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Terminal Node Controller 2400 packet radio modem

Packet radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated or switched circuits.



Aloha and PRNET

Since radio circuits inherently possess a broadcast network topology (i.e., many or all nodes are connected to the network simultaneously), one of the first technical challenges faced in the implementation of packet radio networks was a means to control access to a shared communications channel. Professor Norman Abramson of the University of Hawaii developed a packet radio network known as ALOHAnet and performed a number of experiments around 1970 to develop methods to arbitrate access to a shared radio channel by network nodes. This system operated on UHF frequencies at 9600 baud. From this work the Aloha multiple access protocol was derived. Subsequent enhancements in channel access techniques made by Leonard Kleinrock et al in 1975 would lead Robert Metcalfe to use carrier sense multiple access (CSMA) protocols in the design of the now commonplace Ethernet local area network (LAN) technology.

In 1977, DARPA created a packet radio network called PRNET in the San Francisco Bay area and conducted a series of experiments with SRI to verify the use of ARPANET (a precursor to the Internet) communications protocols (later known as IP) over packet radio links between mobile and fixed network nodes.[1] This system was quite advanced, as it made use of direct sequence spread spectrum (DSSS) modulation and forward error correction (FEC) techniques to provide 100 kpbs and 400 kpbs data channels. These experiments were generally considered to be successful, and also marked the first demonstration of Internetworking, as in these experiments data was routed between the ARPANET, PRNET, and SATNET (a satellite packet radio network) networks. Throughout the 1970s and 1980s, DARPA operated a number of terrestrial and satellite packet radio networks connected to the ARPANET at various military and government installations.

Amateur Packet Radio and the AMPRNet

Amateur radio operators began experimenting with packet radio in 1978, when after obtaining authorization from the Canadian government, Robert Rouleau, VE2PY and The Western Quebec VHF/UHF Amateur Radio Club in Montreal, Canada began experimenting with transmitting ASCII encoded data over VHF amateur radio frequencies using homebuilt equipment.[2] In 1980, Doug Lockhart, VE7APU and the Vancouver Area Digital Communications Group (VADCG) in Vancouver, Canada began producing standardized equipment (Terminal Node Controllers) in quantity for use in amateur radio packet radio networks. In 2003, Rouleau was inducted into CQ Amateur Radio magazine's hall of fame for his work on the Montreal Protocol in 1978.[3]

Not long after this activity began in Canada, amateurs in the US became interested in packet radio, and in 1980, the U.S. Federal Communications Commission (FCC) granted authorization for U.S. amateurs to transmit ASCII codes via amateur radio.[4] The first known amateur packet radio activity in the US occurred in San Francisco in December of 1980 when a packet repeater was put into operation on 2 meters by Hank Magnuski, KA6M and the Pacific Packet Radio Society (PPRS).[5] In keeping with the dominance of DARPA and ARPANET at this time, the nascent amateur packet radio network was dubbed the AMPRNet, in DARPA style and Magnuski had obtained IP address allocations in the network for amateur radio use worldwide.

Many groups of amateur radio operators interested in packet radio soon formed throughout the country including the Pacific Packet Radio Society (PPRS) in California, the Tucson Amateur Packet Radio Corporation (TAPR) in Arizona and the Amateur Radio Research and Development Corporation (AMRAD) in Washington, D.C.[6]

By 1983, TAPR was offering the first TNC available in kit form. Packet radio started becoming more and more popular across North America and by 1984 the first packet based bulletin board systems began to appear. Packet radio proved its value for emergency operations following the crash of an Aeromexico airliner in a neighborhood in Cerritos, California the weekend of Labor Day 1986. Volunteers linked several key sites to pass text traffic via packet radio while keeping the voice frequencies clear.

For an objective description of early developments in amateur packet radio, refer to the article "Packet Radio in the Amateur Service".[7]

The most common use of packet radio today is in amateur radio, to construct wireless computer networks. Its name is a reference to the use of packet switching between network nodes. Packet radio networks use the AX.25 data link layer protocol, derived from the X.25 protocol suite and adapted for amateur radio use.

Commercial Packet Radio[8]

Many commercial operations, particularly those that make use of vehicle dispatch (i.e. taxis, tow trucks, police) were quick to note the value of packet radio systems to provide simple mobile data systems. This led to the rapid development of a number of commercial packet radio systems:

  • MDI (1979)
  • DCS (1984)
  • DRN (1986)
  • Mobitex (1986)
  • ARDIS (1990)
  • CDPD allowed packet data to be carried over AMPS analog cellular telephone networks
  • GPRS is the packet data facility provided by the GSM cellular telephone network

Amateur Packet Radio Evolution

Data not Voice

One of the first challenges faced by amateurs implementing packet radio is that almost all amateur radio equipment (and most surplus commercial/military equipment) has historically been designed to transmit voice, not data. Like any other digital communications system that uses analog media, packet radio systems require a modem. Since the radio equipment to be used with the modem was intended for voice, early amateur packet systems used AFSK modems that followed telephone standards (notably the Bell 202 standard). Needless to say, while this approach worked, it was nowhere near optimal.

In addition, the baseband characteristics of the audio channel provided by voice radios are often quite different from those of telephone audio channels. This led to the need in some cases to enable or disable pre-emphasis or de-emphasis circuits in the radios and/or modems.

Asynchronous Framing

Another problem faced by early "packeteers" was the issue of asynchronous versus synchronous data transfer. Most personal computers from the early days of amateur packet radio had RS-232 serial ports for asynchronous data communications between the computer and devices like modems. The RS-232 standard specifies an asynchronous, start-stop mode of data transmission where data is sent in groups (characters) of 7 or 8 bits. Unfortunately, the simple AFSK modems typically used provide no timing signal to indicate the start of a packet frame. This led to the need for a mechanism to allow the receiver to know when to start assembling each packet frame. The typical approach to accomplishing this is called asynchronous framing, and this is what was used. The receiver looks for the "frame boundary octet" and then begins decoding the packet data that follows it. Another frame boundary octet marks the end of the packet frame.

Sharing the Channel

Station configuration

A basic packet radio station consists of a computer or dumb terminal, a modem, and a transceiver with an antenna. Traditionally, the computer and modem are combined in one unit, the terminal node controller (TNC), with a dumb terminal (or terminal emulator) used to input and display data. Increasingly, however, personal computers are taking over the functions of the TNC, with the modem either a standalone unit or implemented entirely in software. Alternatively, multiple manufacturers (including Kenwood and Alinco) now market handheld or mobile radios with built-in TNCs, allowing connection directly to the serial port of a computer or terminal with no other equipment required.

The computer is responsible for managing network connections, formatting data as AX.25 packets, and controlling the radio channel. Frequently it provides other functionality as well, such as a simple bulletin board system to accept messages while the operator is away.


Following the OSI model, packet radio networks can be described in terms of the physical, data link, and network layer protocols on which they rely.

Physical layer: modem and radio channel

Modems used for packet radio vary in throughput and modulation technique, and are normally selected to match the capabilities of the radio equipment in use. Most commonly used method is one using audio frequency-shift keying (AFSK) within the radio equipment's existing speech bandwidth. The first amateur packet radio stations were constructed using surplus Bell 202 1,200 bit/s modems, and despite its low data rate, Bell 202 modulation has remained the standard for VHF operation in most areas. More recently, 9,600 bit/s has become a popular, albeit more technically demanding, alternative. At HF frequencies, Bell 103 modulation is used, at a rate of 300 bit/s.

Due to historical reasons, all commonly used modulations are based on an idea of minimal modification of the radio itself, usually just connecting the external speaker or headphone output directly to the transmit microphone input and receiver audio output directly to the computer microphone input. Upon adding a turn the transmitter on output signal ("PTT") for transmitter control, one has made a "radio modem". Due to this simplicity, and just having suitable microchips at hand, the Bell 202 modulation became standard way to send the packet radio data over the radio as two distinct tones. The tones are 1200 Hz for Mark and 2200 Hz for space (1000 Hz shift). In the case of Bell 103 modulation, a 200 Hz shift is used. The data is differentially encoded with a NRZI pattern, where a data zero bit is encoded by a change in tones and a data one bit is encoded by no change in tones.

Ways to achieve higher speeds than 1,200 bit/s, include using telephone modem chips via the microphone and audio out connectors. This has been proven to work at speeds up to 4800 bit/s using fax V.27 modems in half-duplex mode. These modems use phase shift keying which works fine when there is no amplitude shift keying, but at faster speeds such as 9600 bit/s, signal levels become critical and they are extremely sensitive to group delay in the radio. These systems were pioneered by Simon Taylor (G1NTX) and Jerry Sandys (G8DXZ) in the 1980s. Other systems which involved small modification of the radio were developed by James Miller (G3RUH) and operated at 9600 bit/s.

Custom modems have been developed which allow throughput rates of 19.2 kbit/s, 56 kbit/s, and even 1.2 Mbit/s over amateur radio links on FCC permitted frequencies of 440 MHz and above. However, special radio equipment is needed to carry data at these speeds. The interface between the "modem" and the "radio" is at intermediate frequency -part of the radio disregarding parts of the radio that were used for voice modulation production and detection. The adoption of these high speed links has been limited.

In commercial data radio uses the intermediate audio stage is dispensed with, and bits are transmitted by directly altering output frequency in between two distinct frequencies (in case of FSK modulation, other alternates do exist). Such radio modem chips are now available, but as they are incompatible with previous Amateur Packet Radio technologies, they have not been adopted into amateur radio use.

High-speed multimedia radio

One notable detail is that 2.4 GHz WLAN band is partially overlapping amateur radio band, and thus WLAN hardware can readily be used by amateur radio licensed operators with higher power radio gear than what the general population "license free" usage allows. The restrictions inherent in Amateur Radio licenses ("signal must be free to receive by anybody", "transmit only between licensed radio amateurs", and "no encryption or other privacy techniques may be used", as well as various content restrictions) prevents this from being an appealing technique for connecting to the internet. Regulation details differ around the world.


Data link layer: AX.25

Packet radio networks rely on the AX.25 data link layer protocol, derived from the X.25 protocol suite and intended specifically for amateur radio use. Despite its name, AX.25 defines both the physical and data link layers of the OSI model. (It also defines a network layer protocol, though this is seldom used.)

Network layer

Packet radio has most often been used for direct, keyboard-to-keyboard connections between stations, either between two live operators or between an operator and a bulletin board system. No network services above the data link layer are required for these applications.

To provide automated routing of data between stations (important for the delivery of electronic mail), several network layer protocols have been developed for use with AX.25. Most prominent among these are NET/ROM, ROSE, and TexNet.

In principle, any network layer protocol may be used, including the ubiquitous Internet protocol.

See also


  1. ^ Okin,J.R. (2005). The Internet Revolution: The Not-for-Dummies Guide to the History, Technology, and Use of the Internet, p.81. Ironbound Press. ISBN 0-9763-8576-7.
  2. ^ Rouleau, Robert and Hodgson, Ian (1981). Packet Radio. Tab Books, Blue Ridge Summit, PA. ISBN 0-8306-9628-8.
  3. ^ "The CQ Amateur Radio Hall of Fame". CQ Amateur Radio. June 2007. 
  4. ^ Mendelsohn, Alex "Amateur Packet - A Brief Chronology: Phase 1 (1970-1986)", FCC Gives The Nod. Retrieved on 2009-08-09.
  5. ^ Kenney, Larry "Introduction to Packet Radio - Part 1", "A Short History - How it all began". Retrieved on 2009-08-09.
  6. ^ American Radio Relay League (2008). "ARRL's VHF Digital Handbook", p 1-2, American Radio Relay League. ISBN 0-8725-9122-0.
  7. ^ Karn, P. Price H. Diersing, R. (May 1985). "Packet Radio in the Amateur Service", pp 431-439, "IEEE Journal on Selected Areas in Communications". ISSN 0733-8716.
  8. ^ DeRose, James F. (1999). "The Wireless Data Handbook", pp.3-7. Wiley-Interscience; 4th edition. ISBN 0-4713-1651-2.
  9. ^ Security & Data Integrity On A Modern Amateur Radio Network - By: Paul J. Toth – NA4AR "HSMM and Information Security," by K8OCL CQ-VHF Fall 2004 - preview via CQ-VHF website "Data Encryption is Legal," N2IRZ, CQ Magazine Aug 2006 - preview from the Summer 2006 TAPR PSR


  • Okin, J.R. (2005). The Internet Revolution: The Not-for-Dummies Guide to the History, Technology, and Use of the Internet. Ironbound Press. ISBN 0-9763-8576-7. 
  • Rouleau, Robert; Hodgson, Ian (1981). Packet Radio. TAB Books. ISBN 0-8306-1345-5. 

External links

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