Field Guide · term

Also known as: LLC, basic link, logical link control

The TETRA LLC (Logical Link Control) sublayer is the thin Layer-2 layer between the MAC and the MLE.12 When the MAC delivers a TM-SDU it is not the MLE PDU directly — it is an LLC PDU: a short basic-link header (the LLC PDU type plus sequence numbers, and an optional 32-bit trailer) wraps the TL-SDU, and the TL-SDU is what carries the 3-bit MLE protocol discriminator and the CMCE call-control message beneath it. Skipping this layer — feeding the raw MAC TM-SDU straight into the CMCE parser — reads every Layer-3 field at the wrong bit offset, because the basic-link header is 4 to 6 bits wide, and that corrupts the decoded ISSI/GSSI.

MAC TM-SDU type 4 N(S)/N(R) TL-SDU → MLE discriminator → CMCE PDU FCS · 32 bits "with FCS" only strip header (+ FCS) → hand the TL-SDU up
The MAC TM-SDU is an LLC basic-link PDU: a 4-bit type field, up to two sequence-number bits, the TL-SDU, and for the with-FCS variants a 32-bit trailer; stripping the header and any FCS leaves the TL-SDU the MLE and CMCE layers parse.

The 4-bit LLC PDU type (§21.2.1, Table 21.1) selects the basic-link PDU and whether a 32-bit Frame Check Sequence trails the PDU. Values 0–7 are the basic-link PDUs GopherTrunk handles; 8–15 are advanced-link, supplementary or Layer-2-signalling PDUs that do not carry CMCE call control.

Type PDU Header after the 4-bit type FCS
0x0 BL-ADATA N(R) + N(S) — 2 bits no
0x1 BL-DATA N(S) — 1 bit no
0x2 BL-UDATA (none — 4-bit type only) no
0x3 BL-ACK N(R) — 1 bit no
0x4 BL-ADATA N(R) + N(S) — 2 bits yes
0x5 BL-DATA N(S) — 1 bit yes
0x6 BL-UDATA (none) yes
0x7 BL-ACK N(R) — 1 bit yes

BL-ADATA carries acknowledged data with both a send and a receive sequence number; BL-DATA carries acknowledged data with only a send sequence number; BL-UDATA is unacknowledged with no sequence numbers (the smallest header, just the 4-bit type); and BL-ACK is the acknowledgement, carrying a receive sequence number and, legitimately, no TL-SDU of its own. The with-FCS variants add a 32-bit CRC computed over the PDU, stripped from the tail before the TL-SDU is handed up.

Stripping the header

Decoding is a matter of reading the 4-bit type, consuming the right number of sequence-number bits, and — for a with-FCS PDU — trimming 32 bits off the end. What remains between the header and the FCS is the TL-SDU, ready for the MLE discriminator and CMCE parse. This is exactly the basic link used for CMCE call-control signalling on the main control channel; the advanced link (AL-*, used for packet data and SNDCP) is not a CMCE carrier and is reported as not-basic-link rather than mis-parsed. osmo-tetra strips this layer in tetra_llc_pdu_parse() before tetra_mle_decode(), and tetra-kit does the same in llc.cc.

Relevance to SDR

internal/radio/tetra/llc.go implements the basic-link strip as a single function, ParseLLC, which returns the TL-SDU and an ok flag. It reports ok=false for a non-basic-link PDU, for a BL-ACK that carries no TL-SDU, and for a slice too short to hold the header. Getting the header width right per PDU type is what keeps the CMCE parser reading from the correct bit offset — a four-bit slip here silently corrupts every source and destination identity in the decoded grant, a failure invisible to any test that encodes and decodes with the same wrong assumption on both ends.

Sources

  1. Terrestrial Trunked Radio — Wikipedia, on the TETRA protocol stack. ↩

  2. Logical link control — Wikipedia, on the LLC sublayer’s place in the data-link layer. ↩

See also