The X.25 protocol is a packet-switching protocol. AX.25 is closely related, except it is used by Ham Radio operators to send signals over the public airwaves.

Packet-switching networks excel in redundant error checking, allowing low-grade public communications mediums to be used instead of expensive data lines and leased lines. The X.25 protocol is the link between the computer and the analog virtual circuit created using public data networks (including phone lines).

X.25 uses Data Terminal Equipment (DTE) to communicate with Data Communications Equipment (DCE).

DTE

Examples of DTE includes computers, dumb terminals, routers, bridges, hubs and data ports.

DCE

Examples of DCE include modems, switches, phone lines and terminal node controllers.

Basically, anything to do with the computer side of the data is classified as DTE, and anything that has to do with the communications between data equipment is DCE.

A Practical Example

In Ham Radio, radio operators use Terminal Node Controllers, or TNCs, to communicate over the airwaves. I have talked to people in Brazil on 10-meter packet radio. An example of DTE and DCE (as used by a Ham Radio operator) is shown below.


Computer               TNC          Radio
--------             --------      --------
|      |____________\|      |_____\|      |
| DTE  |            /| DCE  |     /| DCE  |
--------             --------      --------
  USA                                 \
                                      /
                              Radio   \
                               Waves  /
                                      \
 Japan                                /
--------             --------      --------
|      |/____________|      |/_____|      |
| DTE  |\            | DCE  |\     | DCE  |
--------             --------      --------
Computer               TNC          Radio

The data "Hello" is sent out of the serial port to the Terminal Node Controller. Using the AX.25 protocol installed inside the TNC, it is converted into packets to be transmitted by the radio. The analog signal travels over the air to a radio in Japan. His radio passes the analog audio over to his TNC, which converts the data using the AX.25 protocol into digital data, which is sent to his computer. On his screen, the message "Hello" appears.

The X.25 protocol set has been around since 1974. X.25 is not a very fast protocol, and is comparable with a 56.6 modem in speed. AX.25 communications for Ham Radio operators are limited to 1200baud in the United States. 9600baud communications is common in other countries.

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