolerating a mismatched bit or two) and the adapter then slices the 40-bit CCW that follows. Both the field-position and opcode assignments follow the most-cited public reference (lwvmobile/edacs-fm) rather than a paywalled EDACS specification, so they are best-effort — worth cross-checking against a fresh live capture before trusting an unusual Command or Status interpretation. Strict-mode operators reject any CCW whose Command falls outside the recognised set, since an unknown opcode is a strong signal of bit errors or a misaligned codeword.

Relevance to SDR

internal/radio/edacs/ccw.go holds the CCW packing (ParseCCW, CCWFromBits); opcodes.go interprets the Command and Status fields into structured grants; and internal/radio/framing/bch_edacs.go implements the BCH(40,28,2) encode/decode. Together they let GopherTrunk follow an EDACS system: read the grant CCWs, map each LCN through the band plan, and retune to the assigned voice channel in step with the fielded radios.

Sources

  1. Enhanced Digital Access Communications System — Wikipedia, on the GE/Ericsson EDACS trunking system and its control channel. ↩

  2. BCH code — Wikipedia, on the error-correcting code family that protects the CCW. ↩

See also