Nortel ISDN Signaling Link, Описание, установка и эксплуатация

Страница: 56 / 380

Page 56 of 378 Description
553-3001-202 Standard 1.00 April 2000
“port unreachable” ICMP packet which identifies the destination 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 similar 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 rating, R, for the
network transmission quality. The ITG uses a simplified version of the 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 latency
IP network end-to-end latency consists of several components: routing delay
on the IP network, frame duration delay and Jitter Buffer delay on codec, and
delay on the circuit-switched network. The determination of end-to-end delay
depends on the dynamics of the IP network and the detailed service
specification.
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 ITG node QoS values in MAT.
For instance, assume the end-to-end network latency is 300 milliseconds 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 minimum of 4.3. If the latency
in the IP network increases, the configured QoS is not met and Fallback to
alternate facilities occurs.