X.25

X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks WAN. It was originally defined by the International Telegraph in addition to Telephone Consultative Committee CCITT, now ITU-T in a series of drafts in addition to finalized in a publication call as The Orange Book in 1976.

This allowed it one of the oldest packet-switching communication protocols available; it was developed several years previously IPv4 1981 and the OSI consultation Model 1984. The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI model. It also maintained functionality not found in the OSI network layer.

Networks using X.25 were popular during the gradual 1970s and 1980s with telecommunications companies and in financial transaction systems such(a) as automated teller machines. An X.25 WAN consists of packet-switching exchange PSE nodes as the networking hardware, and leased lines, plain old telephone service connections, or ISDN connections as physical links. However, most users realize moved to Internet Protocol IP systems instead. X.25 was used up to 2015 e.g. by the character card payment industry and is still used by aviation, purchasable from telecoms companies. X.25 was also available in niche application such as Retronet that allow vintage computers to usage the Internet.

Billing

In public networks, X.25 was typically billed as a flat monthly service fee depending on connection speed, and then a price-per-segment on top of this. connective speeds varied, typically from 2400 bit/s up to 2 Mbit/s, although speeds above 64 kbit/s were uncommon in the public networks. A segment was 64 bytes of data rounded up, with no carry-over between packets, charged to the caller or callee in the issue of reverse charged calls, where supported. Calls invoking the Fast Select facility allowing 128 bytes of data in call request, call confirmation and call clearing phases would loosely attract an extra charge, as might use of some of the other X.25 facilities. PVCs would take a monthly rental charge and a lower price-per-segment than VCs, devloping them cheaper only where large volumes of data are passed.