Question

14. Network latency is the sum of four delays: propagation time, transmission time, queuing time, and processing delay. Identify where each of these delays occur and explain what causes the delay for a datagram sent between two hosts over an Ethernet network.

Answer 1

Propagation time - Propagation time or propagation delay is a measure of the time required for a signal to propagate from one end of the circuit to the other. It takes place at the Physical layer.

Excessive propagation delay occurs when the cable is too long.

Transmission time - Transmission time is the amount of time it takes one bit to go from the start of the link to its destination.​​​​​​​ In a network based on packet switching, transmission delay (or store-and-forward delay, also known as packetization delay) is the amount of time required to push all the packet's bits into the wire. It takes place at the Physical layer.

This is the delay caused by the data-rate of the link.​​​​​​​

Queuing time - The time data spends queued inside the buffer is known as queuing time or queuing delay. Each packet traveling over the network will incur many instances of each of these delays. This occurs at the transport layer.

Queuing delay may be caused by delays at the originating switch, intermediate switches, or the call receiver servicing switch. In an Ethernet network, queuing delay is the sum of the delays between the request for service and the establishment of a circuit to the called data terminal equipment.

Processing delay - This is the CPU cycles needed to look at the packet headers and decide what to do with the packet, and process the packet. This occurs at the network layer.

During processing of a packet, routers may check for bit-level errors in the packet that occurred during transmission as well as determining where the packet's next destination is. This contributes to processing delay.