
54 Chapter 2 System telephony networking overview
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Figure 6 Network call redirection path
ISDN Call Connection Limitation
The ICCL (ISDN Call Connection Limitation) feature piggybacks on the call initiation request and
acts as a check at transit PBX points to prevent misconfigured routes or calls with errors from
blocking channels. Also refer to “ISDN overview” on page 709.
This feature adds a transit/tandem counter to a call setup message. This counter is compared at
each transit PBX with a value programmed into the transit PBX, in a range from 0 to 31. If the call
setup counter is higher than the PBX value, the call will be blocked at the PBX system and cleared
back to the network. This prevents calls from creating loops that tie up lines.
Figure 7 demonstrates how a call might loop through a network if the system is not set up
with ICCL.
Figure 7 Call loop on system without ICCL
BCM
Meridian 1
with Meridian Mail
SL-1 MCDN
Telephone A
Telephone B
Call forwarded to Meridian Mail
Original call
BCM
Meridian 1
Telephone A
BCM
BCM
Meridian 1
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