Nortel Networks NN43001-563 - Manual (Page 74)

74 System description
For instance, assume the end-to-end network latency is 300 milliseconds
(ms) and the part of that latency which the IP network contributes is 180 ms.
Furthermore, assume the network has low packet loss. Using the G.711
codec, this means the configured QoS can be a minimum of 4.3. If the
latency in the IP network increases, the configured QoS is not met and
fallback to alternate facilities occurs.
Equipment Impairment factor
Equipment Impairment factors are important parameters used for
transmission planning purposes. They are applicable for the E-Model.
For information on QoS engineering guidelines, refer to "ITG engineering
guidelines" (page 87) .
Fallback to alternate facilities
IP Trunk 3.01 (and later) continuously monitors and analyzes QoS data.
When IP Trunk 3.01 (and later) detects IP network congestion, and the
QoS is below a pre-defined value, new calls routed to the remote gateway
are rejected. Instead, the Meridian 1/CS 1000M routes them over non-IP
facilities. The Stepback on Congestion over ISDN feature provides fallback
to alternate facilities functionality.
Triggering fallback to alternate trunk facilities
A key background activity of IP Trunk 3.01 (and later) is to monitor the
network’s QoS between itself and each remote IP gateway configured in
the dialing plan. When the QoS is below the defined acceptable level for a
given IP Trunk 3.01 (and later) destination node, all outgoing calls from the
near-end Meridian 1/CS 1000M to the far end Leader are re-routed through
alternate circuit-switched trunk facilities. All calls that the switch is trying to
set up are re-rerouted; established calls cannot fall back.
The Meridian 1/CS 1000M provides alternate routing based on BARS or
NARS. BARS/NARS translates the dialed LOC, NPA, NXX, or Special
Number (SPN) into an entry on the Route List Block (RLB) and searches
the trunks in the associated Route Data Block (RDB).
The trigger for fallback to alternate trunk facilities is defined per call, per
customer. The local Active Leader makes the decision to use the fallback
feature. The selection of routes is based on the customer-configured
database. The customer must configure the alternate routing to the PSTN
in the Meridian 1/CS 1000M database.
The fallback to alternate facilities uses an ISDN DCH mechanism. The
Step Back on Congestion over ISDN feature provides fallback to alternate
trunk facilities functionality. When the Meridian 1/CS 1000M presents an
outgoing call and receives a release message back that indicates network
Nortel Communication Server 1000
IP Trunk Fundamentals
NN43001-563 01.01 Standard
Release 5.0 30 May 2007
Copyright © 2007, Nortel Networks
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