Field Guide · term

Also known as: CMCE, circuit mode control entity, MLE PDU

The TETRA CMCE PDUs are the circuit-mode call-control messages that a TETRA infrastructure sends on the control channel to set up, connect and tear down voice calls.12 They ride in the TL-SDU handed up by the LLC layer, beneath a 3-bit MLE protocol discriminator. A receiver following a system reads them to learn who is calling whom and — paired with the MAC channel-allocation element — where the call’s traffic is, which together form a voice grant.

disc 4 = CMCE type call id · source · dest + MAC chan-alloc carrier · timeslot voice grant → retune to traffic slot disc top 2 bits = protocol id: 00 MLE · 01 MM · 10 CMCE · 11 SDS
A Layer-3 PDU opens with a 4-bit discriminator (its top two bits the protocol identifier); a CMCE D-CONNECT names the call's parties, and the MAC channel-allocation element from the same slot supplies the carrier and timeslot — together a voice grant.

The discriminator

Every Layer-3 PDU opens with a 4-bit discriminator (§14.7). Its top two bits are the protocol identifier that selects the sub-protocol, and the low two bits are the upper part of the PDU type within it:

Discriminator Protocol
00xx MLE — Mobile Link Entity (system broadcast)
01xx MM — Mobility Management
10xx CMCE — Circuit Mode Control Entity (call control)
11xx SDS — Short Data Service

Only CMCE and MLE carry trunking-relevant messages a voice follower acts on; MM and SDS are dropped by strict validation.

The CMCE PDUs

Type PDU Role
0x1 D-SETUP Incoming call setup — announces a call is forming
0x2 D-CONNECT Call connected — carries the traffic grant
0x4 D-RELEASE Call released — teardown
0x5 D-TX-CEASED Talker stopped transmitting
0x7 D-TX-GRANTED Late-grant transmission permission
0x9 D-INFO Supplementary services
0xA D-CALL-PROCEEDING Call proceeding

D-SETUP and D-CONNECT are the pair that opens a call: the setup announces it and the connect confirms it and carries the grant. A follower assembles the VoiceGrant field by field — the physical resource (carrier + timeslot + usage marker) from the MAC allocation element, and the call identifier, source and destination SSIs and emergency flag from the CMCE TM-SDU in the same PDU. Whether the destination is a talkgroup GSSI (a group call) or an individual ISSI (a private call) is classified from that destination address so the UI does not surface a radio ID as a phantom talkgroup, and the emergency bit marks an emergency call. D-RELEASE and D-TX-CEASED drive teardown; the latter is treated as idle filler the state machine absorbs.

MLE SYSINFO

Under the MLE discriminator, PDU type 0x3 is SYSINFO, the network-broadcast that carries the system identity: a 10-bit Mobile Country Code, a 14-bit Mobile Network Code, and a 14-bit Location Area. The MCC and MNC uniquely tag a TETRA system, and GopherTrunk treats the first SYSINFO as the trigger that locks the control channel, surfacing the identifier in the lock state.

Relevance to SDR

internal/radio/tetra/cmce.go defines the CMCE and MLE PDU types, the VoiceGrant carrier the MAC path fills, and AsSystemBroadcast for the MLE SYSINFO; pdu.go defines the Discriminator and the PDU shape. A real CMCE TM-SDU is bit-packed (a 3-bit MLE discriminator plus a 5-bit PDU type), so grants are decoded bit-accurately off the MAC layer rather than from the byte-aligned ParsePDU view, which cannot frame the sub-byte fields. IsKnown and strict validation gate the state machine to the documented (discriminator, type) pairs so a PDU whose type field lands in an unallocated range is dropped rather than mis-dispatched.

Sources

  1. Terrestrial Trunked Radio — Wikipedia, on the TETRA call-control signalling. ↩

  2. Protocol data unit — Wikipedia, on the PDU concept the CMCE/MLE messages instantiate. ↩

See also