Internet Radio Linking
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This page describes Internet repeater linking and explains how it works. A working Internet <---> RF gateway is also described. This page will concentrate more on the concepts of repeater linking and the technical issues involved.
What is Internet Radio Linking?
Internet repeater linking is simply the process of using the Internet to act as a voice connection between two or more repeaters. In essence, the Internet is used instead of more traditional media such as leased lines or UHF radio links. Using the Internet in this way allows links to be made economically between repeaters hundreds or thousands of km apart, even on opposite sides of the world, something which would be prohibitively expensive by any other method. Another side effect of some linking systems is that individual amateurs with a soundcard, headset and a microphone can work distant repeaters WITHOUT a radio! This can be of benefit to those in isolated areas, or who are unable to erect suitable antennas.
Types of Internet Radio Links.
Internet repeater links can be made with a variety of software and hardware. However, most fall into a number of broad categories, based on the technology used to make the link:
The first generation of systems used Internet telephony applications such as IPhone for the VoIP function. Thos was popular until late 2000/early 2001, when IPhone ceased to be officially supported by its creator, VocalTec. Because these linking systems were based around closed source applications, functionality was limited and VOX had to be used to interface between the radio and the computer. These early systems also had very limited security (basically none except for manual operator intervention in most cases) from non amateur access.
In the late 1990's, IRLP was designed specifically to address the shortcomings of the systems of the time, and raise the functionality of amateur VoIP systems to a new level. IRLP brought the first dedicated hardware interface between the radio and PC, which allowed for much better control of the radio channel and hardware based DTMF decoding to accept user initiated commands. IRLP was the first system to allow radio users to select their call destination. IRLP also uses the Linux operating system to achieve excellent stability.
2001 saw a lot of changes in amateur Internet linking systems. eQSO and iLINK were developed as Windows based alternatives to the Linux based IRLP. Some amateurs perceived Windows as a more "accessible" platform for them. These systems also offered PC access to the network, which IRLP prohibits, by design. iLINK also offered a custom interface board in a similar concept to that of IRLP for radio control and DTMF decoding. Like IRLP, iLINK allowed radio users to select which node to call. eQSO, by contrast, was and still is based around the concept of conference rooms. eQSO does not have point-point direct links.
In 2002, EchoLink was developed as an iLINK compatible client. During 2002, EchoLink took over from iLINK altogether. The events of 2002 also saw a number of reverse engineering projects which have led to the EchoLink protocols becoming open, with open source implementations available for amateurs. EchoLink became the first cross platform system (with the Linux based EchoLinux client and the multi platform thebridge conference server).
Yaesu also introduced its WIRES and WIRESII Internet linking systems in 2002. The entry of Yaesu into the market is an indication that amateur VoIP technology is here to stay.
How do I link my repeater?
How you link your repeater depends on what you require. If you're after a permanent link with full end user DTMF control and 24x7 availability, IRLP is the way to go. IRLP offers excellent audio quality and a host of useful features. More on my experiences with IRLP here. EchoLink and eQSO offer some interesting alternative systems. As for legality, IRLP meets the ACA's requirements for an Internet linking system in Australia. The release of the Information paper in October 2002 suggests that EchoLink and eQSO may also be used on Australian frequencies. The information paper makes no distinction between simplex and repeater use of any of the systems available, and the ACA does not endorse or prohibit the use of any particular technology, as long as non amateurs are prevented from accessing amateur frequencies.
To assist you with choosing an appropriate linking technology, I have created a review of the leading Internet linking systems that are available as of mid 2002. Please read the Internet Linking Roundup, to get a better overview of the systems available, and their features.
A First Generation Example Working Repeater Link - circa 2001.
This information is for historical purposes only. It is strongly recommended to install one of the amateur specific systems nowadays.
The following illustration shows the block diagram of the Iphone repeater link system that I use here (Jan-Feb 2001). This system, minus the compressor/limiter, was successfully used in the Australia Day repeater linkup on January 26, 2001, and has been used frequently on simplex frequencies since then. This system was last used in March 2001, and has since given way to IRLP, EchoLink and eQSO.
In addition to the basic system outlined above, I have added a compressor/limiter to improve audio fed to the local repeater. I have also shown a buffer amplifier, which was needed to properly drive the FM92 transceiver's auxilliary input. The 23cm link RX is there to allow the local operator to easily access the system without altering mixer settings all the time.
The total cost of the system was surprisingly low. All components except for the tail mute and compressor limiter were found in the shack. The extra cost to build the hardware as shown below was around A$60, which consisted of the timer kit used for the tail mute, the compressor/limiter kit and several audio connectors used in the system. The rest of the parts were scrounged from the junkbox or elsewhere in the shack.
I haven't shown the detailed circuitry for several reasons. Firstly, some of the hardware was built from commercially available kits, and I didn't want to breach any copyright on these circuits. Secondly, the exact configuration will depend on the equipment you have available, so a detailed schematic to suit every situation isn't possible. Thirdly, it is expected that Amateurs attempting a repeater link would have sufficient knowledge to be able to sort out the circuit details. However, if there are any questions, email is always an option. Send any queries to vk3jed@optusnet.com.au
Fig 1. Block Diagram of the VK3JED Internet Repeater Link system in early 2001.
Common Issues
There are a few issues related to Intenret radio linking, especially when linking repeaters. Most of the suggestions below are generic and not tied to a particular system.
Q: What hardware and software do I need to create a temporary link via the Internet?
A: You will need a Pentium 100 PC or faster, a soundcard compatible with your operating system of choice (at this stage, typically Windows 95, 98, ME, NT, 2000, XP, or Linux), a transceiver to link your repeater with, a hardware interface (for the system you're using) and an Internet connection capable of at least 28.8kbps sustained throughput. You will also need Internet linking software compatible with the repeater(s) you're linking to.
Q: I've set it all up but it doesn't connect to the remote system!
A: Check your Internet connection. If you're operating from a corporate network, there may be a firewall preventing the software from working properly. Check with your LAN administrator. Some software requires you to forward ports from the firewall to the Internet gateway machine. Check the help file, system web site and support mailing lists to resolve firewall issues.
Q: How do I find other nodes to link to?
A: It depends on the system. eQSO and EchoLink come with their own inbuilt directory service. IRLP does its own hostname lookups behind the scenes dynamically. However, I maintain a list of links to status pages and node lists for the various systems.
Q: Users on one or both sides of the gateway don't hear any audio!
A: Check your soundcard's mixer settings. The line input of the mixer should be active in record mode, and the Wave output should be selected in the playback side for proper gateway operation. Also check for proper PTT/COS activation. Incorrectly configured firewalls will often be the culprit when audio is only working "one way".
Q: Another gateway operator complains my repeater's tail is jamming the channel or "tail chasing"! How do I fix this?
A: This is a common issue in conference rooms in all systems. here are two ways to fix this problem, using CTCSS on the link to prevent the tail being received by the link receiver, or using a timer to mute received audio after transmitting until after the repeater's tail has dropped. Most experienced gateway operators are happy to help, if you're willing to listen. Most linking software has adjustable timers to help with this problem. However, the real solution is to make sure that no audio is transmitted unless a user on your repeater is actually transmitting.
Q: Someone else's repeater tail is jamming the conference, how can I fix this?
A: It depends on the system. IRLP reflector owners can lock out stations causing problems, as can EchoLink conference owners. I believe eQSO has similar capabilities as well. In all cases, it requires an administrator of the system to perform the lockout.
Q: Whenever I join a conference, everyone else leaves! Don't they like me?
A: This is related to the previous two questions. If you're causing problems to the network, other operators may change conference rooms to avoid the problem. Check that your station is operating properly, with no "tail chasing" and proper audio levels.
Q: Some gateways and individuals have low audio and my users are complaining they can't hear some people. Can this be fixed?
A: This can be a problem as different stations sometimes use different audio levels. Using a compressor/limiter between the soundcard output and the link transmitter can result in a significant improvement. Tests done here have resulted in much better audio reports when the compressor/limiter is inline, and it's hard to tell when someone's audio is low over the Internet.
Q: Distant stations over the Net are timing out my repeater, or are being timed out by my users. Can this be fixed?
A: Other than enforcing proper operation rules, there's not a lot you can do here. IRLP node owners can play with scripts to briefly unkey and re key the system after a certain amount of time has elapsed.
Q: I want to communicate with other operators off air about link problems and other operational issues.
A: Most systems have their own support and discussion lists. Check the home page of your system for more information. Also, there is the generic repeaterlink Yahoo group at http://groups.yahoo.com/group/repeaterlink which is not specific to any linking technology.
A: Internet repeater linking was approved by the ACA in 2000, so this is all legal and above board. Further information can be found in this excellent FAQ by Peter, VK2YX. Update, 2002: The situation with Internet linking is undergoing a review process. At this stage, it appears that the likely outcome is that IRLP and other systems that create a "wormhole" between RF only gateways will be able to link to repeaters. Systems which allow direct Internet access may only be allowed to be run as attended simplex gateways. The release of the Information paper in October 2002 has both clarified and confused the issues somewhat.
Links
Internet Linking Roundup - Which system is the one for you?
Active Nodes for the various Internet linking systems currently in use.
VK3JED IRLP and EchoLink nodes - Experimental systems used for development.
Ian, G3ZHI's Internet Repeater Linking site.
IRLP - The Internet Radio Linking Project