In Digital Mobile Radio (DMR), particularly Tier II (the mode used in amateur radio with repeaters and hotspots), timeslots are a key feature that allows two independent conversations to happen simultaneously on the same 12.5 kHz frequency channel. This is achieved through a technique called Time Division Multiple Access (TDMA).
How TDMA Works in DMR
DMR uses TDMA to divide the radio channel into tiny time slices:
- The full 12.5 kHz channel is not split in frequency (like older FDMA systems).
- Instead, it’s split in time — every second is divided into two alternating 30-millisecond (ms) slots.
- Your radio transmits in one 30 ms burst, then listens for 30 ms, then transmits again, and so on — very rapidly (alternating 30 ms on / 30 ms off).
- This creates two virtual “lanes” on the same frequency:
- Time Slot 1 (TS1) — One conversation path.
- Time Slot 2 (TS2) — A completely separate conversation path.
It’s like having two separate repeaters running on the exact same frequency — one conversation on TS1, another on TS2 — without interfering with each other.
Visual Explanation of TDMA Timeslots
Here’s a simplified diagram of how a DMR signal looks over time:
Time →
[ 30 ms ] [ 30 ms ] [ 30 ms ] [ 30 ms ] [ 30 ms ] ...
TS1 TS2 TS1 TS2 TS1 ...
(User A transmitting) (User B transmitting)- User A’s radio transmits only during TS1 slots (30 ms bursts).
- User B’s radio transmits only during TS2 slots.
- The repeater receives both streams, processes them separately, and rebroadcasts them accordingly.
- Your radio only listens to the timeslot assigned to your channel.
This doubles the capacity: one repeater frequency supports two full-duplex voice paths (or data paths) at once.
Practical Use in Amateur Radio (e.g., BrandMeister Network)
- Repeaters always have two timeslots: TS1 and TS2.
- Hotspots (like Pi-Star or OpenGD77 simplex hotspots):
- Usually use only TS2 (by convention).
- Some duplex hotspots can use both.
- Programming your radio (in CPS like OpenGD77 or Pi-Star):
- Every digital channel must specify TS1 or TS2.
- You choose the timeslot based on how the repeater/hotspot is configured.
- Common conventions (varies by network/repeater, but typical on BrandMeister):
- TS1: Often used for wide-area / global / dynamic talkgroups (e.g., Worldwide 91, North America 93, TAC310).
- TS2: Often used for local / regional / static talkgroups (e.g., Local 2, state/regional TGs, or repeater-specific traffic).
- Many repeaters have static links (always on) on one slot and dynamic/PTT-activated on the other.
Benefits of Timeslots
- Double capacity — Twice as many users/conversations per frequency compared to analog FM.
- Better spectrum efficiency — Fits two voice paths into the same 12.5 kHz bandwidth.
- Battery savings — Your radio only transmits half the time (30 ms bursts), so batteries last longer.
- Independent routing — Each timeslot can carry different talkgroups, routed separately over the internet (e.g., TS1 to BrandMeister worldwide, TS2 to local only).
Key Differences: Analog vs. DMR Timeslots
| Aspect | Analog FM | DMR (TDMA) |
|---|---|---|
| Capacity per freq | 1 conversation | 2 conversations (TS1 + TS2) |
| How it works | Full-time transmission | Alternating 30 ms bursts |
| Interference | One user ties up the frequency | Two users can talk simultaneously |
| Programming | Just frequency + tone | Frequency + Color Code + Timeslot + TG |
In summary: DMR timeslots turn one radio frequency into two virtual channels using rapid time-sharing (TDMA). This is why DMR repeaters feel so efficient — you can have two completely separate conversations (e.g., one local, one worldwide) happening at the same time on the same repeater, without stepping on each other. When programming your OpenGD77 radio, always check the repeater’s info (e.g., on BrandMeister.network) to see which talkgroups are on TS1 vs. TS2!