VoIP and Voice over WLANs
VoIP and Quality of Service (QoS)

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.