IRLP is shorthand for the Internet Radio Linking Project, a way to link radio systems by means of the internet. It is an initiative that got its start with an article in QST magazine which outlined James Millner, WB2REM's experiments with linking amateur radio repeaters via the internet. It has spread beyond its origins with ham radio into commercial and public service radio systems as well. Ham repeaters had been linked before, and this is the basis of all modern trunked radio systems and Cellular Telephone Systems are based on linking of repeaters. In the past this had been done with dedicated phone lines, microwave, or UHF radio links. These first experiments were done with a software package known as VocalTec Iphone, which could be made to work, but it was crash-prone and lacked the controllability that was needed for everyday use.

The stability and controllability issues had to be solved, and a solution was found by David Cameron, VE7LTD using a VoIP package known as Speak Freely, which was a Client Server that ran under Linux. Speak Freely worked well, with excellent stability and clarity, but as word got around about his work, but he had to address the issue of non-hams hacking into the system. Since the source code of Speak Freely was open source software, he was able to make the necessary changes to the source code and came up with a package that forms the basis for a worldwide network of repeaters today.

How it Works

Without being too technical, the software uses the sound card in the computer to turn the audio stream into a compressed audio stream by reducing the sampling rate, then turning it into packets for transmitting onto the net. Control of the system is accomplished by using the PTT and COS and CTCSS signals. The user controls the system using DTMF tones on his radio which can bring up or take down links, reflectors, and nodes.

System Requirements

For the User

The user of the system needs to be a licensed ham and needs only an amateur radio transceiver appropriate for the repeater that is part of the node. To have the ability to control IRLP functions, the transceiver also needs to be able to transmit DTMF tones. Most modern VHF/UHF mobile rigs meet these requirements.

For the Repeater/Node Owner:

The node owner needs to have a link radio with Carrier Operated Squelch. Just about any two way radio has this feature. The node owner will also need a dedicated PC running Linux, with Red Hat 7.3 being the preferred version, with a continuous connection to the internet. The connection must be capable of supporting at least 4 Kilobytes/second transfer rate. In practical terms, this means a DSL or Cable Modem. A Static IP Address is helpful, but not strictly needed. The PC must be at least a Pentium 133 Mhz or better, with 32 Megs of RAM, with a working parallel port, a true Soundblaster sound card, 16 bit or better, a network card, and video card. An IRLP Interface board is also required, which interfaces the computer to the link radio.

Operating Guidelines

Operating on a repeater that is linked to IRLP requires a little more care than regular repeater operation, and are mostly common courtesy. Before bringing up a node, listen for 15 seconds to make sure there is no local traffic, or that the node is already up. Make a call to announce your intentions and bring up the node using the DTMF on your radio. Once the node or reflector is up, listen for any traffic to avoid interrupting a QSO in progress, then announce your presence or make a call like normal. When talking on IRLP, make allowances for delay to allow the packets to propagate. Avoid local traffic when connected to a reflector, though joining in on a roundtable is considered acceptable. Remember that you may be tying up 20 or more repeaters when you talk, so use common sense and keep your local drivetime rountable local. Finally, when you want to disconnect the node, identify yourself and announce your intentions before doing so. This will keep IRLP fun and useful for everyone.

For more information:

See the Home Page for the Internet Radio Linking Project. Here is the URL: