Nortel Networks 8000 - Manual (Page 218)

Nortel Secure Router 8000 Series
Configuration -WAN Access 7 LAPB and X.25 configuration
Setting X.25 VC range
• VC Management
The X.25 protocol can multiplex multiple number of logical virtual connections over a
physically existing link between DTE and DCE. The virtual connections are called VC or
logic channel (LC).
The virtual connections established by X.25 reach a maximum of 4095, and their numbers
range from 1 to 4095. LC Identifier (LCI) or VC Number (VCN) differentiates each VC (or
logic channel).
CQ NOTE
VC and LC are two different concepts. However, at user end, they are generally not strictly
differentiated.
An important part of X.25 operation is to manage the total 4095 VCs. This sequence is a
range of VC channel numbers broken into four ranges (listed here in numerically ascending
order):
- A-Permanent VCs (PVCs) range
- B-Incoming-only channel range
- C-Two-way channel range
- D-Outgoing-only channel range
You must establish the numbers of the VCs by an X.25 call to be set to one of B, C and D.
The permanent VCs must be set in the A range.
• Idle Channel Allocation Rules
According to ITU-T Recommendation X.25, the idle channel allocation rules on call
initiation are as follows:
- Only the DCE can use a call in the incoming-only channel range.
- Only the DTE can use a call in the outgoing-only channel range.
- Both the DCE and the DTE can use a call in the two-way channel range.
- DCE always uses the lowest available LC.
- DTE always uses the highest available LC.
You can prevent a situation where one side of the communication occupies all the channels,
and minimize the possibility of call collision.
• Channel Division
In the X.25 protocol, six parameters are employed to define the four ranges, as shown in
Figure 7-7.
Issue 5.3 (30 March 2009) Nortel Networks Inc. 7-15