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DEFINITY ECS Release 8.2
Administrator’s Guide
555-233-506
Issue 1
April 2000
Managing data calls
431Wideband Switching
15
Glare and blocking
Glare prevention
Glare occurs when both sides of an ISDN interface select the same B-channel for
call initiation. For example, a user side of an interface selects the B-channel for an
outgoing call and, before the switch receives and processes the SETUP message,
the switch selects the same B-channel for call origination. Since any single
wideband call uses more channels, the chances of glare are greater. With proper
and careful administration, glare conditions can be reduced.
To reduce glare probability, the network needs to be administered so both sides of
the interface select channels from opposite ends of facilities. This is called linear
hunting, ascending or descending. For example, on a 23B+D trunk group, the user
side could be administered to select B-channels starting at channel 23 while the
network side would be administered to start selecting at channel 1. Using the same
example, if channel 22 is active but channel 23 is idle, the user side should select
channel 23 for re-use.
Blocking prevention
Blocking occurs when insufficient B-channels required to make a call are
available. Narrowband calls require only one channel so blocking is less likely
than with wideband calls which require multiple B-channels. Blocking also occurs
for wideband calls when bandwidth is not available in the appropriate format (that
is, fixed, floating, or flexible).
To reduce blocking, the switch selects trunks for both wideband and narrowband
calls to maximize availability of idle fixed channels for H0, H11, and H12 calls
and idle floating channels for N x DS0 calls that require a contiguous bandwidth.
The strategy for preserving idle channels to minimize blocking depends on the
channel type. The chances for blocking are reduced if you use a flexible
algorithm, assuming it is supported on the other end.
Channel Type Blocking Minimization Strategy
H0 Preserve idle quadrants
H11 Preserve idle facilities
H12 Preserve idle facilities
Flexible NxDS0 Preserve idle facilities
Floating NxDS0 Preserve idle facilities as first priority