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Internet Repeater Linking

Worldwide communication from your VHF/UHF handheld transceiver. That's the promise of the Internet Radio Linking Project, amateur radio's fastest-growing mode. There are more than one hundred IRLP-capable repeaters worldwide, and their numbers are growing daily. In Australia, amateurs in Sydney, Melbourne, Perth and Ipswich can already access IRLP-equipped repeaters with many future systems planned. Repeater sponsors report record activity, with it being hard to get a word in edgeways during busy times.

Participation in IRLP requires nothing more than a standard two metre or seventy centimetre handheld or mobile transceiver. Amateurs of all Australian licence grades can access IRLP if there is a node in their area.

This month's we explore the history of IRLP and its development in Australia. Operating procedures are also discussed, with an emphasis on how they differ from standard voice repeater etiquette. The article concludes by considering the future of IRLP and how it relates to other aspects of amateur radio.


A key aim for many amateurs has been to communicate over long distances. For most of amateur radio's history, most long-distance communication has been on the high-frequency part of the spectrum. This began to change with the advent of amateur satellites. However short pass times, restricted footprints and the need for specialised equipment and antennas ensured that HF, with its modest equipment and antenna demands, remains dominant for long-distance amateur communication.

The growth of packet radio from the late 1980s and the rapid spread of the internet several years later led to amateurs linking the two networks together. The use of packet 'converse bridges' linked to 'wormholes' allowed amateurs to have keyboard-to-keyboard chats via their local bulletin board system. However network congestion and low data transfer speeds often made this mode no faster than slow-speed Morse.

Amateurs soon started to experiment with using the internet for voice communication. A system called I-Phone (internet-phone) allowed voice repeaters to be linked via the internet. I-Phone proved an instant hit and soon spread worldwide. It saw greatest use in Australia during a special Australia-Day link-up on January 26 this year. Links in most state capitals allowed hundreds of amateurs to communicate across Australia in a manner reminiscent to the Aussat Jamboree of the Air satellite links in the early 1990s.

I-Phone had several disadvantages. These can be summarised as instability and lack of security. The instability was due to it being based on the Windows operating system. Links were frequently lost and control operators had to 'babysit' the link to ensure that it remained operational. The security of I-Phone was also poor, with it being possible for non-amateurs to break into an amateur link via the internet. Control operators again had the burden of supervising their system to ensure that non-licensed persons were not illegally accessing amateur repeaters.

A Canadian amateur, David Cameron VE7LTD, who had been experimenting with I-Phone, developed a new internet-based radio linking system that was without I-Phone's problems. Basing the software on the stable Linux operating system cured the instability observed with I-Phone. Security was strengthened by using the PGP key encryption system to prevent pirate hackers breaking into radio links via the internet. Additional features of IRLP include user-selectable links (via the DTMF keypad provided on many transceivers) and the ability for participating link stations (or nodes) to receive automatic software updates. A further benefit (from an amateur viewpoint) is that all participants must access the system via a radio link. This contrasts with I-Phone, which is less radio-based as users can log in via the web.

Many involved in I-Phone have converted their repeaters to IRLP. Canada and the US were the first countries to switch, and the last three months have seen phenomenal growth in Australia. IRLP is now available in some UK cities, with great interest also being shown by amateurs in other parts of Europe.

IRLP in Australia

A rudimentary internet-radio link was established in Sydney in 1992/3 (Reference two). The experiment lasted only a short time, but raised considerable interest in the possibility of combining amateur radio voice and data communication. To put things in perspective, at the time many PC users had not even switched to Windows, the World Wide Web was hardly born and e-mail was only known in academic, research and computer enthusiast circles.

A few years later, I-Phone was introduced to Sydney's VK2RBM repeater, operated by the Blue Mountains Amateur Radio Club. I-Phone was based on Internet Telephone Package software. Amateurs could plug their headset into their sound card and work through I-Phone equipped repeaters from anywhere with a internet connection. As mentioned before, I-Phone's popularity in Australia peaked in January, when it was successfully used for a nation-wide hook-up.

Australia's first IRLP node was VK6RNC, run by Perth's Northern Corridor Radio Group. It opened in February. VK2RBM switched from I-Phone to IRLP in April. Melbourne's VK3RGL came third when it opened in June 2001 after a week of tests on a simplex frequency.

At the time of writing (early July 2001) six Australian repeaters were equipped with IRLP. These are listed below.

*VK2RBM Blue Mountains 147.050 MHz

*VK2RMP Wollongong 146.800 MHz

*VK3RGL Geelong 147.000 MHz

*VK4RKP Ipswich 146.725 MHz

*VK6RNC Perth North 146.625 MHz

*VK6RFM Fremantle 146.950 MHz

IRLP nodes are planned for Adelaide, Darwin, Launceston, Canberra, Bendigo, Mildura, Penrith, Plumpton, Terry Hills and Antarctica. Some may be on air by the time this article appears.

How IRLP works (by David Cameron VE7LTD)

An IRLP node consists of a radio transceiver (to provide an RF link into the node), an IRLP interface board, and a personal computer connected to broadband Internet. IRLP can also be used over a telephone line Internet connection, but performance is not as impressive.

The computer uses Voice-Over-IP streaming software called Speak Freely, which operates under the Linux operating system. The software digitises and compresses audio received from the radio. At the other end of the link it decompresses the audio and converts it to analogue. This audio is then fed to the radio transceiver. The process reverses when the station at the distant node responds.

In more detail, Voice-Over-IP works as follows:

*Sample the audio using an analogue to digital (A/D) converter. The A/D converter used by IRLP is the input source of a standard PC sound card. This creates a continuous mono 8-bit digital stream of raw audio at 8000Hz (64000 bps).

*Compress the audio by downsampling the stream and using an 8-bit ULAW algorithm to reduce the size of the stream by a factor of two (32000 bps) with little degradation of the audio.

*Split the sample into small chucks (or packets).

*Transmit the packets to the remote host using a User Datagram Protocol (UDP) stream. UDP does NOT confirm the reception of packets, so it uses a "fire and forget" method.

*Receive the packets on the remote host.

*Join the split packets back into an 8-bit ULAW stream.

*Uncompress the ULAW stream back into an 8-bit raw stream of audio.

*Play the raw audio stream through a digital to analogue (D/A) converter (the output device of your sound card).

The control software controls the stream using carrier operated squelch (COS) or continuous tone coded subaudible squelch signals (CTCSS) to start and stop the stream. When COS is present, the computer detects it through the IRLP interface board.

The buffer that joins the split packets back into the audio stream controls the PTT. The IRLP interface board receives a "key" signal from the computer while there are packets in the buffer, and an "unkey" command when the buffer is empty.

The user interfaces to the IRLP computer using DTMF (dual tone multi frequency or 'touchtone') signals sent over the repeater. DTMF sequences are owner programmable, and can accomplish almost any function imaginable. The DTMF signals are detected on the IRLP interface board and sent directly to the computer in binary, where they are converted into numbers. A DTMF software program then runs commands on the computer depending on the code entered.

These commands are sent to various software scripts that start and stop Speak Freely, basically establishing and breaking the link.

Operating etiquette

To get the most from IRLP, operators should be unselfish and share the link with others who have equal right to use the facility. Commonsense and good repeater operating manners will generally serve the IRLP user well. However you should be aware of the following differences between IRLP and standard repeater operation.

* Large groups and high activity. As with any large on-air gathering, confusion can reign if operators are unclear as to who is next in line. Clearly identify the next station when you conclude a transmission. Before calling in, listen for a few minutes to get an idea of order. Also avoid using an IRLP-linked repeater for lengthy local chats if contact can be maintained on simplex or via other unlinked repeaters.

* Presence of interstate and foreign stations. Regular users of a local repeater know each other's voices and heavily rely on this to fill gaps caused sloppy pronunciation. Also usually only a two or three letter callsign suffix needs to be remembered. The presence of interstate and overseas stations makes clear pronunciation with standard phonetics imperative on IRLP. Foreign stations sometimes have difficulty understanding our accent. Speaking slightly slower than usual will often assist here.

* Time delay. An unlinked voice repeater has almost no time delay. However as IRLP links make use of data processing and long-distance transmission, delays can be up to three seconds. Before talking, count to five, press your PTT, wait a second and then talk. The delay allows links to stabilise and reset.

* Multiple time-outs. Normal repeater usage requires that operators abide by the timeout of the repeater or their transmissions are cut off. A successful IRLP contact requires operators abide by the time-outs of both repeaters plus that of the IRLP link itself. If one side remains inactive after a specified duration, the link between the repeaters will automatically drop out. To prevent this, operators should 'ping-pong' transmissions between the local and distant repeater so that each end maintains sufficient activity to maintain the link.

* Programmable links. Operating voice repeaters in Australia is a matter of selecting the correct frequency and offset and making a call. Using an IRLP-equipped repeater whose link is active requires no extra access tones or codes. On the other hand, if an IRLP link is inactive, or you wish to choose which repeater you wish to link to, you will need to enter the correct DTMF code for that link. This matter is discussed in detail later in the article.

* Existence of a 'reflector'. Normal IRLP links join two repeaters only. However, a 'reflector' can allow multiple repeaters to be linked via IRLP. The record for the number of repeaters linked at any one time currently stands at over 20! This has great potential for special-interest groups (eg youth, ALARA or old timers on-air gatherings), special events such as JOTA or amateur news broadcasts.

IRLP codes and link selection

From the user's perspective, the largest difference between IRLP and conventional repeaters is the use of user-selectable links. These use DTMF codes, as used by standard touch-tone telephones. If you wish to operate through an open IRLP link, you do not need to send DTMF tones or have DTMF equipped on your transceiver. Tones are only needed if you wish to open a link that has closed or reset the link to another repeater or node. Each node has a unique four-digit code that must be entered to allow linkage to it.

There are differing opinions over the extent to which people should be given the access codes required to activate links. Groups in small cities may opt to make codes available to everyone. In densely populated areas (such as the eastern seaboard) sponsoring clubs may opt for codes to be available to designated control operators or members only. Another possibility, being discussed as this article was being written, is to release node codes, but add a prefix available to designated operators only. The matter of codes is likely to be controversial, with many different views being expressed. However there is general consensus among Australian clubs that IRLP should be open to all, and members listening will enter the code for non-members wishing to be linked to a particular repeater.

Most modern amateur hand-held transceivers can transmit DTMF codes. Those without suitable equipment or who don't know the codes have several options. These include:

*Ask another station (especially a member of the club sponsoring the repeater) to key the code for you

*Homebrew a DTMF encoder or salvage one from telephone equipment

*Purchase a touch tone keypad (at the time of writing, Tandy was selling them very cheaply)

Before you enter a code check that the repeater is not in use. Wait a few seconds, identify yourself, announce that you are connecting to another node and send the DTMF code. If you are successful, you will hear a voice announcement confirming the link connection.

IRLP's impact on amateur activity patterns

Almost every emerging mode in amateur radio has an effect on existing activities and modes. In some cases the new mode supplants the old mode and the old mode is seldom used or utilised mainly for nostalgic purposes. This became true for AM when HF SSB emerged dominant in the 1960s. In other cases interest in the established activity continues, but on a smaller scale. This is perhaps true for VHF SSB/AM tunable operation when FM net frequencies and repeaters spread across the country in the 1970s. The extent to which IRLP will influence other amateur activities is not yet known.

Many of the concerns expressed about IRLP are remarkably similar to those expressed when repeaters became popular. These concerns include a centralisation of station capability (from many individual stations to a few repeater sites) and technical expertise (from many individuals to a tiny number of software developers and repeater maintainers). Those who value amateur radio's possible contribution to emergency preparedness should also be concerned that our HF equipment and antenna capability is maintained despite any swing away from HF towards IRLP.

On the credit side, IRLP offers many benefits for amateur activity. Those who establish IRLP nodes learn about data and voice communications technology and have the potential to spread the benefit of their knowledge throughout the amateur service. IRLP has reignited activity amongst many licensed amateurs who have let their involvement lapse. It offers particular benefits to the increasing number of amateurs unable to fully enjoy international HF communication due to space and interference constraints at home. IRLP is also an excellent drawcard when promoting amateur radio to the general public. If the amateur service is to remain a technological activity, it cannot afford to let itself be bypassed by developments such as IRLP.

IRLP also requires the amateur service to maturely manage its affairs in a co-operative fashion. Again the growth of repeaters in the 1970s and packet radio in the 1980s provide precedents for this type of co-operation. No doubt there will be some testing moments as those involved seek to find a workable balance between central standards and local initiative and handle potentially controversial matters such as access to control codes! The need to involve individual amateurs worldwide in IRLP development and to lessen the division between the tiny number of innovators and the broader mass of the amateur population by raising the expertise of the latter is another challenge faced by proponents of all modern communication techniques, not just IRLP.


There is little doubt that IRLP and allied techniques will have an influence on amateur radio activity at least as significant as the growth of FM and repeaters in the 1970s. It is hoped that radio amateurs are imaginative in their use of this technology and are able to exploit it for their collective good.


The author acknowledges the assistance of IRLP developer David Cameron VE7LTD and local pioneers Peter Illmayer VK2YX and Tony Langdon VK3JED in the preparation of this article. Much of the material was presented in abridged form by VK3JED at the Moorabbin & District Radio Club on July 6, 2001.


*The Official Home of the Internet Repeater Linking Project http://www.irlp.net

*Bell & Illmayer, Radio and Communications, July 2001

This article appeared in Amateur Radio August 2001. No updates have been made and it appears solely for historical interest.



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