Packet Radio is simply digital communications
over radio waves. There are many digital communication modes for radio, packet
being one of the less sophisticated methods. Packet modems (Terminal Node
Controllers or TNC) essentially do the same function as a phone modem,
modulating/de-modulating a signal. Packet radio is mostly operated in simplex,
that is, one transmits at a time. There are duplex packet modems that function
very similarly to phone modems.
Simplex - One way communication only. There is only one transmission and no reply expected.
Half-Duplex - Two way communications, however only one transmission at a time.
Full-duplex - Two way simultaneous communication. Both stations can talk and receive at the same time. Such as Telephones.
Packet modems are A LOT more expensive than phone modems, a 56kbs phone modem could cost £50 and a 1200bps packet modem could cost £150 or more.
communications also have drastically reduced throughput due to the stop-start
transmission style because both can't talk to each other at the same time.
Packet radio provides similar facilities to the Bulletin Board & the internet such as electronic mail and menu driven information retrieval and file transfer facilities. Packet radio email networks also span the world, and packet radio email addresses have a similar form to an internet email address. Packet radio operates on a network protocol called AX.25 (An 'Amateur' version of the X.25 protocol).
What can be done on Packet Radio?
Like other digital communications modes, packet radio can be used to talk to other amateurs. For those who cannot use HF frequencies, 2 amateurs can talk to each other from long distances using the packet radio network.
Packet BBS operations:
Many cities have a packet Bulletin Board System (BBS) attached to their local packet network. Amateurs can check into the BBS's and read messages from other packet users on almost any topic. BBS's are networked together over the packet network to allow messages to reach a broader audience than your local BBS users. Private messages may also be sent to other packet operators, either locally or who use other BBSs. BBS's have the latest ARRL, AMSAT, and propagation bulletins. Many BBS's have a file section containing various text files full of information on amateur radio in general.
DX Packet Cluster:
This is where Packet radio is used for 'DX spotting'. HF operators connect to the local DX Packet Cluster for the latest reports on DX. Often a user will 'spot' some hot DX and distribute the DX report real time.
With special software, amateurs can pass any binary files to other amateurs. Currently, this is done with TCP/IP communications, YAPP, and other specialized protocols.
Many of the amateur radio satellites contain microcomputer systems that can provide special information to amateurs. Some satellites contain CCD cameras on board and you can download images of the earth and the stars. Others provide store and forward packet mailboxes to allow rapid message transfers over long distances. Some satellites use AX.25, some use special packet protocols developed for satellite communications. A few transmit AX.25 packets over FM transmitters, but most use SSB transmissions.
What is AX.25 ?
AX.25 (Amateur X.25) is the communications protocol used for packet radio. A protocol is a standard for how two computer systems are to communicate with each other, somewhat analogous to using business format when writing a business letter. AX.25 was developed in the 1970's and based of the wired network protocol X.25. Because of the difference in the transport medium (radios vs wires) and because of different addressing schemes, X.25 was modified to suit amateur radio's needs. AX.25 includes a digipeater field to allow other stations to automatically repeat packets to extend the range of transmitters. One advantage of AX.25 is that every packet sent contains the senders and recipients amateur radio callsign, thus providing station identification with every transmission.
AX.25 specifies channel access (ability to transmit on the channel) to be handled by CSMA (Carrier Sense Multiple Access) If you need to transmit, your TNC monitors the channel to see if someone else is transmitting. If no one else is transmitting, then the radio keys up and the TNC sends it's packet. All the other stations hear the packet and do not transmit until you are done. Unfortunately, 2 stations could accidentally transmit at the same time. This is called a collision. If a collision occurs, neither TNC will receive a reply back from the last packet it sent. Each TNC will wait a random amount of time and then retransmit the packet. CSMA/CD (The CD = Collision Detection) is an access method used on Ethernet networks, and Appletalk networks empty the use of CSMA/CA (The CA = Collision Avoidance)
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