16 CONFIGURING LAPB AND X.25
This chapter contains information on the following topics:
■X.25 and LAPB Protocols Overview
■Configure LAPB
■Configure X.25
■Configure X.25 over Other Protocols
■Display and Debug LAPB and X.25
■Typical LAPB Configuration Example
■Typical X.25 Configuration Example
■Fault Diagnosis and Troubleshooting of LAPB
■Fault Diagnosis and Troubleshooting of X.25
X.25 and LAPB
Protocols Overview X.25 protocol is the interface procedure between the data terminal equipment
(DTE) and data circuit-terminating equipment (DCE). In 1974, CCITT issued the
first draft of X.25, whose initial files were based on the experiences and
recommendations of Telenet and Tymnet of USA and Datapac packet-switched
networks of Canada. It was revised in 1976, 1978, 1980 and 1984, added many
optional service functions and facilities.
With X.25, two DTE can communicate with each other via the existing telephone
network. X.25 sessions are established when one DTE device contacts another to
request a communication session. The DTE device that receives the request can
either accept or refuse the connection. If the request is accepted, the two systems
begin full-duplex information transfer. Either DTE device can terminate the
connection. After the session is terminated, any further communication requires
the establishment of a new session.
X.25 is the protocol of point-to-point interaction between DTE and DCE. DTE
usually refers to the host or terminal at the user side, and DCE usually refers to the
synchronous modem. DTE is connected with DCE directly, DCE is connected to a
port of packet switching exchange, and some connections are established
between the packet switching exchanges, thus forming the paths between
different DTE. In an X.25 network, the relation between entities is shown in the
following diagram: