Packet radio Guide, Meaning , Facts, Information and Description
Packet Radio (PR in the following) is a data transmission mode used in Amateur Radio, which is based on protocols similar to those on the internet, in particular AX.25, which is a subform of the X.25 protocol.Version 2.0 and earlier of the specification had limited packet lengths, and for connected virtual circuits, could only transmit eight packets before having to wait for an acknowledgement. These limitations made sense in the context of the noisy, bandwidth-limited channels encountered in earlier packet radio implementations. While good for slower links, however, it was inefficient for faster channels. Today's AX.25, however, supports up to 128 outstanding packets before an acknowledgement is necessary, and packets with up to 8192 bytes of payload data. This makes AX.25 suitable for use on multi-megabit per second networks.
AX.25 version 2.0 and later also supports raw datagram mode of operation too. While using virtual circuits for slower links (e.g., 5 Mbit/s or slower) makes more efficient use of bandwidth and improves end-to-end throughput, the 128 packet window is a limiting factor on faster networks. Therefore, one can use AX.25 entirely in datagram mode as well, thus allowing AX.25 to rival Frame Relay and Ethernet in performance. Support for 8192 byte payloads makes AX.25 suitable for use over multi-gigabit per second networks as well.
See also the latest specification for the AX.25 protocol (as of this writing). Note well that AX.25 is not a toy protocol. AX.25 can and has been adopted for use in non-amateur radio communications channels and applications as well.
AX.25 is a layer-2 protocol, fulfilling the same basic purpose as Ethernet, Frame Relay, or ATM networking. In the past, amateur radio operators didn't have much need for data communications beyond ordinary BBS use, and so traditional applications have tended to rely on using AX.25 directly as a reliable data transport to connect with BBSes. No further protocols were needed, since links were always point-to-point in nature, and always involved plain text.
Today, many layer-3 protocols, such as IP, ROSE, TheNET, and many more, exist for AX.25 specializing in the needs of amateur radio. IP packets in particular are regularly routed on AX.25 networks, providing a routable layer on top of AX.25. This, therefore, allows for TCP/IP, UDP/IP, and ICMP/IP networking, including but not limited to HTTP, FTP, etc.
Each packet radio station consists of at least one transceiver with antenna and a modem. Modems range from 1200 bit/s to several megabits per second in throughputs, with 1200 and 9600 bit/s speeds being the most common.
In more simple setups, a TNC (Terminal Node Controller) is also present. TNCs serve the same function as X.25 Packet Assembler Disassemblers, but often have enhanced functionality, such as integrated e-mail and miniature BBSes built right into them. These packet stations also have a terminal (which may be emulated by a computer), to allow operator interaction with the TNC, but automatic packet radio stations do not need a terminal. Today however one often finds packet stations where the modem is directly interfaced with the computer which has its own protocol implementation instead of relying on the TNC.
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