Introduction to Tandem X.25 Capabilities

Data-Link-Layer Components
What is X.25?
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Data-Link-Layer
Components
The Data Link Layer (layer 2) includes the procedures and data frame formats used to
establish the data-link-layer connection between the DTE and the DCE. The Data Link
Layer must set up the link, initiate the communication, and handle error-recovery.
At the originating or source location, the Data Link Layer receives packets from the
Packet Layer. It appends frame-header information to this packet to create a frame.
The frame is then transmitted across the data link to the destination DTE.
At the destination DTE, the framing process is reversed. The Data Link Layer at the
remote site strips the frame-header information and passes the packet up to the Packet
Layer. The Packet Layer strips off the packet header and passes any user data to the
upper layers (user application or other process) for further handling.
The Data Link Layer defines two classes of procedures that dictate the
communications protocol for transferring frames from one station to another station.
These procedures are the Link Access Procedure (LAP) and the Link Access Procedure
Balanced (LAPB). LAP and LAPB use the frame format and terminology of the High
Layer Data Link Control (HDLC) procedure specified by the International
Organization for Standardization (ISO).
LAP LAP was the original X.25 data link control procedure. However, it is not used much
and was not listed in the CCITT X.25 recommendation for 1988. (Tandem X25AM
does not support LAP as of software release C30.08.)
LAPB Later, LAPB was adopted and it is the procedure used most commonly today.
(Tandem X25AM supports only LAPB as of software release C30.08 .) LAPB is similar
to the Asynchronous Balanced Mode (ABM) of HDLC.
In LAPB, two computer systems are connected in a point-to-point configuration and
share equal control of the link. Either end of the link can set up the link, transmit
information, or disconnect the link. In this type of configuration, each computer
system or station is referred to as a combined station, because it can perform the
functions of both a primary and a secondary station.
When it is issuing commands to set up the link, to disconnect the link, or to transmit
data, the system is performing the function of a primary station. When the system is
responding to commands, it is performing the function of a secondary station.
In X.25, the data communications link is described in terms of the DTE and the DCE.
The address field identifies which side of the DTE/DCE link has issued the command
or response (see Figure 1-17).