Glossary of digital & trunked radio terms
Every term used across the Digital & Trunked Radio path, defined in plain language and linked to the lesson where it’s explained in full. Skim it as a refresher, or use your browser’s find (Ctrl/Cmd-F) to jump to a word. Terms are grouped by theme and ordered roughly from concepts to specific systems.
How to use this page This is a reference, not a reading order. If you’re new, start with why radio went digital and follow the path; come back here whenever a piece of jargon trips you up. Terms link to the lesson that explains them, and many also have a live panel in the app you can watch.
Trunking concepts
Trunking — A system where many user groups share a small pool of frequencies, with a computer assigning a free channel to each call on demand and reclaiming it afterward. See Conventional vs trunked and Birth of trunking
Conventional radio — Non-trunked radio where each group uses a fixed, permanent frequency. Simple to scan, but wasteful of spectrum. See Conventional vs trunked
Control channel — The data-only frequency that coordinates a trunked system, announcing call setups and the voice channel each is assigned to. GopherTrunk decodes it first. See The control channel and the CC Activity panel
Voice / traffic channel — A frequency temporarily assigned to carry an actual call. Released back to the pool when the call ends. See The control channel
Talkgroup — A virtual channel — a number with a label, like “Police Dispatch” — identifying a group of users. You follow talkgroups, not frequencies. See Talkgroups, IDs & affiliation
Radio ID / Unit ID — The unique identifier of an individual radio transmitting on a system, distinct from the talkgroup it’s using. See Talkgroups, IDs & affiliation and the Radio IDs panel
Affiliation / Registration — A radio registering with the trunked system over the control channel when it powers on or changes talkgroups, so the system knows it’s active and where to route calls. See Talkgroups, IDs & affiliation
Channel grant — The control-channel message that tells radios “this talkgroup, go to this voice channel.” The event that starts every call. See Anatomy of a call
Hang time — A short interval after a transmission ends during which the system keeps the voice channel reserved for the talkgroup, so a quick reply doesn’t need a fresh grant. See Anatomy of a call
Message trunking — A grant strategy where a voice channel stays assigned to a talkgroup for an entire conversation (including hang time), not just one transmission. See Trunking flavors
Transmission trunking — A grant strategy where the voice channel is released the instant each transmission ends, so every push-to-talk needs a new grant. More efficient, but harder to follow. See Trunking flavors
Dedicated control channel — A trunking design where one frequency is permanently set aside to carry only control data. Most large P25 and Motorola systems work this way. See Trunking flavors
Distributed control channel — A design where the control role rotates among channels or shares time with voice, rather than living on one fixed frequency. Common in DMR Tier III and some LTR-style systems. See Trunking flavors
Sites, coverage & roaming
Site — One physical transmitter location (one set of towers and channels) within a larger system. A system can have many sites. See Sites, simulcast & roaming
Simulcast — Several transmitters broadcasting the same signal on the same frequency at once, time-aligned to cover a wide area as if it were one site. Powerful, but the overlapping signals can stress a decoder. See Sites, simulcast & roaming and Multisite & simulcast in practice
Roaming — A radio automatically moving from one site to another as the user travels, re-affiliating at each new site. See Sites, simulcast & roaming
WACN (Wide Area Communications Network) — In P25, a top-level identifier for a network operator’s whole system-of-systems. Combined with the System ID it uniquely names a network. See Identifying the system
System ID — A number identifying a particular trunked system, used (with the WACN in P25) to tell one system apart from another on the air. See Identifying the system
RFSS (RF Sub-System) — A P25 grouping of one or more sites under common control, sitting between the whole system and an individual site. See Sites, simulcast & roaming
NAC (Network Access Code) — A 12-bit code carried in every P25 frame, acting like a “squelch code” so receivers ignore signals from other systems on the same frequency. See Identifying the system
Color code — DMR’s equivalent of the NAC: a small number (0–15) that distinguishes co-channel systems so radios only act on their own. See Identifying the system
Digital voice & vocoders
Vocoder — A speech codec that analyzes voice and compresses it into tiny data frames for transmission, then reconstructs it at the other end. See Voice to bits: vocoders and the Vocoders panel
IMBE (Improved Multi-Band Excitation) — The vocoder used by P25 Phase 1, encoding voice at 4.4 kbit/s plus error correction. See Voice to bits: vocoders
AMBE+2 — A later, more efficient vocoder used by DMR, NXDN, and P25 Phase 2, fitting voice into a smaller data rate. See Voice to bits: vocoders
ACELP (Algebraic Code-Excited Linear Prediction) — The vocoder family used by TETRA, a different lineage from the MBE codecs above. See Voice to bits: vocoders
Analog voice — Voice sent as a continuous FM signal, which degrades gradually with distance and noise. Contrast with digital voice, which is crisp until it fails. See Analog vs digital voice
Modulation for trunked radio
C4FM (Compatible 4-level FM) — The four-level FSK used by P25 Phase 1 and the basis of DMR’s modulation. See Digital modulation for trunking
CQPSK / LSM (Linear Simulcast Modulation) — A phase-and-amplitude (linear) way of sending the same P25 Phase 1 symbols as C4FM, designed to behave better on simulcast systems. See Digital modulation for trunking
π/4-DQPSK — The differential phase-shift keying used by P25 Phase 2 and TETRA, sending two bits per symbol by shifting phase. See Digital modulation for trunking
4FSK (four-level FSK) — Frequency-shift keying with four tone levels, carrying two bits per symbol; underlies C4FM, DMR, and NXDN. See Digital modulation for trunking and the Constellation panel
GMSK (Gaussian Minimum-Shift Keying) — A smoothed frequency-shift modulation used by D-STAR (and many other digital systems) that keeps the signal spectrally tidy. See Digital modulation for trunking
Symbol rate — How many symbols per second a signal sends; with bits-per-symbol it sets the bit rate. P25 and DMR run at 4800 symbols/s. See Digital modulation for trunking and the Symbol scope
FDMA (Frequency-Division Multiple Access) — Giving each call its own narrow frequency channel. P25 Phase 1 and NXDN are FDMA. See TDMA vs FDMA
TDMA (Time-Division Multiple Access) — Splitting one frequency into repeating time slots so several calls share it. DMR and P25 Phase 2 are TDMA. See TDMA vs FDMA
Time slot — One of the alternating time windows on a TDMA channel; DMR has two (slot 1 and slot 2), each able to carry a separate call. See TDMA vs FDMA
Framing, error control & decoding
Frame / burst — A fixed-size package of bits sent over the air, containing voice or data plus sync and signaling. DMR calls its slot-sized unit a burst. See Framing, FEC & interleaving
Forward error correction (FEC) — Extra redundant bits added before transmission so the receiver can fix errors without asking for a resend — what lets digital voice survive noise. See Framing, FEC & interleaving
Interleaving — Scrambling the order of bits before sending so that a short burst of interference damages many separate, recoverable bits instead of one unrecoverable chunk. See Framing, FEC & interleaving
CRC (Cyclic Redundancy Check) — A checksum attached to a frame so the receiver can tell whether the bits arrived intact. See Framing, FEC & interleaving
BER (Bit Error Rate) — The fraction of received bits that are wrong; a key health metric for a decode, climbing as the signal degrades. See Troubleshooting a decode
Digital cliff — The point where a digital signal stops gracefully degrading and suddenly fails completely: clear audio one moment, silence the next. See Analog vs digital voice
Security & control
Encryption — Scrambling voice or data so only authorized radios can recover it; encrypted talkgroups can be detected but not decoded. See Encryption & authentication and the DMR encryption panel
AES-256 — A strong, standardized encryption cipher (256-bit key) used by P25 and others for secure traffic; not breakable by monitoring. See Encryption & authentication
DES-OFB — An older 56-bit DES cipher in output-feedback mode, once common on P25; weak by modern standards but still seen in the field. See Encryption & authentication
ADP (Advanced Digital Privacy) — Motorola’s RC4-based 40-bit encryption option, lighter than AES and intended as basic privacy rather than strong security. See Encryption & authentication
Authentication — A system verifying that a radio is a legitimate, authorized unit before granting it service, separate from encrypting the traffic itself. See Encryption & authentication
Stun / Kill — Over-the-air commands a system can send to disable a radio temporarily (stun) or permanently (kill), used when a unit is lost or stolen. See Encryption & authentication
P25 (Project 25)
P25 (Project 25) — A North American suite of standards for public-safety digital radio, defining both conventional and trunked operation. See P25 Phase 1
P25 Phase 1 — The first generation of P25: FDMA, 12.5 kHz channels, C4FM modulation, and the IMBE vocoder. See P25 Phase 1
P25 Phase 2 — The newer generation: two-slot TDMA on a 12.5 kHz channel (effectively 6.25 kHz per call) using π/4-DQPSK and the AMBE+2 vocoder. See P25 Phase 2
TSBK (Trunking Signaling Block) — The control-channel message format that carries P25 trunking signaling — grants, affiliations, and system data. See Control-channel signaling
LDU (Logical Data Unit) — A P25 Phase 1 voice frame structure (LDU1/LDU2) that packages IMBE voice along with embedded signaling and link control. See P25 Phase 1
DMR
DMR (Digital Mobile Radio) — An ETSI standard for digital land-mobile radio using two-slot TDMA on 12.5 kHz channels, popular for business and amateur use. See DMR Tier II & III
Tier II — Conventional (non-trunked) DMR: licensed two-slot TDMA repeaters with no central trunking controller. See DMR Tier II & III
Tier III — Trunked DMR, adding a control-channel-driven trunking layer on top of DMR’s TDMA. See DMR Tier II & III
CSBK (Control Signaling Block) — DMR’s signaling message unit, used for trunking control and other system messages — DMR’s counterpart to P25’s TSBK. See Control-channel signaling
TETRA
TETRA (Terrestrial Trunked Radio) — A European ETSI standard for trunked digital radio, using four-slot TDMA and π/4-DQPSK, common for public safety and transit outside North America. See TETRA
TMO (Trunked Mode Operation) — TETRA radios operating through the infrastructure (sites and controllers), the normal trunked mode. See TETRA
DMO (Direct Mode Operation) — TETRA (and DMR) radios talking directly to each other without infrastructure, like a walkie-talkie. See TETRA
Other digital standards
NXDN — A narrowband FDMA digital standard (6.25 kHz) developed by Icom and Kenwood, available in conventional and trunked forms. See NXDN
dPMR (digital Private Mobile Radio) — An ETSI 6.25 kHz FDMA standard closely related to NXDN, aimed at light commercial use. See dPMR, D-STAR & YSF
D-STAR — An amateur-radio digital voice and data standard (developed with Icom) using GMSK and the AMBE vocoder. See dPMR, D-STAR & YSF
System Fusion / YSF (Yaesu System Fusion) — Yaesu’s amateur C4FM digital voice system, also known as YSF. See dPMR, D-STAR & YSF
Legacy & proprietary systems
SmartNet / SmartZone — Motorola’s earlier trunking systems: SmartNet for a single site, SmartZone adding multi-site networking, both using an analog or digital voice payload. See Motorola SmartNet
Type I / Type II — Two Motorola trunking fleet-management schemes. Type I uses fleet/subfleet addressing; Type II uses flat talkgroup IDs and is far more common. See Motorola SmartNet
EDACS (Enhanced Digital Access Communications System) — A GE/Ericsson/Harris trunking system known for fast channel access and a distinctive control channel. See EDACS, LTR & MPT-1327
ProVoice — The digital voice option layered onto EDACS systems, using the AMBE vocoder. See EDACS, LTR & MPT-1327
LTR (Logic Trunked Radio) — An EF Johnson trunking scheme with distributed control — signaling is embedded on each channel rather than a dedicated control channel. See EDACS, LTR & MPT-1327
Passport — A proprietary trunking extension of LTR developed by Trident/SmarTrunk, adding features on top of the LTR scheme. See EDACS, LTR & MPT-1327
MPT-1327 — A British signaling standard for analog trunked radio, widely used internationally before digital systems took over. See EDACS, LTR & MPT-1327
Standards bodies
APCO (Association of Public-Safety Communications Officials) — The North American body that drove the creation of Project 25 (the “APCO-25” name). See Standards & bodies
ETSI (European Telecommunications Standards Institute) — The standards body behind DMR, TETRA, and dPMR. See Standards & bodies
TIA (Telecommunications Industry Association) — The U.S. standards organization that publishes the P25 (TIA-102) document suite. See Standards & bodies
ITU (International Telecommunication Union) — The UN agency that allocates spectrum and sets worldwide radio regulations within which all these systems operate. See Standards & bodies
DMR Association — The industry group that promotes DMR interoperability and maintains shared identifier resources for the standard. See Standards & bodies
Didn’t find a term? It’s probably explained in one of the Digital & Trunked Radio lessons — or open an issue on GitHub and we’ll add it.