Packet radio is a mode of communications that will link Amateur Radio stations together directly or by means of a network. In contrast to the case with the packet-switched networks, the key issue is transmission capacity allocation, and has the following characteristics:
An effort to standardize these networks has been underway since 1982, under the sponsorship of the American Radio Relay League [Karn et al 85] and [Diersiong \& Ward 89], which has produced a standard for a link level protocol suitable for packet-radio networks known as AX.25 [WB4JFI 84].
An ARRL-type network is a distributed network, organized into clusters of
stations connected by repeaters. All stations and repeaters share a single
frequency for transmission and reception. The AX.25 standard does not
specify the frequency to be used, but a typical network uses the 220-MHz band,
using FSK and with a bandwidth of 20 kHz or 100 kHz. A typical data rate is
4800 bps, and a fixed routing scheme is used. In this case, the route to be
followed is specified by the source station.
AX.25 Link-Layer Protocol is designed to be used between two Amateur Radio stations in a point-to-point communications environment. The Link Protocol is based on, and very close to, HDLC [Black 93]. The following subsections explains the AX.25 frame structure very briefly.