Networking Protocols.

X. 25 Protocol

X.25 is a protocol used for packet switching across computer networks and it is the most common form of virtual circuit packet switching. It works at the data link, physical, and network layers (Layers 1 to 3) of the OSI (Open Systems Interconnection) model (http://compnetworking.about.com/cs/designosimodel/g/bldef_osi.htm). When it was created, it was based on an analog network infrastructure. X.25 provides error control as a utility within the network. Because X.25 packet-switching networks were for data only, it was not important to be able to tightly control delays or losses; the endpoints would resolve any such problems (Goleniewski, L. 2007). In X.25, small packets were desirable due to noise factor and they were 128 bytes or 256 bytes long and it can support speeds of 9.6 Kbps to 2 Mbps. When there was noise in the network, errors would populate therefore retransmissions were necessary. Data Terminal Equipment (DTE), Data Circuit-Terminating Equipment (DCE), and Packet Switching Exchange (PSE) are X.25 devices. X.25 protocol uses serial communications in either asynchronous or synchronous modes. There are several standards used for X.25 networks such as X.21bis which uses a 15-pin connector supporting up to 2 Mpbs; there is RS-232 (EIA/TIA-232) uses a 25-pin connector supporting up to 19.2 Kbps; RS-449 (EIA/TIA-449) uses a 37-pin connector supporting up to 64 Kbps; and V.35 uses a 34-pin connector supporting up to 2 Mbps.