Quality of Service 73
Packet InterNet Groper (PING)
PacketInterNet Groper (PING) sends an Internet Control Message Protocol
(ICMP) echo request message to a host, expecting an ICMP echo reply.
This allows the measurement of the round-trip time to a selected host. By
sending repeated ICMP echo request messages, the percentage of packet
loss for a route can be measured.
Traceroute
Tracerouteuses 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 Trunk3.01 (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 voicequality. When MOS is equal to
5.0, voice quality is good. When MOS is equal to 0.0, voice quality is bad.
For packetloss 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 Trunk3.01 (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 bufferdelay 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.
Nortel Communication Server 1000
IP TrunkFundamentals
NN43001-563 02.01 Standard
Release 5.5 21December 2007
Copyright© 2007, Nor tel Networks
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