X.25

X.25 is a protocol that defines packets switched networking for exchanging data over connection-oriented service. X.25 also defines the control of information that is passed between a user device, called the DTE (Data Terminal Equipment) and another network workstation, called a DCE (Data Communications Equipment). X.25 and LAPB layers put great emphasis on flow control and error checking these functions are very nice to have on slow unreliable analog connection. But in the new world with high-speed reliable digital service these functions are a drawback. Each X.25 link consists of a DTE at one end and a DCE at the other end.

When configuring a Cisco router with X.25 there are several commands you need to remember.

encapsulation x25 dte or dce x25 address x.121 –_address x25 map protocol address x.121-address

X.25 has three layers that correspond to the bottom three layers of the OSI model.

X.25 (Layer 3) also called the packet level, creates packets which contain both control information and user data. This level also includes procedures for creating and maintaining two permanent associations between two DTE’s