Quality of Service (QoS) in networks :
A stream of packets from a source to destination is called a flow. Quality of Service is defined as something a flow seeks to attain. In connection oriented network, all the packets belonging to a flow follow the same order. In a connectionless network, all the packets may follow different routes.
The needs of each flow can be characterized by four primary parameters :
- Reliability, Lack of reliability means losing a packet or acknowledgement which entertains retransmission.
- Delay, Increase in delay means destination will find the packet later than expected, Importance of delay changes according to the various application.
- Jitter, Variation of the delay is jitter, If the delay is not at a constant rate, it may result in poor quality.
- Bandwidth, Increase in bandwidth means increase in the amount of data which can be transferred in given amount of time, Importance of bandwidth also varies according to various application.
Application Reliability Delay Jitter Bandwidth High Low Low Low File transfer High Low Low Medium Web access High Medium Low Medium Remote login High Medium Medium Low Audio on demand Low Low High Medium Video on demand Low Low High High Telephony Low High High Low Videoconferencing Low High High High
Techniques for achieving good Quality of Service :
- Overprovisioning –
The logic of overprovisioning is to provide greater router capacity, buffer space and bandwidth. It is an expensive technique as the resources are costly. Eg: Telephone System.
- Buffering –
Flows can be buffered on the receiving side before being delivered. It will not affect reliability or bandwidth, but helps to smooth out jitter. This technique can be used at uniform intervals.
- Traffic Shaping –
It is defined as about regulating the average rate of data transmission. It smooths the traffic on server side other than client side. When a connection is set up, the user machine and subnet agree on a certain traffic pattern for that circuit called as Service Level Agreement. It reduces congestion and thus helps the carrier to deliver the packets in the agreed pattern.
Andrew S. Tanenbaum, 4rth ed, Computer Networks. Pearson Education.