Nortel Networks 8000 - Manual (Page 208)

Nortel Secure Router 8000 Series
Configuration -WAN Access 7 LAPB and X.25 configuration
The link set up between DTE and DCE by X.25 layer 2 (LAPB) is multiplexed by X.25 layer
3, and several usable VCs are the final output to users.
Figure 7-3 shows the relation between packets and frames in various X.25 layers.
Figure 7-3 X.25 packet and LAPB frame
X.25 Layer3
X.25 Layer2
X.25 Layer1
Packet
Head
UserData
f ''
FB Head Data FCS FB
BitFlow
7.1.3 LAPB
X.25 link layer protocol
The Link Access Procedure Balanced (LAPB), at layer 2 of X.25, defines the format and
procedure of interactive frames between DTE and DCE.
X.25 link layer specifies the frame switching process between DTE and DCE.
In terms of hierarchy, the link layer bridges the packet layer interface of DTE and that of DCE.
Through this bridge, you can transmit the packets continuously between the packet layer of
DTE and that of DCE.
The following are the major functions of the link layer:
Transmits the data effectively between DTE and DCE
Ensures the synchronization of information between the receiver and transmitter
Detects and corrects the error in the transmission
Identifies and reports the procedure error to the higher layer protocol
Informs the packet layer of the link layer state
LAPB application
As specified in international standards, X.25 link layer protocol LAPB adopts the frame
structure of High-Level Data Link Control (HDLC) and LAPB is a subset of HDLC.
You set up the bi-directional link when either site sends an SABM (Set Asynchronous
Balanced Mode) command and the other replies with UA response frame.
Defined as the X.25 layer-2 protocol, LAPB is actually a separate link layer protocol, which
can transmit data with LAPB bearing non-X.25 upper layer protocol.
With the SR8000, you can set up the link layer protocol on a serial interface as LAPB to
implement simple local data transmission.
Issue 5.3 (30 March 2009) Nortel Networks Inc. 7-5