Field Guide · term

Also known as: SLOT TYPE, DMR slot type, data type field

The DMR slot type (SLOT TYPE) is the 20-bit field that surrounds the sync word of every data/control burst: 10 bits immediately before the sync and 10 immediately after.1 It carries just two things a decoder needs before it can interpret the payload — the 4-bit Color Code that separates co-channel systems, and the 4-bit Data Type that names what kind of block the burst holds — but wraps them in a strong FEC so both survive a fading channel. Only 8 of the 20 bits are information; the other 12 are parity from a (20,8,7) Hamming / Golay code.2

CC · 4 Data Type · 4 (20,8,7) parity · 12 bits 8 info bits, 12 parity · d=7 → corrects 3 errors, detects 4 split 10 bits before sync + 10 bits after sync
Eight information bits — color code and data type — are protected by a distance-7 code and split into two 10-bit fields on either side of the sync, so a decoder recovers both even after up to three bit errors.

The Data Type enum

The Data Type names the block, per ETSI TS 102 361-1 §9.1.2 Table 9.6. GopherTrunk enumerates the valid values in internal/radio/dmr/slottype.go:

Value Data Type Meaning
0x0 PI Header Privacy (encryption) header
0x1 Voice LC Header Full Link Control opening a voice call
0x2 Terminator with LC Full LC closing a voice call
0x3 CSBK Control Signalling Block
0x4 MBC Header Multi-Block Control header
0x5 MBC Continuation Multi-Block Control continuation
0x6 Data Header Packet-data header
0x7 Rate ½ Data Confirmed/unconfirmed rate-½ data block
0x8 Rate ¾ Data Rate-¾ data block
0x9 Idle Idle block
0xA Rate 1 Data Rate-1 data block
0xB Reserved (0xC–0xF also reserved)

The (20,8,7) code

The eight information bits are laid out as [CC(4) | DataType(4)] in the high byte, followed by 12 parity bits. The code has minimum distance 7, so it corrects up to 3 bit errors and detects 4. GopherTrunk’s framing.HammingDecode20_8 (internal/radio/framing/hamming20.go) does a minimum-distance search over all 256 valid codewords rather than a syndrome table — 256 entries fit in cache and keep the decoder auditable against the spec’s generator. ParseSlotType runs that decode, returns the corrected bit count (0 on a clean codeword), and reports ErrSlotTypeUncorrectable when the nearest codeword is more than 3 bits away. Some references call this Hamming(20,8) and others Golay(20,8); the wire format is identical.

Relevance to SDR

The slot type is the router of GopherTrunk’s DMR control path. Because it decodes independently of the 196-bit payload, a receiver can read the Color Code and Data Type even when the payload is too corrupt to use, and the Data Type is what tells the deframer whether to hand the two payload halves to the CSBK parser, the voice-LC / RS(12,9) path, the MBC assembler, or a data-block decoder. It is also one of the arbiters that resolves the sync-word polarity ambiguity (issue #264): a burst decoded at the wrong polarity yields an uncorrectable slot type and is dropped, so getting the (20,8,7) code exactly right is part of what keeps a spectrum-inverted stream from producing phantom blocks.

Sources

  1. Digital mobile radio — Wikipedia, on the DMR standard and its burst framing. ↩

  2. Hamming code — Wikipedia, on the linear block codes underlying the slot-type FEC. ↩

See also