Packet radio

thumb|Terminal Node Controller 2400 baud packet radio modem

Packet radio is the application of packet switching techniques to digital radio communications.

Packet radio is frequently used by amateur radio operators. The AX.25 (Amateur X.25) protocol was derived from the X.25 protocol and adapted for amateur radio use. Every AX.25 packet includes the sender's amateur radio callsign, which satisfies the United States Federal Communications Commission (FCC) requirements for amateur radio station identification. Using AX.25, it is possible for any packet station to act as a digipeater, linking distant stations with each other through ad hoc networks. This makes packet radio especially useful for emergency communications.

History

Earlier digital radio communications modes were telegraphy (using Morse code), teleprinter (using Baudot code) and facsimile.

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 communication channel to avoid collisions of signals. Professor Norman Abramson of the University of Hawaiʻi led development of a packet radio network known as ALOHAnet and performed a number of experiments beginning in the 1970s to develop methods to arbitrate access to a shared radio channel by network nodes. This system operated on UHF frequencies at 9,600 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.

Over 1973–76, 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 Internet Protocol (IP)) over packet radio links between mobile and fixed network nodes. This system was quite advanced, as it made use of direct sequence spread spectrum (DSSS) modulation and forward error correction (FEC) techniques to provide and 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; Bram Frank, VE2BFH; Norm Pearl, VE2BQS; and Jacques Orsali, VE2EHP of the Montreal Amateur Radio Club Montreal, Quebec, began experimenting with transmitting ASCII encoded data over VHF amateur radio frequencies using homebuilt equipment. In 1980, Doug Lockhart VE7APU, and the Vancouver Area Digital Communications Group (VADCG) in Vancouver, British Columbia began producing standardized equipment (Terminal Node Controllers) in quantity for use in amateur packet radio networks. In 1989, Lockhart was inducted by the Canadian Amateur Radio Federation (CARF, now Radio Amateurs of Canada), as its First Member of the Canadian Amateur Radio Hall of Fame (CARHOF) for his outstanding contribution to the development of Amateur Packet Radio Communications. In 2003, Rouleau was inducted into CQ Amateur Radio magazine's hall of fame for his work on the Montreal Protocol in 1978.

Not long after this activity began in Canada, amateurs in the US became interested in packet radio. In 1980, the United States Federal Communications Commission (FCC) granted authorization for United States amateurs to transmit ASCII codes via amateur radio. Repeaters may be designed for amateur packet radio, these are dubbed "digipeaters". The first known amateur packet radio activity in the US occurred in San Francisco during December 1980, when a packet repeater was put into operation on 2 meters by Hank Magnuski KA6M, and the Pacific Packet Radio Society (PPRS). In keeping with the dominance of DARPA and ARPANET at the time, the nascent amateur packet radio network was dubbed the AMPRNet in DARPA style. Magnuski 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.

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, in August 1986. Volunteers linked several key sites to pass text traffic via packet radio, which kept voice frequencies clear.

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

Data link

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

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 network layer protocols are NET/ROM & TheNET, ROSE, FlexNet and TexNet.

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

Implementations

Many commercial operations, particularly those that make use of vehicle dispatch (e.g., 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

Other uses

Packet radio can be used in mobile communications. Some mobile packet radio stations transmit their location periodically using the Automatic Packet Reporting System (APRS). If the APRS packet is received by an i-gate station, position reports and other messages can be routed to an Internet server and made accessible on a public web page. This allows amateur radio operators to track the locations along with telemetry and other messages of vehicles, hikers, high-altitude balloons, etc., around the world.

Some packet radio implementations also use dedicated point-to-point links such as TARPN. In cases such as this, new protocols have emerged, such as Improved Layer 2 Protocol (IL2P), supporting forward error correction for noisy and weak signal links.

References

External links

"Packet Radio Physical Layer" Useful notes taken by N1VG during the development of the OpenTracker TNC/APRS node
Packet radio software
Fast Packet Systems
44.0.0.0
AMRAD
TARPN
Packet Radio Simulator