5-38 Call control procedures
297-2521-106 Standard 23.01 March 2001
B-channel glare
Glare is the simultaneous seizure of a trunk at both ends. Glare occurs when
the DMS-250 switch tries to select an idle PRI trunk member for termination
just as a SETUP message is being received from the PBX requesting the
same trunk member. When this happens, the following rules apply:
• If during processing of a termination, the switch detects an origination,
the origination is either denied or allowed to proceed. This is based on
the BCHGLARE parameter (can be datafilled) in table TRKSGRP. If the
BCHGLARE parameter indicates that the DMS-250 switch should
YIELD, the switch selects another member to complete the call.
The DMS-250 switch network provides the ability to datafill a PRI
interface so that it yields to glare conditions. However, yielding to glare
conditions is discouraged because it conflicts with the Q.931 standard
that states the user always yields to the DMS-250 switch network.
• If the switch attempts to route to a B-channel that has already begun to
process an origination, the switch does not terminate to the B-channel
regardless of BCHGLARE datafill. The terminating call selects another
member to complete the call.
Response to datafill
If yielding or standing is based on BCHGLARE (BCHGLARE = STAND),
the following applies:
• If BCHGLARE is datafilled as STAND, the DMS-250 switch continues
processing on the terminating call.
• The switch responds to the SETUP message with a REL COM message
containing CSE value 34, no circuit/channel available (this indicates that
the requested B-channel is not available).
• The REL COM message is followed by a SETUP message that contains
the call reference for the terminating call, as shown in Figure 5-21.
To Next Page