3Com MSR 50 Series Configuration Manual page 287

X.25, and their numbers range from 1 to 4095. The number used to differentiate
each virtual circuit (or logic channel) is called Logic Channel Identifier (LCI) or
Virtual Circuit Number (VCN).
n
Strictly speaking, VC and LC are different. However, at the user end, they are
generally not distinguished strictly.
An important part of X.25 operation is how to manage the total 4,095 virtual
circuits. All the virtual circuit numbers are divided into four ranges (listed here in
ascending order):
A-Permanent virtual circuits (PVCs) range
■
B-Incoming-only channel range
■
C-Two-way channel range
■
D-Outgoing-only channel range
■
The numbers of the virtual circuits established by an X.25 call must be set in the
ranges of B, C and D. The permanent virtual circuits must be set in the A range.
According to ITU-T Recommendation X.25, the idle channel allocation rules in
initiating calls are as follows:
Only the DCE can initiate a call using a channel in the incoming-only channel
■
range.
Only the DTE can initiate a call using a channel in the outgoing-only channel
■
range.
Both the DCE and the DTE can initiate a call using a channel in the two-way
■
channel range.
DCE always uses the lowest available logic channel.
■
DTE always uses the highest available logic channel.
■
Thus, we can avoid the case that one side of the communication occupies all the
channels, and minimize the possibility of call collision.
In X.25 protocol, six parameters are employed to define the four ranges, as shown
in the following figure.
Figure 60 X.25 channel delimitation
1
LIC
HIC
LTC
HTC
LOC
HOC
4095
PVC range
Incoming-only
channel range
unused
Two-way
channel range
Unused
Outgoing-only
channel range
Unused
Configuring X.25 287