• QoS is an overall performance measure of the computer network.

• QoS provides guarantees on the ability of a network to deliver predictable results.

• QoS is a concept used by International Standards Organization (ISO) that specifies how “GOOD” networking services are.

• QoS is especially important for the new generation of Internet Applications such as VOIP, Video- on-demand etc.

• A stream of packets from source to destination is called a flow.

Important flow characteristics of the QoS are given below :

• Reliability

If a packet gets lost or acknowledgement is not received (at sender), the re-transmission of data will be needed. This decreases the reliability. The importance of the reliability can differ according to the application.

For example: E- mail and file transfer need to have a reliable transmission as compared to that of an audio conferencing.

• Delay

Delay of a message from source to destination is a very important characteristic. However, delay can be tolerated differently by the different applications.

For example: The time delay cannot be tolerated in audio conferencing (needs a minimum time delay), while the time delay in the e-mail or file transfer has less importance.

• Jitter

The jitter is the variation in the packet delay. If the difference between delays is large, then it is called as high jitter. On the contrary, if the difference between delays is small, it is known as low jitter. Real time audio and video applications cannot tolerate high jitter.

Example: Case1: If 3 packets are sent at times 0, 1, 2 and received at 10, 11, 12. Here, the delay is same for all packets and it is acceptable for the telephonic conversation. Case2: If 3 packets 0, 1, 2 are sent and received at 31, 34, 39, so the delay is different for all packets. In this case, the time delay is not acceptable for the telephonic conversation.

• Bandwidth

Bandwidth is measured by bits per second. Different applications need the different bandwidth.

For example: Video conferencing needs more bandwidth in comparison to that of sending an e-mail.

• Throughput

Amount of data transferred from one place to another or processed in a specified amount of time. Data transfer rate for networks are measured in terms of throughput (in terms of Kbps, Mbps).