7.2 Configuring X.25
7.2.5 Configuring X.25 VC ranges
The Table summarizes additional configuration tasks for VC assignment. The complete range of VCs is allocated to PVCs or SVCs, depending on your requirements. SVCs are commonly used.

Decode the abbreviations used in the command column of the Table, as follows:

i incoming
t two-way
o outgoing
l low
h high
c circuit

The circuit numbers must be assigned so that an incoming range comes before a two-way range, both of which come before an outgoing range. Any PVCs must take a circuit number that comes before any SVC range. The following numbering scheme lists the proper order for these VC assignment commands:

1 <= PVCs < (lic <= hic) < (ltc <= htc) < (loc <= hoc) <= 4095

(Where lic is a low incoming circuit number, hic is a high incoming circuit number, ltc is a low two-way circuit number, htc is a high two-way circuit number, loc is a low outgoing circuit number, and hoc is a high outgoing circuit number.)

If both limits of a range are zero, the range is unused.

X.25 ignores any events on a VC number that are not in an assigned VC range; it considers the out-of-range VC as a protocol error. The network administrator specifies the VC ranges for an X.25 attachment. For correct operation, the X.25 DTE and DCE devices must have identically configured ranges. Numbers configured for any PVCs must also agree on both sides of an attachment (not necessarily end to end).

The following example may help you understand why you would configure VC ranges. If you acquire ten circuits from your SVC provider and use all ten on a given situation, incoming calls cannot get through. You can partition the ten circuits to be dedicated to different calling situations-incoming, outgoing, or either:

  1. Incoming
  2. Incoming
  3. Incoming
  4. Either
  5. Either
  6. Either
  7. Either
  8. Outgoing
  9. Outgoing
  10. Outgoing

If many incoming calls are coming in, some circuits are still available for outgoing calls.