Delay in packet switched network

Circuit switching In Circuit switched networks the resources needed along a path (buffers and link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end systems.

Packet switched networks In packet switched networks resources (buffers and transmission rate) are not reserved, a session’s message use the resources on demand, and as a consequences, may have to wait for access to a communication link.

Delay Packet start in a host (source), passes through a series of routers and ends its journey in another host (destination), during this period packet suffers from several types of delays which are 1.Nodal processing delay 2.Queuing delay 3.Transmission delay 4.Propagation delay 5.Total nodal delay

Processing Delay The time require to examine the packet's header and determine where to direct the packet is part of the processing delay. Processing delay also include other factors such as the time needed to check for bit level errors in the packet that occurred in the transmitting the packets bits from the upstream node to the Router at down end.

Queuing delay At the queue, the packet experience a queuing delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on a number of earlier- arriving packets that are queued and waiting for transmission across the link. 0 packets in the queue then queuing delay will be zero increasing number of packets in a queue will increase this delay.

Queuing delay and traffic intensity Suppose ‘a’ is the average rate of packet arrival ‘L’ is the length of the packet (i.e. number of bits) Then Traffic intensity is La/R If La/R>1 then the average rate at which bits arrives at the queue exceeds the rate at which bits can be transmitted from the queue. If La/R≤1 then every packet will arrive at an empty queue and there will be no queuing delay.

Transmission delay The transmission delay (also called store and forward delay) is the amount of time required to transmit (push) all the packets bits onto the link. For example “L” is the number of bits in a packet, “R” is a transmission rate then transmission delay is L/R.

Propagation Delay The time required to propagate from the beginning of the link to its destination. Bit propagates at the speed of the link. Its range is from 2*10 8 meter/sec to 3*10 8 Propagation delay is the distance between the two routers divided by the speed.

Total nodal delay Total nodal delay is the sum of all the delays. d nodal =d proc +d queue +d trans +d prop