Page 56 of 378 Description
553-3001-202 Standard 1.00 April 2000
“port unreachable” ICMP packet which identifies the destina tion host.
Traceroute can be used to measure round-trip times to all hops along a route,
identifying bottlenecks in the network.
E-Model
The ITG uses the E-Model, a method simi lar to the ITU-T Recommendation
G.107, to determine voice quality. This model evaluates the end-to-end
network transmission performance and outputs a scalar rat ing, R, for the
network transmission quality. The ITG uses a simplified version of t he model
to correlate the network QoS to the subjective Mean Opinion Score (MOS).
MOS is a numerical scale used to rate voice quality. When MOS is equal to
5.0, voice quality is good. When MOS is equal to 0.0, voice quality is bad.
For packet loss over 16%, the MOS value is set to 0, and the remote node is
considered to be in fallback mode.
End-to-end latenc
y
IP network end-to-end latency consists of several components: routing del ay
on the IP network, frame duration de la y and Ji tt er Buffer de lay o n c odec, an d
delay on the circuit-switched ne twork. The dete rminatio n of end-to -end dela y
depends on the dynamics of the IP network and the detailed service
specification.
MOS values are calculated based on the routing de lay and fr ame 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 ITG node QoS values in MAT.
For instance, assume the end-to-end network l atenc y is 30 0 mill iseconds and
the part of that latency which the IP network can contribute is 180 ms.
Furthermore, assume the network has low packet loss. Using the G.711
codec, this means the configured QoS can be a m inimum of 4.3. If the laten cy
in the IP network increases, the configured QoS is not met and Fallback to
alternate facilitie s occurs.