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Packet radio

From iA wiki

Packet radio is a form of digital data transmission used in amateur radio to construct wireless computer networks. Its name is a reference to the use of packet switching between network nodes, which allows multiple virtual circuits to coexist on a single radio channel. Packet radio networks use the AX.25 data link layer protocol, derived from the X.25 protocol suite and designed for amateur radio use.

Note that while the term "packet radio" also was used for early version of mobile ad hoc networks, this technology has little in common with the description below.

1 Station configuration
2 Layers
3 External links
4 See also

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Station configuration

A basic packet radio station consists of a computer, 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.

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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.

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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 radio equipment is one using Frequency modulation to transmit narrowish speech bandwidth. The first amateur packet radio stations were constructed using surplus Bell 202 1,200 bit/s modems, and despite its low data rate, Bell 202 modulation has remained the standard for VHF operation in most areas. More recently, 9,600 bit/s has become a popular alternative. At HF frequencies, Bell 103 modulation is used, at a rate of 300 bit/s.

Due to historical reasons, all commonly used modulations are based on an idea of minimal modification of the radio itself, usually just connecting transmit modulator output directly to the external microphone input and receiver demodulator input directly to the external speaker output with all distortions that are included in the audio amplifiers of the radio. Just adding a turn the transmitter on signal ("PTT") for the outputs, 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 sounds for each bit. (One tone for zero, other tone for one.)

Ways to achieve higher speeds than 1,200 bit/s, include using telephone modem chips via the microphone and audio out connectors. This has been proven to work at speeds up to 4800bps 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 9600bps, 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. (link to article) Other systems which involved small modification of the radio were developed by James Miller (G3RUH) and operated at 9600bps.

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. However, special radio equipment is needed to carry data at these speeds, and their adoption has been limited. In these special radios, the interface in between the "modem" and the "radio" is at intermediate frequency -part of the radio disregarding parts of the radio that were used for voice modulation production and detection.

One notable detail is that 2.4 GHz WLAN band is partially overlapping amateur radio band, and thus WLAN hardware can readily be used by amateur radio licensed people with higher power radio gear than what the general population "license free" usage allows. ("Free to receive by anybody", "transmit only in between radio amateurs", and "no encryption" -rules usually make these very unappealing to spend time on.) Regulation details differ around the world.

In commercial data radio uses the intermediate audio stage is dispended with, and bits are transmitted by directly altering output frequency in between two distrinct frequencies (in case of FSK modulation, other alternates do exist). Such radio modem chips are cheap in the -00s, but as they are incompatible with previous Amateur Packet Radio technologies, they have not been adopted into amateur radio use.

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Data link layer: AX.25

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

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Network layer

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

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

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

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