Quality of Service

The Internet was originally designed for best-effort service without guarantee of predictable performance. Best-effort service is often sufficient for a traffic that is not sensitive to delay, such as file transfers and e-mail. Such a traffic is called elastic because it can stretch to work under delay conditions; it is also called available bit rate because applications can speed up or slow down according to the available bit rate.

The real-time traffic generated by some multimedia applications is delay sensitive and therefore requires guaranteed and predictable performance. Quality of service (QoS) is an internetworking issue that refers to a set of techniques and mechanisms that guarantee the performance of the network to deliver predictable service to an application program.

DATA-FLOW CHARACTERISTICS

Traditionally, four types of characteristics are attributed to a flow: reliability, delay, jitter, and bandwidth. To provide quality of server (QoS)

Reliability
Reliability is a characteristic that a flow needs in order to deliver the packets safe and sound to the destination. Lack of reliability means losing a packet or acknowledgment, which entails retransmission. However, the sensitivity of different application programs to reliability varies. For example, reliable transmission is more important for electronic mail, file transfer, and Internet access than for telephony or audio conferencing.
Delay
Source-to-destination delay is another flow characteristic. Again, applications can tolerate delay in different degrees. In this case, telephony, audio conferencing, video conferencing, and remote logging need minimum delay, while delay in file transfer or e-mail is less important.
Jitter
Jitter is the variation in delay for packets belonging to the same flow. For example, if four packets depart at times 0, 1, 2, 3 and arrive at 20, 21, 22, 23, all have the same delay, 20 units of time. On the other hand, if the above four packets arrive at 21, 23, 24, and 28, they will have different delays. For applications such as audio and video, the first case is completely acceptable; the second case is not. For these applications, it does not matter if the packets arrive with a short or long delay as long as the delay is the same for all packets. These types of applications do not tolerate jitter.

Bandwidth
Different applications need different bandwidths. In video conferencing we need to send millions of bits per second to refresh a color screen while the total number of bits in an e-mail may not reach even a million.