Field Guide · term

Also known as: color code, DMR color code, CC

A color code (CC) is a 4-bit number, 0 through 15, carried in every DMR burst that identifies which system the transmission belongs to, so a repeater or radio accepts only its own traffic on a shared frequency.1 It is DMR’s direct counterpart to the P25 Network Access Code and the NXDN RAN: a small per-burst “colour” that keeps two co-channel systems from keying each other’s repeaters or unmuting each other’s radios.

slot 1 (CC = 1) slot 2 (CC = 1) 30 ms TDMA frame — two timeslots share one 12.5 kHz channel repeater repeats the burst only if CC matches its configured value
DMR carries a 4-bit color code in each burst; a repeater keys up only for bursts whose color code matches its own.

How it works

DMR splits one 12.5 kHz channel into two TDMA timeslots. The color code appears in the burst signalling (the slot-type and embedded-signalling fields, and in the CACH), so it is present on both timeslots and on the control channel of trunked systems. A repeater is configured with one color code and will only repeat bursts that carry it; a subscriber radio likewise mutes anything with the wrong code. Because it is only 4 bits, planners assign color codes so that neighbouring repeaters on the same frequency use different values — the same reuse logic as CTCSS tones on analog channels.

The color code is an access filter, not an address. Talkgroup and radio identity are separate fields, and full network identity on a trunked DMR Tier III system comes from the system-identity signalling on the control channel. The color code is simply the fast gate that runs first.

In practice

  • On a conventional DMR repeater the color code is fixed and set once; on a trunked DMR Tier III system every site’s control channel and voice channels carry it, and it stays constant across the system.
  • Two repeaters on the same output frequency must use different color codes to coexist, so hobbyists sometimes deduce coverage overlap by watching which codes appear on a channel.
  • Because it is only 4 bits it is trivial to enumerate — a monitor can simply report whatever code arrives rather than needing a lookup table.

Relevance to SDR

For a monitor the color code labels which system a decoded burst belongs to and helps separate two DMR signals sharing a frequency. GopherTrunk reads the color code from each burst it decodes and reports it; it does not need to be pre-set to a system’s color code the way a subscriber radio does, because a receiver simply observes whatever value is present. Like the P25 NAC, it is metadata, not encryption — a matching color code gives access to signalling and clear voice but does nothing against keyed traffic.

Sources

  1. Digital mobile radio — Wikipedia, on the DMR air interface and its color code. ↩

See also