1 introduction, 1 introduction -3, 1 x.25 – Nortel Networks 8000 User Manual
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Nortel Secure Router 8000 Series
Configuration -WAN Access
7 LAPB and X.25 configuration
7.1 Introduction
7.1.1 X.25
This section covers the following topics that you need to know before you configure X.25 and
LAPB:
X.25 is a procedure of point-to-point interworking between Data Terminal Equipment (DTE)
and Data Circuit-terminal Equipment (DCE). It defines three layers starting from physical
layer to packet layer.
In 1974, CCITT issued the first draft of X.25. The initial files were based on the experience
and recommendations of Telnet and Tymnet of USA and Datapac packet-switched networks
of Canada. It was further revised in 1976, 1978, 1980, and 1984 with many added optional
service functions and facilities.
DTE refers to the host or terminal on the user side and DCE refers to devices such as the
synchronous modem.
DTE and DCE are directly connected. DCE is connected with a port on a packet switch. The
connection is set up between the packet switches. Thus, the communication channel between
DTE and DTE is set up.
In an X.25 network, the relation between entities is shown in Figure 7-1.
Figure 7-1 X.25 network model
PSN
DTE: Data Terminal Equipment DCE: Data Circuit-terminating Equipment
PSE: Packet Switching Equipment PSN: Packet Switching Network
Issue 5.3 (30 March 2009)
Nortel Networks Inc.
7-3