digital radio Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services. Types In digital broadcasting syst ...

, packet radio is the application of

packet switching In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...

techniques to digital radio communications. Packet radio uses a

protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technolog ...

as opposed to

circuit switching Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full b ...

message switching In telecommunications, message switching involves messages routed in their entirety, one hop at a time. It evolved from circuit switching and was the precursor of packet switching. History Western Union operated a message switching system, Plan ...

protocols to transmit digital data via a

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...

communication link A data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a transmitter and a recei ...

. 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 Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum ( radio waves), in a one-to-many model. Broadcasting began ...

) * A packet may be stored and subsequently forwarded towards its destination by a network

node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines, ...

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

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

. Packet radio is frequently used by

amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...

operators. The

AX.25 AX.25 (Amateur X.25) is a data link layer protocol originally derived from layer 2 of the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks. AX.25 v2.0 and later occupies t ...

(Amateur X.25) protocol was derived from the

X.25 X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts ...

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

protocol and adapted for amateur radio use. Every AX.25 packet includes the sender's amateur radio callsign, which satisfies the US FCC requirements for amateur radio station identification. AX.25 allows other stations to automatically repeat packets to extend the range of transmissions. It is possible for any packet station to act as a

digipeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...

, linking distant stations with each other through

ad hoc network An ad hoc network refers to technologies that allow network communications on an ad hoc basis. Associated technologies include: *Wireless ad hoc network *Mobile ad hoc network * Vehicular ad hoc network ** Intelligent vehicular ad hoc network * Prot ...

s. This makes packet radio especially useful for emergency communications. Packet radio can be used in

mobile communications Mobile telephony is the provision of telephone services to phones which may move around freely rather than stay fixed in one location. Telephony is supposed to specifically point to a voice-only service or connection, though sometimes the l ...

. Some mobile packet radio stations transmit their location periodically using the

Automatic Packet Reporting System Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time digital communications of information of immediate value in the local area. Data can include object Global Positioning System (GPS) coordinates, weather st ...

(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

Improved Layer 2 Protocol IL2P (Improved Layer 2 Protocol) is a data link layer protocol originally derived from layer 2 of the X.25 protocol suite and designed for use by amateur radio operators. It is used exclusively on amateur packet radio networks. IL2P occupies the ...

(IL2P) supporting

forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...

for noisy and weak signal links.

Timeline

Earlier

communications modes were

telegraphy Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...

(using

Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...

teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initia ...

(using

Baudot code The Baudot code is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII ...

) and

facsimile A facsimile (from Latin ''fac simile'', "to make alike") is a copy or reproduction of an old book, manuscript, map, Old master print, art print, or other item of historical value that is as true to the original source as possible. It differs from ...

Aloha and PRNET

Since radio circuits inherently possess a

network topology Network topology is the arrangement of the elements ( links, nodes, etc.) of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, including command and contro ...

(i.e., many or all

nodes In general, a node is a localized swelling (a "knot") or a point of intersection (a Vertex (graph theory), vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two ...

are connected to the

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

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 A communication 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 in telecommunications and computer networking. A channel is used for informa ...

to avoid collisions of signals. Professor

Norman Abramson Norman Manuel Abramson (April 1, 1932biography
from

University of Hawaii A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, th ...

led development of a packet radio network known as

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 packe ...

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 ''Aloha'' ( , ) is the Hawaiian language, Hawaiian word for love, affection, peace, compassion and mercy, that is commonly used as a simple greeting but has a deeper cultural and spiritual significance to native Hawaiians, for whom the term is u ...

multiple access protocol was derived. Subsequent enhancements in channel access techniques made by

Leonard Kleinrock Leonard Kleinrock (born June 13, 1934) is an American computer scientist and a long-tenured professor at UCLA's Henry Samueli School of Engineering and Applied Science. In the early 1960s, Kleinrock pioneered the application of queueing theory ...

''et al.'' in 1975 would lead

Robert Metcalfe Robert Melancton Metcalfe (born April 7, 1946) is an engineer and entrepreneur from the United States who helped pioneer the Internet starting in 1970. He co-invented Ethernet, co-founded 3Com and formulated Metcalfe's law, which describes the e ...

to use

carrier-sense multiple access Carrier-sense multiple access (CSMA) is a medium access control (MAC) 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 s ...

(CSMA) protocols in the design of the now commonplace

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a larger ...

(LAN) technology. Over 1973–76,

DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adv ...

created a packet radio network called

PRNET The Packet Radio Network (PRNET) was a set of early, experimental mobile ad hoc networks whose technologies evolved over time. It was funded by the Advanced Research Projects Agency (ARPA). Major participants in the project included BBN Technol ...

in the

San Francisco Bay San Francisco Bay is a large tidal estuary in the U.S. state of California, and gives its name to the San Francisco Bay Area. It is dominated by the big cities of San Francisco, San Jose, and Oakland. San Francisco Bay drains water from a ...

area and conducted a series of experiments with

SRI Shri (; , ) is a Sanskrit term denoting resplendence, wealth and prosperity, primarily used as an honorific. The word is widely used in South and Southeast Asian languages such as Marathi, Malay (including Indonesian and Malaysian), Javanes ...

to verify the use of

ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical fou ...

(a precursor to the

)

communications protocols A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

(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 In telecommunication and radio communication, spread-spectrum techniques are methods by which a signal (e.g., an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency dom ...

(DSSS) modulation and forward error correction ( 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 Internetworking is the practice of interconnecting multiple computer networks, such that any pair of hosts in the connected networks can exchange messages irrespective of their hardware-level networking technology. The resulting system of interc ...

, as in these experiments data was routed between the ARPANET, PRNET, and

SATNET SATNET, also known as the Atlantic Packet Satellite Network, was an early satellite network that formed an initial segment of the Internet. It was implemented by BBN Technologies under the direction of the Advanced Research Projects Agency. T ...

(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 Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...

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 Montreal ( ; officially Montréal, ) is the List of the largest municipalities in Canada by population, second-most populous city in Canada and List of towns in Quebec, most populous city in the Provinces and territories of Canada, Canadian ...

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

began experimenting with transmitting

ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...

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 ( ) is a major city in western Canada, located in the Lower Mainland region of British Columbia. As the List of cities in British Columbia, most populous city in the province, the 2021 Canadian census recorded 662,248 people in the ...

, 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 ''CQ Amateur Radio'' (also known simply as ''CQ'' or ''CQ magazine'', and formerly as ''CQ: The Radio Amateur's Journal'') is a magazine for amateur radio enthusiasts first published in 1945. The English language edition is read worldwide; Spani ...

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 The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdiction ...

(FCC) granted authorization for United States amateurs to transmit ASCII codes via amateur radio. See ''FCC Gives The Nod'' and ''Making Modifications'' 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 San Francisco (; Spanish language, Spanish for "Francis of Assisi, Saint Francis"), officially the City and County of San Francisco, is the commercial, financial, and cultural center of Northern California. The city proper is the List of Ca ...

during December 1980, when a packet repeater was put into operation on 2 meters by

Hank Magnuski Henry "Hank" Stanley Magnuski is an American engineer and was the co-founder and CEO of GammaLink, an early pioneer in PC-to-fax technology. He also founded Internet Video Services, a video service provider; MediaMart, an electronic commerce site ...

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 The AMPRNet (AMateur Packet Radio Network) or Network 44 is used in amateur radio for packet radio and digital communications between computer networks managed by amateur radio operators. Like other amateur radio frequency allocations, an IP r ...

in DARPA style. Magnuski obtained

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

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 California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

, the Tucson Amateur Packet Radio Corporation (TAPR) in

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

and the Amateur Radio Research and Development Corporation (AMRAD) in

Washington, D.C. ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...

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 is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

and by 1984 the first packet-based

bulletin board system A bulletin board system (BBS), also called computer bulletin board service (CBBS), is a computer server running software that allows users to connect to the system using a terminal program. Once logged in, the user can perform functions such as ...

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 Cerritos (Spanish for "Little hills") 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. As of 20 ...

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".

Commercial systems

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 Mobitex 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. It was developed in t ...

(1986) * ARDIS (1990) *

CDPD Cellular Digital Packet Data (CDPD) was a wide-area mobile data service which used unused bandwidth normally used by Advanced Mobile Phone System (AMPS) mobile phones between 800 and 900 MHz to transfer data. Speeds up to 19.2 kbit/s were ...

allowed packet data to be carried over AMPS analog cellular telephone networks * GPRS is the packet data facility provided by the

GSM The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...

cellular telephone network

Technical details

Voice not data

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 Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...

modems that followed telephone standards (notably the

Bell 202 The Bell 202 modem was an early (1976) modem standard developed by the Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second, half-duplex. It has separate sets of circuits fo ...

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 FSK may refer to: * FSK (band), a German band * Federal Counterintelligence Service, (Russian ') of Russia * Fiskerton railway station, in England * Forskolin, a diterpene * Forsvarets Spesialkommando, a Norwegian special forces unit * Fort Scott M ...

modulation like G3RUH's packet radio modem, a 9,600 baud transmission is easily made in the same channel. In addition, the

baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into a ...

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 Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...

data transfer. At the time, most personal computers had asynchronous

RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such a ...

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 A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...

. 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

computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...

or dumb terminal, a

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

, and a

transceiver In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. The ...

with an

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

. Traditionally, the computer and modem are combined in one unit, the

terminal node controller A terminal node controller (TNC) 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 ...

(TNC), with a

dumb terminal A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. The teletype was an example of an early-day hard-copy terminal a ...

(or

terminal emulator A terminal emulator, or terminal application, is a computer program that emulates a video terminal within some other display architecture. Though typically synonymous with a shell or text terminal, the term ''terminal'' covers all remote termin ...

) used to input and display data. Increasingly,

personal computers A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or techn ...

are taking over the functions of the TNC, with the modem either a standalone unit or implemented entirely in

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...

. Alternatively, multiple manufacturers (including Kenwood and Alinco) now market handheld or mobile radios with built-in TNCs, allowing connection directly to the

serial port In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. ...

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

to accept messages while the operator is away. Following the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

, 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 (

) within the radio equipment's existing speech bandwidth. The first amateur packet radio stations were constructed using surplus

1,200

bit/s In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...

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, although more technically demanding, alternative. At HF frequencies,

Bell 103 The Bell 103 modem or Bell 103 dataset was the second commercial modem for computers, released by AT&T Corporation in 1962. It allowed digital data to be transmitted over regular unconditioned telephone lines at a speed of 300 bits per second. ...

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 bits/s, include using telephone modem chips via the microphone and audio out connectors. This has been proven to work at speeds up to 4,800 bit/s using fax V.27 modems in half-duplex mode. These modems use

phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...

which works fine when there is no

amplitude-shift keying Amplitude-shift keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In an ASK system, a symbol, representing one or more bits, is sent by transmitting a fixed-amplitude car ...

, but at faster speeds such as 9,600 bit/s, signal levels become critical and they are extremely sensitive to

group delay In signal processing, group delay and phase delay are delay times experienced by a signal's various frequency components when the signal passes through a system that is linear time-invariant (LTI), such as a microphone, coaxial cable, amplifie ...

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).

High-speed multimedia radio

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.

Data link layer: AX.25

Packet radio networks rely on the

protocol, derived from the

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.)AX.25 Link Access Protocol for Amateur Packet Radio
the official specification, from

Tucson Amateur Packet Radio TAPR 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 (TNC) for amateur use. Thus, the group was named Tucson Amateur Packet Ra ...

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

. 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 Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronics, electronic (digital media, digital) version of, or counterpart to, 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 The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. IP h ...

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

AMRAD

TARPN
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