Nortel Networks 8000 - Manual (Page 220)

Nortel Secure Router 8000 Series
Configuration -WAN Access 7 LAPB and X.25 configuration
The X.25 protocol requires DTE and DCE to have the same VC range parameter;
therefore, the new configuration cannot take effect immediately after successful X.25
negotiation. It is necessary to use the
shutdown and undo shutdown commands.
Setting X.25 packet numbering modulus
X.25 procedure requires DTE and DCE to have the same packet numbering mode.The
configuration takes effect when you run the shutdown and undo shutdown commands.
The packet sequence numbering mode of X.25 layer 3 is different from the frame sequence
numbering mode of LAPB (X.25 layer 2).
When you employ the modulo 128 numbering mode in the DTE or DCE interface with high
throughput rate for LAPB, you affect only the efficiency of the local DTE or DCE interface
by making an increase in point-to-point efficiency. For X.25 layer 3, you affect the end-to-end
by increasing the efficiency of communication between two sets DTEs.
Configuring the default flow control parameter
The X.25 protocol is a reliable transmission protocol with powerful flow control capability by
providing the parameters "window size" and "packet size".
Only correct configuration brings about effective and correct X.25 flow control. Any
improper configuration will result in such cases as "clearing" and "resetting". Most public
X.25 packet networks use the default window size and maximum packet size specified in the
ITU-T X.25 Recommendation, which is also true for the SR8000 making this task optional
without special requirements of service provider.
Only correct configuration brings about effective and correct X.25 flow control. Any
improper configuration results in cases such as "clearing" and "resetting". Most public X.25
packet networks, however, use the default window size and maximum packet size specified in
ITU-T X.25 Recommendation, which is also applicable for the SR8000. Therefore, this task
may be optional without special requirements of service providers.
After setting the default window size and maximum packet size:
• If related parameters are not negotiated in the call process, you can establish the SVCs
with call process that use these default values. The following section describes the
parameter negotiation during call process.
• If a window size or packet size option is not assigned, you can establish PVCs without
call process that use these default values. For the details about the PVC setting, refer to
the subsequent sections.
The X.25 transmitting end fragments the exceedingly long data packet of the upper layer
according to the maximum packet size, and marks the last fragment packet ( the M bit of the
last fragment packet is not set ). When the packet reaches the receiving end, X.25 reassembles
all these fragment packets, and judges whether a complete packet is received according to the
M bit marker. Therefore, a very small value of the maximum packet size consumes more
router resources, which reduces the efficiency.
Note the following points:
• Maximum packet size < MTU x 8 < LAPB N1.
• Due to the reasons mentioned above, new configuration does not take effect until the
shutdown and undo shutdown commands are run.
Issue 5.3 (30 March 2009) Nortel Networks Inc. 7-17