Simplex and Duplex Explained in Plain Words

When two people talk over a radio, one speaks while the other stays quiet and listens. To reply, you have to press the PTT (push-to-talk) button. That is simplex in its purest form. Understanding the difference between simplex, half-duplex and full duplex isn't hard — and for anyone interested in digital radio and the DMR protocol, it's foundational knowledge: without it you can't make sense of repeaters or hotspots.

Simplex: one road in both directions

Simplex communication means transmitting on a single frequency, one at a time. At any given moment the channel is occupied by either the first party or the second. The two physically cannot speak at the same time: if both press PTT, the signals collide and neither can be understood.

Classic examples: point-to-point walkie-talkies, taxi dispatch, security teams, construction sites. The telltale sign is the word "over" at the end of a transmission: the speaker is signalling that they're done and have released the PTT. Both parties are tuned to the same frequency (RX = TX).

In practice, simplex radios operate in the LPD, PMR446 and amateur VHF/UHF bands. Range is limited by power and terrain: it's line-of-sight. For more on the frequencies allowed in Russia, see the article "Frequencies and the law".

Half-duplex: same turn-taking, different frequencies

Half-duplex works on the same logic — one speaks, the other listens — but reception and transmission happen on different frequencies. This is exactly how operation through a repeater works: the radio transmits on one frequency (the repeater's "input") and the repeater retransmits on another (its "output"). The user still presses the PTT and waits for a reply — there's no simultaneous two-way speech.

Users usually can't tell simplex and half-duplex apart by ear. The fundamental difference lies inside the equipment and in the network's frequency plan.

Duplex: two streams at once

Full duplex means simultaneous two-way communication. The most familiar example is an ordinary phone call: you speak and hear the other person at the same time, with no "overs" needed. This requires two different frequencies (or two different time intervals in digital systems), spaced so the signals don't interfere with each other.

In analog radio, full duplex requires either two separate antennas with a duplex filter (a duplexer), or a large frequency separation — a shift. On the 2-metre band (144 MHz), the standard repeater offset is 600 kHz; on the 70-centimetre band (430–440 MHz) it's 1.6 MHz. The repeater receives on the "input" frequency and simultaneously retransmits on the "output" frequency.

How it works in DMR: the magic of TDMA

The DMR (Digital Mobile Radio) protocol adds an important twist: it uses TDMA — Time Division Multiple Access. On a single physical frequency 12.5 kHz wide, DMR fits two independent time slots (Time Slot 1 and Time Slot 2), alternating with a period of 30 ms each. Two 30-millisecond slots form a 60 ms frame.

What does this mean in practice? A repeater or hotspot can simultaneously handle two conversations on a single frequency — each on its own time slot. This fundamentally sets DMR apart from analog FM, where one channel = one conversation.

In direct (simplex) communication between radios without a repeater, DMR radios also work on a single frequency, but in this mode only one time slot is used. The second slot never goes on the air — there's nothing to fill the channel with.

Simplex vs. duplex hotspot: what's the difference

A hotspot is a personal "repeater in your pocket" that connects your radio to an internet-based network like DMRhub. And here the distinction between a simplex and a duplex hotspot becomes practically important.

A simplex hotspot (for example, an MMDVM on a single module) uses one frequency for both reception and transmission (RX = TX). It's simpler and cheaper to build and is suited to personal "one conversation" use. But since DMR is TDMA with two time slots, a hotspot that's "simplex" on the RF side still processes both slots in software: a received packet is decoded and relayed to the network, while an incoming packet from the network is transmitted on the air according to the TDMA schedule. In effect the hotspot works as a "pseudo half-duplex" node — but both slots are available in turn within a single frequency channel.

A duplex hotspot (Duplex Hotspot, covered in detail in a separate article) uses two different frequencies — one to receive from the radio, another to transmit on the air — and two radio modules. This fully reproduces the logic of a repeater: the radio receives and transmits simultaneously, with no "switching" delays. A duplex hotspot ensures both DMR time slots work correctly at the same time and is better suited to small groups or more complex configurations.

The choice between them is driven by your use case. If you just want to monitor the network and talk from your radio, a simplex hotspot will do. If you need full two-slot operation or several users, look toward a duplex one. A detailed breakdown of the options is in the article "Which hotspot to choose".

A repeater is always duplex

A repeater is a fixed station that receives a signal on one frequency and immediately retransmits it on another. By definition it's a duplex device: both frequencies operate at the same time. That's exactly why, to work through a repeater, a radio is always configured with two frequencies (RX and TX) and a shift: the radio transmits on the repeater's "input" and listens to its "output".

In DMR, you also add the Color Code and the time slot number — parameters without which the repeater will simply ignore your radio.

DMRhub: communication without an over-the-air frequency

The DMRhub network offers an interesting capability: the Android app lets you take part in conversations without a radio and without any over-the-air frequency at all. Your smartphone connects directly to the DMRhub server over the internet and works as a full-fledged DMR terminal. From the network's point of view, it's just another user — like a radio behind a hotspot.

If you already have a radio and a hotspot, they connect to the same network. The RadioStar hotspot (available in simplex and duplex variants) is registered on the devices page and immediately gets access to the network's talkgroups. You can register your DMR ID and join the network through your dashboard.