Nortel Networks NN43001-563 - Manual (Page 73)

Quality of Service 73
Traceroute
Traceroute uses the IP Time-To-Live (TTL) field to forward router hops to
a specific IP address. A router must not forward an IP packet with a TTL
field of 0 or 1. It must, instead, discard the packet and return an ICMP
"time exceeded" message to the originating IP address. Traceroute uses
this mechanism by sending an IP datagram with a TTL of 1 to the specified
destination host. The first router to handle the datagram returns a "time
exceeded" message. This identifies the first router on the route. Traceroute
sends out a datagram with a TTL of 2. This causes the second router on the
route to return a "time exceeded" message, and so on, until all hops have
been identified. The Traceroute IP datagram has a port number unlikely to
be in use at the destination (usually >30,000). This causes the destination
to return a "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
IP Trunk 3.01 (and later) 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. IP Trunk 3.01 (and later)
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 Trunk 3.01 (and later) network end-to-end latency consists of several
components: routing delay on the IP Trunk 3.01 (and later) network, frame
duration delay and jitter buffer delay on the codec, and delay on the
circuit-switched network. The determination of end-to-end delay depends
on the dynamics of the IP Trunk 3.01 (and later) 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 IP Trunk 3.01 (and later) node QoS values in TM 3.1.
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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