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.
History
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.
Conclusion
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.
Acknowledgements
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.
References
*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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