In simple terms, QoS refers to providing data with the type of transport
service that it needs. For example, in bank transactions, 100 percent
reliability is what matters most. For file transfers, sufficient bandwidth is
more important than low delay. And for voice, delay and variation in the delay
are most important. There are four factors that contribute to QoS:
- End-to-end delay
- Reliability
- Variation in the delay from packet to packet, or jitter
- Bandwidth
These factors are shown and described as they apply to VoIP in Figure
.
QoS is
implemented by classifying data, often by setting bits in the IP header called
either the Type of Service (TOS) bits or the Differentiated Services Code Point
(DSCP). Then the different classes of data are treated differently. Certain
classes can be given priority over other classes. Delay can be minimized
through various queuing techniques. A minimum bandwidth during congestion can
be guaranteed to some classes.