Overview

Recall that a packet starts in a host, passes through a series of routers, and ends its journey in another host. As a packet travels from one node to the subsequent node along this path, the packet suffers from several types of delays at each node along the path. The most important of these delays are the nodal processing delay, queuing delay, transmission delay, and propagation delay; together, these delays accumulate to give a total nodal delay. The performance of many Internet applications are greatly affected by network delays. In order to acquire a deep understanding of packet switching and computer networks, we must understand the nature and importance of these delays.

Types of Delay

1. Processing Delay : The time required to examine the packet’s header and determine where to direct the packet is part of the processing delay. The processing delay can also include other factors, such as the time needed to check for bit-level errors in the packet that occured in transmitting the packet’s bits from the upstream node to the router A. Processing delays in high-speed routers are typically on the order of microseconds or less. After this nodal processing, the router directs the packet to the queue that precedes the link to router B.
2. Queuing Delay : At the queu, the packet experiences a queuing delay as it waits to be transmited onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet’s queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Queuing delays can be on the order of microseconds to milliseconds in practice.
3. Transmission Delay : Assuming that packets are transmitted in a first-come-first-served manner, as is common in packet-switched networks, our packet can be transmitted only after all the packets that have arrived before it have been transmitted. Denote the length of the packet by L bits, and denote the transmission rate if the link from router A to router B by R bits/sec. For example, for a 10 Mbps Ethernet link, the rate is R = 10 Mbps, for a 100 Mbps Ethernet link, the rate is R = 100 Mbps. The transmission delay is L/R. This is the amount of time required to push(that is, transmità all of the packet’s bits into the link. Transmission delays are typically on the order of microseconds to milliseconds in practice.
4. Propagation Delay : Once a bit is pushed into the link, it needs to propagate to router B. The time required to propagate from the beginning of the link to router B is the propagation delay. The bit propagates at the propagation speed of the link. The propagation speed depends on the physical medium of the link (that is, fiber optics, twisted-pair copper wire, and so on) and is in the range of 2.10⁸ meters/sec to 3.10⁸ meters/sec which is equal to, or a little less than,the speed of light. The propagation delay is the distance between two routers divided by the propagation speed. That is the propagation delay is d/s, where d is the distancce between router A and router B and s is the propagation speed of the link. Once the last bit of the packet propagates to node B, it and all the preceding bits of the packet are stored in router B. The whole process then continues with router B now performing the forwarding. In wide-area networks, propagation delays are on the order of milliseconds.

Comparing Transmission and Propagation Delay

Newcomers to the field of computer networking sometimes have difficulty understanding the difference between transmission delay and propagation delay. The difference is subtle but important. The transmission delay is the amount of time required for the router to push out the packet; it is a function of the packet’s length and the transmission rate of the link, but has nothing to do with the distance between the two routers. The propagation delay, on the other hand, is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers, but has nothing to do with the packet’s length or the transmission rate of the link. An analogy might clarify the notions of transmission and propagation dela. Consider a highway that has a tollbooth every 100 kilometers,