Chapter 5 Network Design Considerations - NEC Univerge SV9100 Manual

5-2
of all the delays at the different network devices and across the network links
through which voice traffic passes. Many factors contribute to end-to-end delay,
which are covered next.
The buffering, queuing, and switching or routing delay of IP routers primarily
determines IP network delay. Specifically, IP network delay is comprised of the
following:
Packet Capture Delay

Packet capture delay is the time required to receive the entire packet before
processing and forwarding it through the router. This delay is determined by the
packet length and transmission speed. Using short packets over high-speed
networks can easily shorten the delay but potentially decrease network
efficiency.
Switching/Routing Delay

Switching/routing delay is the time the router takes to switch the packet. This
time is needed to analyze the packet header, check the routing table, and route
the packet to the output port. This delay depends on the architecture of the
switches/routers and the size of the routing table.
Queuing Time

Due to the statistical multiplexing nature of IP networks and to the
asynchronous nature of packet arrivals, some queuing, thus delay, is required at
the input and output ports of a packet switch. This delay is a function of the
traffic load on a packet switch, the length of the packets and the statistical
distribution over the ports. Designing very large router and link capacities can
reduce but not completely eliminate this delay.
Jitter
Delay variation is the difference in delay exhibited by different packets that are
part of the same traffic flow. High frequency delay variation is known as jitter.
Jitter is caused primarily by differences in queue wait times for consecutive
packets in a flow, and is the most significant issue for QoS. Certain traffic types,
especially real-time traffic such as voice, are very intolerant of jitter. Differences
in packet arrival times cause choppiness in the voice.
All transport systems exhibit some jitter. As long as jitter falls within defined
tolerances, it does not impact service quality. Excessive jitter can be overcome
by buffering, but this increases delay, which can cause other problems. With
intelligent discard mechanisms, IP telephony/VoIP systems try to synchronize a
communication flow by selective packet discard, in an effort to avoid the
walkie-talkie phenomenon caused when two sides of a conversation have
significant latency. UNIVERGE SV9100 incorporates a Jitter Buffer to avoid
these problems.
Packet Loss
During a voice transmission, loss of multiple bits or packets of stream may cause
an audible pop that can become annoying to the user. In a data transmission,
loss of a single bit or multiple packets of information is almost never noticed by
Issue 2.0
Network Design Considerations