PoC stations and the app: DMR from a phone and Android radios

You can get on the air without a hotspot or a repeater nearby — straight from a smartphone, or from a «radio» that is really just an Android phone inside. Such devices are called PoC stations, and they work over the cellular network rather than on a radio frequency. Let's take an honest look at what kind of beast this is, how PoC fits into a DMR network, where it beats a classic radio and where it is plainly weaker — and how the DMRhub app turns a phone into a full-fledged network node.

What PoC is and how it differs from a radio

PoC (Push-to-Talk over Cellular) is a way of voice communication where you press PTT, your voice is digitized and sent out as packets over the internet: via 4G/5G/LTE or Wi-Fi to a server, and from there on to the other parties. The radio doesn't transmit any carrier of its own on the air: the phone's modem carries it to the cell tower, and from there the carrier's network and the internet take over.

On the outside, a PoC station often looks like a familiar radio: a body, a stubby antenna, a big PTT button. But under the hood it runs Android with a SIM card — essentially a rugged smartphone without a screen (or with a tiny one), built around a single button. Hence the key difference from a real radio:

• A classic DMR/analog radio transmits a radio signal on VHF/UHF itself and can talk directly to another radio (simplex) even where there is neither cellular coverage nor internet.
• A PoC station is «mute» on its own: without a cellular network or Wi-Fi it can't reach anyone, even if the other person is standing three meters away. But wherever there is cellular coverage, range is practically unlimited — even across the whole country.

How PoC fits into a DMR network

PoC and DMR are two different worlds: one lives in cellular packets, the other on the air in 12.5 kHz channels. To make them talk to each other, you need a gateway (gateway / RoIP bridge) that takes voice from one world and pours it into the other. Technically there is a chain between them:

1. The PoC device (or the app on the phone) encodes the voice and sends it to the server over the internet.
2. The gateway server, when needed, transcodes the audio into the DMR vocoder (AMBE) — the very format that digital radios «speak».
3. From there the stream goes into the DMR network: to a talkgroup, into a private call, or — through a repeater/hotspot — onto the real air.

In «big» amateur networks such bridges are built on server software — combinations like MMDVM_Bridge, Analog_Bridge and transcoders on AMBE chips (ThumbDV, AMBEd). It works, but the setup scares off a newcomer: you have to stand up a server, configure reflectors, set up a vocoder. In DMRhub all of this plumbing is hidden inside the network — from the user's side, only the app remains.

The upsides of PoC — where it really helps

• Coverage wherever there is cellular. Range isn't limited by terrain or transmitter power: as long as 4G/Wi-Fi has a signal, you're connected even from another city.
• Cheap and quick to get started. No hotspot, antenna or SWR tuning needed — just a phone with internet and an app. The perfect entry point to «get a feel» for the network.
• Rich features out of the box. GPS position, text messages, group and private calls, Bluetooth headsets — PoC devices have all of this by default.
• One account, many devices. Today you're on the network from the phone in your pocket, tomorrow from a desktop hotspot under the same DMR ID.

The downsides — to be honest about

• Total dependence on the internet. Lose cellular or Wi-Fi, and PoC goes silent. For emergency communication, a trip into the wilderness or a coverage-free zone, this is no option.
• No real RF on the air. From a smartphone you communicate through the network, not «over the air». Two PoC stations next to each other without internet won't hear one another — no simplex at all.
• Latency and quality depend on the channel. On weak 3G the voice may «bubble» and arrive with lag — the price of working over a packet network.
• It's not a replacement for a radio, but a complement. PoC covers «long-distance» communication and getting started beautifully, but local independent communication without a network is still only delivered by a regular radio on the air.

The DMRhub app: a smartphone as a network node

The DMRhub app is a PoC client for our network. Install it on an Android smartphone or right on a PoC station, log in with your DMR ID — and the phone becomes a full-fledged node of the network on par with a hotspot. What works:

• Voice with PTT. A big push-to-talk button, talkgroup selection, hearing everyone currently in the group — just like on a real radio.
• Private calls. One-on-one by DMR ID — to reach a specific person without raising the whole group.
• DMR-SMS. Text messages within the network, reaching both the radios on the air and other phones.
• Contacts and Last Heard. You can see who's talking and who's online, right on the screen.

A handy scenario: at home you're on the air through a hotspot, but you step outside — and continue the same conversation from your phone via the app, losing neither the group nor your ID. PoC and a hotspot don't compete, they complement each other.