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• The X.25 physical layer defines the physical and electrical specifications between DTE and
DCE.
• The X.25 data link layer defines the format and specifications of the frames exchanged between
DTE and DCE. Link Access Procedure, Balanced (LAPB) is the data link protocol of X.25.
• The X.25 packet layer defines the packet format and rules for Layer 3 packet switching.
The X.25 packet layer can multiplex multiple virtual circuits over the Layer 2 link between DTE and
DCE to provide separate point-to-point connections for subscribers.
Figure 122 sh
ows the data forms at different X.25 layers.
Figure 122 X.25 packet and LAPB frame
LAPB
LAPB is the data link layer protocol of X.25. It manages communication and packet framing between
DTE and DCE.
From the perspective of layering, the link layer is like a bridge, across which the packet layers of DTE
and DCE exchange packets continuously.
Use the link layer functions to perform the following tasks:
• Transmit data effectively between DTE and DCE.
• Ensure the synchronization of information between receiver and sender.
• Detect and correct errors during transmission.
• Identify and report procedural errors to the higher layer protocols.
• Inform the packet layer of the link layer state.
LAPB adopts the framing structure of HDLC and is a subset of HDLC. LAPB uses the SABM
command to initiate link setup. Two devices establish a two-way link after one of them sends an
SABM command and the other replies with an UA response.
Although initially defined for X.25, LAPB, as an independent link layer protocol, can directly carry
non-X.25 upper layer protocols. You can set the link layer protocol of a serial interface as LAPB for
local data transmission.
Virtual circuit
X.25 enables two DTEs to communicate across a telephone network. The following describes the
communication flow:
1. One DTE sends a connection request call to another DTE.
2. The called DTE accepts or rejects the request.
3. Once the connection is set up, the DTEs can transmit data in full duplex mode, and either end
can disconnect the connection at any time.
The connection established by X.25 between a pair of DTEs is called a virtual circuit. It provides a
virtual channel for communication between the two ends.
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