X.25

The X.25 protocol is an ITU (originally CCITT) recommendation that defines connections of  terminals and computers to packet-switching networks. Packet-switching networks route packets of data through a network of switches to destination nodes. In the case of X.25, packets of data up to 128 bytes in size are inserted in HDLC frames, addressed, and forwarded to the intended destination across the X.25 network. Because the service is packet-oriented, many users can share the service simultaneously. It provides any-to-any connections for                       

 simultaneous users. X.25 is a well-established protocol, but is now somewhat dated. Traditionally, X.25 has been used in place of dial-up or leased-line circuits as a way to set up links to remote offices or remote users.

X.400 Message-Handling System

The ITU (formerly CCITT) defined the X.400 MHS standard, an electronic system for exchanging messages among store-and-forward mail systems. In ISO terminology, X.400 is  called MOTIS (Message-Oriented Text Interchange System). The goal of the standard is to

provide compatibility among multi-vendor products and interfaces as well as public and private message services.X.400 was first introduced in 1984 and has been through several enhancements. It outlines the protocols, procedures, components, terminology, and testing methods required to build interoperable e-mail systems. X.400 is based on a distributed client/server model.

xDSL

See DSL (Digital Subscriber Line).