User guide

EDA System Services
50 1/1551-HSC 901 35/3 Uen C 2005-12-02
6.2 Quality of Service
Quality of Service (QoS) of a network deals with the ability of the network to
provide transport services suitable for the applications using the network.
There are three main parameters that affect the quality of the transmission:
• Packet loss – packets of information that get lost in the network and do
not reach the receiver
• Delay – the time it takes for a packet to traverse through the network
from the sender to the receiver
• Jitter – variation in the delay within the same traffic flow
Table 2 illustrates different requirements that different services have:
Table 2 QoS Requirements of different Service Types
Packet loss Delay Jitter
Video
Very sensitive (Not) sensitive Not sensitive
Voice
Not sensitive Sensitive Very sensitive
Data
Not sensitive Not sensitive Not sensitive
The Very sensitive adjectives in Table 2 indicate which parameter will
cause the perceived quality of the application using the transport to
deteriorate. Not sensitive means that as long as the deterioration is
reasonable (there is no definite definition for limits), the perceived quality
will not be changed.
Video applications – As long as the video is only streaming video, only the
packet loss is important for the perceived quality. Delay will only affect the
time elapse when changing a channel. However, if the video service is
interactive video, the delay becomes more important, and should be kept at
a time that is acceptable as response time.
Voice applications – Such applications like telephony, are very sensitive,
especially to delay and jitter. The jitter is especially important since voice
packets may arrive in the wrong order if they have different delay length.
Buffering the packets can help this, but the buffer will then increase the
delay. Loss of packets can be handled by a telephony application without
substantial service deterioration, provided that the loss is limited to a small
fraction of the packets, and that the packets are not lost in bursts.
Data applications – Such as Internet surfing, using the TCP/IP protocol
stack, are generally not very sensitive, neither to packet loss nor delay.