Configuring Lapb - 3Com MSR 50 Series Configuration Manual

Configuring LAPB

established between DTE and DCE by X.25 layer 2 (LAPB) is multiplexed by X.25
layer 3, and those finally presented to users are several usable virtual circuits.
The relation between packets and frames in the X.25 layers is shown in the
following diagram.
Figure 58 X.25 packet and LAPB frame
X.25 Layer 3
Packet
X.25 Layer 2
Frame Frame
delimiter header
Frame
X.25 Layer 1
X.25 link layer specifies the frame switching process between DTE and DCE. From
the perspective of layering, the link layer is just like a bridge interconnecting the
packet layer interface of DTE and that of DCE. Through this bridge, packets can be
transmitted continuously between the packet layer of DTE and that of DCE. The
link layer has the following main functions:
Transmit the data effectively between DTE and DCE
■
Ensure the synchronization of information between the receiver and sender
■
Detect and correct errors in the transmission
■
Identify and report the procedure error to the higher layer protocol
■
Inform the packet layer of the link layer state
■
As specified in international standards, the link layer protocol LAPB of X.25 adopts
the frame structure of High-level Data Link Control (HDLC) and is a subset of
HDLC. It requires for setting up a link by making use of the Set Asynchronous
Balanced Mode (SABM) command. A two-way link can be established after either
site sends an SABM command and the other replies with a UA response.
Although defined for X.25, as a separate link layer protocol, LAPB can directly
carry non-X.25 upper layer protocols for data transmission. You can set the link
layer protocol of serial interface as LAPB and transmit data locally. Meanwhile, the
X.25 implementation has switching function. Therefore, the device can be used as
a small-sized X.25 packet switch, thus protecting users' investment in X.25. The
following figure describes the relation between LAPB, X.25 and X.25 switching.
Figure 59 Relation between LAPB, X.25 and X.25 switching
IP
X.25
switching
X.25
LAPB
Follow these steps to configure LAPB:
Packet
User data
header
Data
Bit stream
Configuring LAPB 285
Frame
Frame
check
delimiter
sequence