Packet radio
Encyclopedia
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
.
By the early 1990s, packet radio was not only recognized as a way to send text, but also to send files (including small computer programs), handle repetitive transmissions, control remote systems, etc.
The technology itself was a leap forward, making it possible for nearly any packet station to act as a digipeater, linking distant stations with each other through ad hoc networks. This makes packet especially useful for emergency communications. In addition, mobile packet radio stations can automatically transmit their location, and check in periodically with the network to show that they are still operating.
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. 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.
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. 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 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.
Not long after this activity began in Canada, amateurs in the US became interested in packet radio. In 1980, the U.S. Federal Communications Commission
(FCC) granted authorization for U.S. amateurs to transmit ASCII codes via amateur radio. 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 44.0.0.0 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 system
s began to appear. Packet radio proved its value for emergency operations following the crash of an Aeromexico
airliner in a neighborhood in Cerritos, California
Labor Day
weekend, 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".
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 node
s. Packet radio networks use the AX.25
data link layer
protocol, derived from the X.25
protocol suite and adapted for amateur radio use.
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.
data transfer. At the time, most personal computers had asynchronous RS-232
serial ports for data communications between the computer and devices such as 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
. That led to the need for a mechanism to enable the receiver to know when to start assembling each packet frame. The method used is called 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.
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.
, packet radio networks can be described in terms of the physical, data link, and network layer protocols on which they rely.
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 bits/s has become a popular, albeit more technically demanding, alternative. At HF
frequencies, Bell 103 modulation is used, at a rate of 300 bits/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 bits/s, include using telephone modem chips via the microphone and audio out connectors. This has been proven to work at speeds up to 4800 bits/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 bits/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 bits/s.
Custom modems have been developed which allow throughput rates of 19.2 kbits/s, 56 kbits/s, and even 1.2 Mbits/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 1200 bit/s operation. The adoption of these high speed links has been limited.
In commercial data radio applications, the intermediate audio stage is not used. Data is transmitted by directly altering output frequency between two distinct frequencies (in the case of FSK modulation, other alternates exist). Such radio modem chips are now available, but as they are incompatible with existing Amateur Packet Radio technology, they have not been adopted into amateur radio use.
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.)
. 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 & TheNET, ROSE, FlexNet and TexNet.
In principle, any network layer protocol may be used, including the ubiquitous Internet protocol
.
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
technology used to transmit digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
via radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
or wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
communications links
Data link
In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver and the interconnecting data telecommunication circuit...
. It uses the same concepts of data transmission via Datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
that are fundamental to communications via the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, as opposed to the older techniques used by dedicated or switched circuits
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
.
Purpose and advantages
Packet radio is the fourth major digital radio communications mode. Earlier modes were telegraphy (Morse Code), teleprinter (Baudot) and facsimile. Like those earlier modes, packet was intended as a way to reliably transmit written information. The primary advantage was initially expected to be increased speed, but as the protocol developed, other capabilities surfaced.By the early 1990s, packet radio was not only recognized as a way to send text, but also to send files (including small computer programs), handle repetitive transmissions, control remote systems, etc.
The technology itself was a leap forward, making it possible for nearly any packet station to act as a digipeater, linking distant stations with each other through ad hoc networks. This makes packet especially useful for emergency communications. In addition, mobile packet radio stations can automatically transmit their location, and check in periodically with the network to show that they are still operating.
Aloha and PRNET
Since radio circuitsTelecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
inherently possess a broadcast network topology
Network topology
Network topology is the layout pattern of interconnections of the various elements of a computer or biological network....
(i.e., many or all nodes
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
are connected to the network
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
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
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
. Professor Norman Abramson
Norman Abramson
Norman Abramson is an American engineer and computer scientist, most known for developing the ALOHAnet system for wireless computer communication....
of the University of Hawaii
University of Hawaii
The University of Hawaii System, formally the University of Hawaii and popularly known as UH, is a public, co-educational college and university system that confers associate, bachelor, master, and doctoral degrees through three university campuses, seven community college campuses, an employment...
developed a packet radio network known as ALOHAnet
ALOHAnet
ALOHAnet, also known as the ALOHA System, or simply ALOHA, was a pioneering computer networking system developed at the University of Hawaii. ALOHAnet became operational in June, 1971, providing the first public demonstration of a wireless packet data network.The ALOHAnet used a new method of...
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
Leonard Kleinrock
Leonard Kleinrock is an American engineer and computer scientist. A computer science professor at UCLA's Henry Samueli School of Engineering and Applied Science, he made several important contributions to the field of computer networking, in particular to the theoretical side of computer networking...
et al in 1975 would lead Robert Metcalfe
Robert Metcalfe
Robert Melancton Metcalfe is an electrical engineer from the United States who co-invented Ethernet, founded 3Com and formulated Metcalfe's Law., he is a general partner of Polaris Venture Partners...
to use carrier sense multiple access
Carrier Sense Multiple Access
Carrier Sense Multiple Access is a probabilistic Media Access Control protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum."Carrier Sense" describes the fact that a...
(CSMA) protocols in the design of the now commonplace Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
local area network
Län
Län and lääni refer to the administrative divisions used in Sweden and previously in Finland. The provinces of Finland were abolished on January 1, 2010....
(LAN) technology.
In 1977, DARPA created a packet radio network called PRNET in the San Francisco Bay
San Francisco Bay
San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...
area and conducted a series of experiments with SRI
SRI International
SRI International , founded as Stanford Research Institute, is one of the world's largest contract research institutes. Based in Menlo Park, California, the trustees of Stanford University established it in 1946 as a center of innovation to support economic development in the region. It was later...
to verify the use of ARPANET
ARPANET
The Advanced Research Projects Agency Network , was the world's first operational packet switching network and the core network of a set that came to compose the global Internet...
(a precursor to the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
) communications protocols
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
(later known as IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
) over packet radio links between mobile and fixed network nodes. This system was quite advanced, as it made use of direct sequence spread spectrum
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
(DSSS) modulation and forward error correction (FEC
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
) 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
Internetworking
Internetworking is the practice of connecting a computer network with other networks through the use of gateways that provide a common method of routing information packets between the networks...
, 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 radioAmateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
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
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
began experimenting with transmitting ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
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
Vancouver
Vancouver is a coastal seaport city on the mainland of British Columbia, Canada. It is the hub of Greater Vancouver, which, with over 2.3 million residents, is the third most populous metropolitan area in the country,...
, Canada began producing standardized equipment (Terminal Node Controllers
Terminal node controller
A terminal node controller is a device used by amateur radio operators to participate in AX.25 packet radio networks. It is similar in function to the Packet Assembler/Disassemblers used on X.25 networks, with the addition of a modem to convert baseband digital signals to audio tones.The TNC was...
) in quantity for use in amateur packet radio networks. In 2003, Rouleau was inducted into CQ Amateur Radio
CQ Amateur Radio
CQ Amateur Radio is a magazine for amateur radio enthusiasts. The magazine has a worldwide readership base for its English language edition. A Spanish language edition is published in Spain with some translations of articles from the English language edition and some original European content...
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 U.S. Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) granted authorization for U.S. amateurs to transmit ASCII codes via amateur radio. 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
2 meters
The 2-meter amateur radio band is a portion of the VHF radio spectrum, comprising frequencies stretching from 144.000 MHz to 148.000 MHz in International Telecommunication Union region Regions 2 and 3 and from 144.000 MHz to 146.000 MHz in ITU Region 1...
by Hank Magnuski
Hank Magnuski
Prior to founding NCast, Dr. Hank Magnuski was co-founder and CEO of GammaLink. He invented the industry's first PC-to-fax communications technology in 1985. In 1994, GammaLink merged with Dialogic Corporation of Parsippany, New Jersey, a leading manufacturer of voice cards for the PC. In 1999,...
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
AMPRNet
The AMPRNet is a name used by amateur radio operators for computer networks connected over amateur radio. Other names for the network include IPv4 Network 44/8 and Network 44...
in DARPA style. Magnuski obtained IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
address allocations in the 44.0.0.0 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
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, the Tucson Amateur Packet Radio Corporation (TAPR) in Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
and the Amateur Radio Research and Development Corporation (AMRAD) in Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
By 1983, TAPR was offering the first TNC available in kit form. Packet radio started becoming more and more popular across North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and by 1984 the first packet based bulletin board system
Bulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
s began to appear. Packet radio proved its value for emergency operations following the crash of an Aeromexico
Aeroméxico Flight 498
Aeroméxico Flight 498, registration , was a Douglas DC-9-32 on route from Mexico City, Mexico to Los Angeles International Airport, Los Angeles, California, United States on August 31, 1986. N4891F was a privately-operated Piper PA-28-181 Archer owned by the Kramer family en route from Torrance to...
airliner in a neighborhood in Cerritos, California
Cerritos, California
Cerritos is a city in Los Angeles County, California, United States, and is one of several cities that constitute the Gateway Cities of southeast Los Angeles County. It was incorporated on April 24, 1956...
Labor Day
Labor Day
Labor Day is a United States federal holiday observed on the first Monday in September that celebrates the economic and social contributions of workers.-History:...
weekend, 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".
The most common use of packet radio today is in amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
, to construct wireless computer networks. Its name is a reference to the use of packet switching
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
between network node
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
s. Packet radio networks use the AX.25
AX.25
AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks....
data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
protocol, derived from the X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
protocol suite and adapted for amateur radio use.
Commercial Packet Radio
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)
- MobitexMobitexMobitex is an OSI based open standard, national public access wireless packet-switched data network. Mobitex puts great emphasis on safety and reliability with its use by military, police, firefighters and ambulance services. Mobitex was developed in the beginning of the 1980s by the Swedish...
(1986) - ARDIS (1990)
- CDPD allowed packet data to be carried over AMPSAdvanced Mobile Phone SystemAdvanced Mobile Phone System was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas in 1983, Israel in 1986, and Australia in 1987. It was the primary analog mobile phone system in North America through the 1980s and into the 2000s...
analog cellular telephone networks - GPRS is the packet data facility provided by the GSM cellular telephone network
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). While this approach worked, it was not optimal.In addition, the baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
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 synchronousComparison of synchronous and asynchronous signalling
Synchronous and asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the transmission medium.-The need for...
data transfer. At the time, most personal computers had asynchronous RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
serial ports for data communications between the computer and devices such as 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
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
. That led to the need for a mechanism to enable the receiver to know when to start assembling each packet frame. The method used is called 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.
Sharing the Channel
A number of data 'conversations' are possible on a single radio channel over a finite period.Station configuration
A basic packet radio station consists of a computerComputer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
or dumb terminal, a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
, and a transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
with an antenna. Traditionally, the computer and modem are combined in one unit, the terminal node controller
Terminal node controller
A terminal node controller is a device used by amateur radio operators to participate in AX.25 packet radio networks. It is similar in function to the Packet Assembler/Disassemblers used on X.25 networks, with the addition of a modem to convert baseband digital signals to audio tones.The TNC was...
(TNC), with a dumb terminal (or terminal emulator
Terminal emulator
A terminal emulator, terminal application, term, or tty for short, is a program that emulates a video terminal within some other display architecture....
) 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
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
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
Bulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
to accept messages while the operator is away.
Layers
Following the OSI modelOSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
, 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 bits/s has become a popular, albeit more technically demanding, alternative. At HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
frequencies, Bell 103 modulation is used, at a rate of 300 bits/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 bits/s, include using telephone modem chips via the microphone and audio out connectors. This has been proven to work at speeds up to 4800 bits/s using fax V.27
ITU V-Series
The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces....
modems in half-duplex mode. These modems use phase shift keying which works fine when there is no amplitude shift keying
Amplitude-shift keying
Amplitude-shift keying is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.Any digital modulation scheme uses a finite number of distinct signals to represent digital data. ASK uses a finite number of amplitudes, each assigned a unique pattern of...
, but at faster speeds such as 9600 bits/s, signal levels become critical and they are extremely sensitive to group delay
Group delay
Group delay is a measure of the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component...
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 bits/s.
Custom modems have been developed which allow throughput rates of 19.2 kbits/s, 56 kbits/s, and even 1.2 Mbits/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 1200 bit/s operation. The adoption of these high speed links has been limited.
In commercial data radio applications, the intermediate audio stage is not used. Data is transmitted by directly altering output frequency between two distinct frequencies (in the case of FSK modulation, other alternates exist). Such radio modem chips are now available, but as they are incompatible with existing Amateur Packet Radio technology, they have not been adopted into amateur radio use.
High-speed multimedia radio
One notable detail is the 2.4 GHz WLAN band partially overlaps an amateur radio band, Thus WLAN hardware can readily be used by licensed amateur radio operators at higher power levels than the "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.25AX.25
AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks....
data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
protocol, derived from the X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
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 systemBulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
. 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 & TheNET, ROSE, FlexNet and TexNet.
In principle, any network layer protocol may be used, including the ubiquitous Internet protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
.
See also
- AMPRNetAMPRNetThe AMPRNet is a name used by amateur radio operators for computer networks connected over amateur radio. Other names for the network include IPv4 Network 44/8 and Network 44...
- Automatic Packet Reporting System
- D-STARD-STARD-STAR is a digital voice and data protocol specification developed as the result of research by the Japan Amateur Radio League to investigate digital technologies for amateur radio...
- List of packet radio nodes
- Tucson Amateur Packet RadioTucson Amateur Packet RadioTAPR is an international amateur radio organization. It was founded in Tucson, Arizona, in 1981 by a group of amateurs interested in developing a terminal node controller for amateur use. Thus, the group was named Tucson Amateur Packet Radio, Inc. After developing one of the first widely...
- WinlinkWinlinkWinlink, also known as the Winlink 2000 Network, is a worldwide radio messaging system that mixes internet technology and appropriate amateur radio radio frequency technologies. The system provides radio interconnection services including: email with attachments, position reporting, graphic and...
- Spartan Packet Radio ExperimentSpartan Packet Radio ExperimentThe Spartan Packet Radio Experiment was an Amateur Radio communications experiment that flew on the Space Shuttle Endeavor's STS-72 mission as part of NASA's Spartan/OAST-Flyer spacecraft in January 1996...
- An experiment intended to test the tracking of satellites via amateur packet radio, flown on Space ShuttleSpace ShuttleThe Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
mission STS-72STS-72STS-72 was a Space Shuttle Endeavour mission to capture and return to Earth a Japanese microgravity research spacecraft known as Space Flyer Unit...
.
External links
- AX.25 Link Access Protocol for Amateur Packet Radio: the official specification, from Tucson Amateur Packet RadioTucson Amateur Packet RadioTAPR is an international amateur radio organization. It was founded in Tucson, Arizona, in 1981 by a group of amateurs interested in developing a terminal node controller for amateur use. Thus, the group was named Tucson Amateur Packet Radio, Inc. After developing one of the first widely...
- "Packet Radio Physical Layer" Useful notes taken by N1VG during the development of the OpenTracker TNC/APRS node
- Pacific Packet Radio Society
- Fast Packet Systems
- Ham Radio India Digital FAQ
- San Francisco Packet Radio Net
- 44.0.0.0
- AMRAD
