Before this:TDMA vs. FDMA: fitting more calls on a channelControl-channel signaling: what the data says
DMR: Tier II & Tier III
Key takeaways DMR (Digital Mobile Radio) is the open ETSI TS 102 361 standard that dominates business, utility and amateur digital radio. It packs two-slot TDMA into a 12.5 kHz channel — ≈6.25 kHz equivalent per call — using 4FSK at 4800 symbols/s (9600 bps) and the AMBE+2 vocoder. It comes in three tiers: Tier I (license-free simplex), Tier II (conventional two-slot repeaters), and Tier III (trunked, with a dedicated control channel and CSBK signaling). A color code keeps repeaters apart like a NAC. Vendors also built proprietary trunked variants — Capacity Plus, Connect Plus, Capacity Max, Hytera XPT — that differ from open Tier III.
Where P25 rules North-American public safety, DMR rules almost everything else digital: taxis, warehouses, utilities, security, and a vast amateur network. Its appeal is simple — it is an open standard with cheap radios that doubles capacity on existing 12.5 kHz channels. This lesson unpacks how it works and the tiers you will meet.
The physical layer: two slots in 12.5 kHz
DMR uses 4FSK — the same four-level FSK family as P25’s C4FM — running at 4800 symbols/s, which (two bits per symbol) gives 9600 bps. The clever part is two-slot TDMA: a single 12.5 kHz channel is split in time into two alternating slots, so it carries two simultaneous calls. That is the same 6.25 kHz equivalent efficiency idea as P25 Phase 2, but DMR built it in from the start across the whole standard. Voice rides in the AMBE+2 vocoder — specifically the 3600×2450 variant, distinct from the 3600×2400 variant P25 Phase 2 and NXDN use.
A color code (0–15) is carried in the bursts and works like a P25 NAC or an analog CTCSS tone: a radio ignores traffic whose color code does not match, so neighbouring repeaters can reuse a frequency without interfering. GopherTrunk reads it to confirm it is locked to the intended repeater.
The three tiers
DMR is defined as three tiers, escalating in capability:
- Tier I — license-free, low-power simplex (radio to radio, no repeater). Think consumer and very light commercial use. There is no infrastructure to follow.
- Tier II — conventional, licensed operation through two-slot repeaters. Each repeater sits on a fixed pair of frequencies and carries two slots. This is the workhorse of commercial DMR and amateur DMR repeaters. There is no control channel — you monitor it much like a conventional channel, decoding whichever slot is active.
- Tier III — trunked DMR. A dedicated control channel carries CSBK (Control Signaling Block) messages that grant calls across a pool of channels, exactly the trunking pattern you know from P25 but with DMR’s signaling vocabulary.
Conventional Tier II vs. trunked Tier III
The practical difference is whether there is a control channel to follow.
| DMR Tier II | DMR Tier III | |
|---|---|---|
| Operation | Conventional | Trunked |
| Control channel | None | Dedicated, CSBK signaling |
| Channel assignment | Fixed per repeater | Assigned per call from a pool |
| Signaling | Embedded link control | CSBK grants on control channel |
| How you monitor | Watch the repeater’s two slots | Decode control channel, follow grants |
| GopherTrunk grant | dmr-tier2 |
dmr-tier3 |
For Tier II, GopherTrunk treats the repeater like a conventional digital channel: lock the carrier, read the embedded link control to learn the talkgroup and source ID per slot, and decode AMBE+2 for whichever slot is transmitting. For Tier III, it does the full trunking dance — lock the control channel, read CSBKs on the CC Activity panel, and follow each grant to the assigned voice channel and slot, exactly as it does for P25 but with DMR framing.
CSBKs: DMR’s trunking signaling
The CSBK is to Tier III what the TSBK is to P25: a fixed-size control message. On a Tier III control channel a continuous stream of CSBKs announces channel grants, registrations, and system parameters. The grant CSBKs tell a follower which physical channel and time slot a talkgroup’s call was assigned. Because DMR is two-slot, a Tier III grant — like P25 Phase 2 — references a slot as well as a frequency, so the decoder must tune the channel and then lock the correct slot.
The proprietary variants
Here is the wrinkle that makes DMR trunking messy in the field: several major trunked DMR systems are not open Tier III at all. Vendors built their own trunking on top of the DMR physical layer — often before Tier III was finalized — using vendor-specific control signaling rather than standard CSBK grants:
- Motorola Capacity Plus — single-site (and Multi-Site) trunking that spreads calls across a repeater’s slots without a separate dedicated control channel in the Tier III sense.
- Motorola Connect Plus — multi-site trunking using a dedicated control channel, but with Motorola’s own signaling, not open CSBKs.
- Motorola Capacity Max — Motorola’s later, larger multi-site trunked architecture.
- Hytera XPT — Hytera’s distributed trunking scheme.
All sit on the same 4FSK + AMBE+2 physical layer, so they look like DMR on a scope, but their control signaling differs from the open standard. A decoder therefore needs system-specific support to follow each one; the open Tier III CSBK logic does not automatically apply. Conventional Tier II, by contrast, is well covered by the open standard and the most reliably decoded.
Encryption on DMR
Many commercial DMR systems protect voice with RC4/ARC4 “Enhanced Privacy.”
GopherTrunk supports known-key decode: an operator supplies a key they are authorized
to hold, declared per system with a key_id, algorithm: rc4, and the hex key. It reads
the encrypted flag from the link control before voice starts, and for unencrypted calls
decodes AMBE+2 end to end. The full configuration and current pipeline status — including
which descramble step is still pending — are in DMR encryption.
GopherTrunk performs no key recovery; only monitor systems you are legally permitted
to monitor.
Quick check: which DMR tier is trunked, with a dedicated control channel?
Recap
- DMR is the open ETSI TS 102 361 standard: two-slot TDMA in 12.5 kHz, 4FSK at 9600 bps, AMBE+2 voice, with a color code that keeps repeaters apart.
- Tier I is license-free simplex; Tier II is conventional two-slot repeaters; Tier III is trunked with a control channel and CSBK signaling.
- GopherTrunk reads embedded link control on Tier II and follows CSBK grants on Tier III, including the per-call slot.
- Proprietary Capacity Plus / Connect Plus / Capacity Max / Hytera XPT share DMR’s physical layer but use vendor signaling, needing system-specific support.
- DMR RC4 Enhanced Privacy is supported in a known-key model only.
That closes the P25 and DMR module. The next module opens with another major trunked family rooted in Europe: TETRA.
Frequently asked questions
What is DMR?
DMR (Digital Mobile Radio) is an open ETSI standard, TS 102 361, widely used in business, utility and amateur radio. It fits two-slot TDMA into a 12.5 kHz channel — about 6.25 kHz equivalent per call — using 4FSK at 4800 symbols per second and the AMBE+2 vocoder. Its low cost made it the dominant digital standard outside public safety.
What are the three DMR tiers?
Tier I is license-free, low-power simplex for personal and light commercial use. Tier II is conventional licensed operation with two-slot TDMA repeaters — the most common business DMR. Tier III is trunked DMR, with a dedicated control channel and CSBK signaling that assigns calls across a channel pool.
What is a DMR color code?
A color code is a value carried in DMR bursts that works like a NAC or CTCSS tone — radios ignore traffic whose color code does not match. It lets neighboring repeaters share a frequency without interfering. GopherTrunk reads the color code to confirm it is decoding the right repeater.
How do proprietary DMR trunking systems differ from Tier III?
Vendors built their own trunked schemes on the DMR physical layer — Motorola Capacity Plus, Connect Plus and Capacity Max, and Hytera XPT — before or alongside open Tier III. They use vendor-specific control signaling rather than standard Tier III CSBKs, so a decoder needs system-specific support to follow them, while conventional Tier II DMR is well covered by the open standard.