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X.25 is an international data communications protocol.
Literally, X.25 defines the lower three layers of the OSI
data communications stack. The X.25 packet layer is roughly equivalent to the
OSI Network layer. The X.25 frame layer uses LAP-B. This layer is connection-
oriented. Its mechanics are similar to a telephone call, with call-setup, data,
and call-disconnect phases. The physical layer defined by the standard is X.21, a 15-pin
plug that did not gain wide acceptance. X.21 bis was introduced,
which is electrically equivalent to RS-232-C.
In the U.S., X.25 is implemented either through RS-232
for low-speed connections, or V.35 for high-speed connections.
X.25 was designed during times of noisy phone lines and speeds
of up to 64Kbits. This shows in its overhead. Its primary
features are its immunity to noisy lines and its world-wide acceptance.
People think X.25 is old and obsolete. I generally agree with this, but
I'm sure that many countries still use it for packet (data) communications,
and I have a feeling that most Automatic Teller Machines still use it.
The features that are built into the protocol are tailor-made for
the transaction environment that ATM machines (and Point-Of-Sale terminals)
operate in. It's not a sexy, media-hyped protocol, but is still useful.