Guide P25 trunking

Trunking follower (P25, DMR Tier III)

Receive only. OnAir decodes a trunked system's control channel and can follow the voice calls it announces. Open the P25 tab, Follow, and turn on Follow calls.

What it does

• System model. From the control channel's TSBKs (TIA-102.AABC): WACN, system, RFSS, site, NAC, the channel identifier table (IDEN_UP, IDEN_UP_VU, IDEN_UP_TDMA), adjacent sites, secondary control channels (SCCB), and the control-channel candidates to try when it is lost.
• Calls. Group voice grants (new and update messages, including the explicit update), unit-to-unit grants. Each becomes a call-log entry with talkgroup, source unit, frequency, duration, and an encrypted flag.
• Voice following.
  • Wide capture (whole-span decoding on): when the granted channel is inside the captured span the sound moves to that carrier. Up to N calls (1 to 4, default 2) are followed at once, limited by CPU. When all are busy a higher-priority talkgroup preempts the lowest one.
  • Single radio: the radio leaves the control channel for the call, and returns when the call ends (no voice for the hang time, or a terminator). It does not hop away from a call that is already followed, and never hops when "retune the radio" is off. In both cases the entry says needs a wider capture or a second radio and counts as not followed.
  • If the control channel is lost the radio tries the known candidates in turn.
• Talkgroup list. Name, tag, category, priority (1 highest, 0 none), lockout, listen, scan list. Lockout talkgroups are logged but not followed. With scan list only only listed talkgroups with Listen on (and in an enabled list, when any list is enabled) are followed. The list is saved to trunk_talkgroups.csv in the OnAir data folder.
• Call log with CSV export, now talking table, per-call recording (WAV, gated) when the vocoder add-on is present.

Encrypted calls

P25 service options bit 0x40, or a talkgroup you marked with an E mode in the list, and DMR privacy flags: the call is logged as encrypted and is never followed, recorded, decoded or attacked. IDs and talkgroups are still shown.

Voice

Voice sound needs the user-installed vocoder add-on (see CODECS.md). Without it everything else works; recordings are skipped because there is no sound to save.

CSV columns

Import reads the header row, case-insensitive, any column order: Decimal (also DEC, TGID, Talkgroup), Hex, Alpha Tag (Name), Mode (D, DE, T, A), Description, Tag, Category (Group), and the OnAir extras Priority, Lockout, List, Listen. The usual RadioReference-style export (Decimal, Hex, Alpha Tag, Mode, Description, Tag, Category) imports as it is; without a header that order is assumed. Quoted fields, doubled quotes, LF/CRLF and a UTF-8 BOM are accepted. A row without a valid number (1 to 65535) is skipped and counted. OnAir downloads no data: you bring the file.

Honest scope

• P25 Phase 1 control channel: implemented and tested with synthetic signals (through the real C4FM receiver, and as message logic). Not tested on a real system.
• P25 Phase 2: the control channel is the same TSBK format, and IDEN_UP_TDMA is parsed (only channel type 3, 2 slots per carrier, is placed; other types are shown as not placed). There is no Phase 2 voice demodulator, so Phase 2 TDMA voice grants are logged and not followed.
• Message layouts are written from the field layouts as understood from TIA-102.AABC, which is not available offline here; the explicit update (0x03) and SCCB (0x39) layouts in particular need a check against the document.
• DMR Tier III: the follow logic is shared, but the CSBK opcodes (DmrOpcodes: 0x30 / 0x31 / 0x32 for talkgroup / private / broadcast grants) and the grant field layout are unverified against ETSI TS 102 361-4 and are kept in one table. The grants carry logical channel numbers only, so you must supply the LCN to frequency table (DmrModel::setChannel); there is no UI for it yet and the UI feeds only P25. Connect Plus and Capacity Plus are manufacturer protocols and are not supported.
• Site map: the control channel carries no coordinates; SystemModel::setSitePosition takes them from a user table, but there is no map view yet.
• Transcription hook: not wired. The call log entries carry the recording path for it.
• Only the selected carrier's sound plays, so two simultaneous followed calls are decoded and logged but one is heard (the highest priority).
• The Follow tab's text is not translated yet (like the rest of the P25 tab).

Tests

test_p25_trunk (system model, follow state machine, priority, lockout, scan lists, rotating control channel, CSV, encrypted handling, 20000 random messages and junk CSV), test_p25_trunk_rf (the synthetic control channel through the receiver into the controller).

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See also: Tutorials and guides, FAQ, Troubleshooting.