Fallback To Alternate Facilities - Nortel 1000 Description, Installation And Operation Manual

Page 102 of 644

Fallback to alternate facilities

553-3001-363 Standard 2.00
System description
MOS values are calculated based on the routing delay and frame duration and
jitter buffer delay on the codec. These latencies must be taken into
consideration during the engineering of the total network's latency. If the
end-to-end latency of the network is specified and the latency of the PSTN
circuit-switched components is removed, the remainder is the latency
available for the IP trunks. This latency value plays a large role when
configuring IP Trunk 3.01 (and later) node QoS values in OTM.
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.
Note: For information on QoS engineering guidelines, refer to "ITG
engineering guidelines" on
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.
September 2004
page 119.