Nortel Networks NN43001-563 Fallback to alternate facilities

74 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 Trunk3.01 (and later) node QoS values in TM 3.1.

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 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 Trunk3.01 (and later) continuously monitors and analyzes QoS data.

When IP Trunk3.01 (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.

A key background activity of IP Trunk3.01 (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 Trunk3.01 (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).

Nortel Communication Server 1000

IP TrunkFundamentals

NN43001-563 02.01 Standard

Release 5.5 21 December 2007