Packet radio

Source: Wikipedia, the free encyclopedia.
Terminal Node Controller 2400 baud packet radio modem

In

communication link
.

Packet radio is frequently used by

digipeater, linking distant stations with each other through ad hoc networks
. This makes packet radio especially useful for emergency communications.

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 "igate" 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 of vehicles, hikers, high-altitude balloons, etc., along with telemetry and other messages 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

forward error correction
for noisy and weak signal links.

History

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

Aloha and PRNET

Since radio

local area network
(LAN) technology.

Over 1973–76,

FEC) techniques to provide 100 kbit/s and 400 kbit/s 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

Main article: AMPRNet

VHF amateur radio frequencies using homebuilt equipment.[3] 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 2003, Rouleau was inducted into CQ Amateur Radio magazine's hall of fame for his work on the Montreal Protocol in 1978.[4]

Not long after this activity began in Canada, amateurs in the US became interested in packet radio. In 1980, the United States

44.0.0.0/8 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

Tucson Amateur Packet Radio Corporation (TAPR) in Arizona and the Amateur Radio Research and Development Corporation (AMRAD) in Washington, D.C.[7]

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".[8][5]

Concepts

Packet radio can be differentiated from other digital radio switching schemes by the following attributes:

  • Transmitted data is broken into packets, each of which contains a destination (and typically the source) address
  • A transmitted message may be broken into a sequence of packets before transmission, which are then re-assembled into the original message upon reception
  • Packets for multiple destinations can be transmitted on the same radio link in an asynchronous fashion
  • A packet may be addressed to all possible recipients rather than a specific one (broadcast)
  • A packet may be stored and subsequently forwarded towards its destination by a network node

This is very similar to how packets of data are transferred between nodes on the Internet.

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

Bell 202 standard). While this approach worked, it was not optimal, because it used a 25 kHz FM channel to transmit at 1,200 baud. When using a direct FSK modulation like G3RUH's packet radio modem, a 9,600 baud transmission is easily made in the same channel. 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.

Another problem faced by early "packeteers" was the issue of

asynchronous framing
. The receiver looks for the "frame boundary octet," then begins decoding the packet data that follows it. Another frame boundary octet marks the end of the packet frame.

A number of data "conversations" are possible on a single radio channel over a finite period.

A basic packet radio station consists of a

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.

Layers

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

Example of packet radio transmission in 1,200 bits per second
Problems playing this file? See media help.

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 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 to the radio itself, usually just connecting the computer's audio output directly to the transmitter's microphone input and receiver's audio output directly to the computer's 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 1,200 Hz for Mark and 2,200 Hz for space (1,000 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

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 9,600 bit/s.

1,200 bit/s AFSK node controllers on 2 meters (144–148 MHz) are the most commonly found packet radio. For 1,200/2,400 bit/s UHF/VHF packet radio, amateurs use commonly available narrow band FM voice radios. For HF packet, 300 bit/s data is used over single sideband (SSB) modulation. For high speed packet (9,600 bit/s upwards), special radios or modified FM radios must be used.

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 the intermediate frequency part of the radio as opposed to the audio section used for 1,200 bit/s operation. The adoption of these high-speed links has been limited.

In many commercial data radio applications, audio baseband modulation is not used. Data is transmitted by altering the transmitter output frequency between two distinct frequencies (in the case of FSK modulation, other alternates exist).

The 2.4 GHz "Wi-Fi" band partially overlaps an amateur radio band, so commercial Wi-Fi hardware can be adapted and used by licensed amateur radio operators at higher power levels, although restrictions on amateur radio limit the appeal of using packet radio to connect to the internet. US FCC regulations do not allow amateur radio communications to be encrypted or private, in addition to other content restrictions.[9]

See also: High-speed multimedia radio

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.)[10]

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:[11]

See also

References

  1. ISBN 0-9763857-6-7
    .
  2. ^ I wrote the code for the demo on May 31st 1978 at the M.A.R.C. meeting in Montreal
  3. ISBN 0-8306-9628-8
    .
  4. ^ "The CQ Amateur Radio Hall of Fame" (PDF). CQ Amateur Radio. June 2007. Archived from the original (PDF) on 2008-12-03. Retrieved 2009-05-02.
  5. ^ a b Mendelsohn, Alex. "Amateur Packet – A Brief Chronology: Phase 1 (1970–1986)". Archived from the original on 2001-01-29. Retrieved 2009-08-09. See FCC Gives The Nod and Making Modifications
  6. ^ Kenney, Larry "Introduction to Packet Radio – Part 1", "A Short History – How it all began". Retrieved 2009-08-09.
  7. ISBN 0-87259-122-0
    .
  8. ^ 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.
  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 http://www.arrl.org/files/file/About%20ARRL/Committee%20Reports/2004/July/HSMM.pdf
  10. ^ AX.25 Link Access Protocol for Amateur Packet Radio: the official specification, from Tucson Amateur Packet Radio
  11. ISBN 0-471-31651-2
    .

Further reading

External links

Wikimedia Commons has media related to Packet radio.
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