NEC UNIVERGE SV9100 Manual page 7

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
analyse 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.
2.3 Jitter (Delay Variation)?
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
will 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.
SV9100 incorporates a Jitter Buffer to avoid these problems.
2.4 Packet Loss?
IP is an unreliable protocol which means that in some circumstances packets of data can be
discarded (dropped) by the network. This usually occurs when the network is particularly busy.
Loss of multiple packets of a voice stream may cause an audible pop that will become annoying to
the user. To maintain voice quality, packet loss should not exceed around 1% of all packets.
Obviously this figure should be as close to 0% as possible.
2.5 CODEC Selection?
The CODEC used will affect the voice quality due to the different compression algorithms used, and
the amount of bandwidth required (see:
For example, on a low bandwidth WAN link, using a high bandwidth CODEC (such as G.711) may
Part 1: VoIP Reference Manual
mechanisms, IP telephony/VoIP systems
Bandwidth and CODECs).
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