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 f 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 delay. Consider a highway that has a tollbooth every 100 kilometers, as shown in fig. 1.17. You can think of the highway segments between tollbooths as links and the tollbooths as routers. Suppose that cars travel (that is , propagate) on the highway at a rate of 100 km/hr (that is, when a car leaves a tollbooth, it instantaneously accelerates to 100 km/hr and maintains that speed between tollbooths). Suppose next that 10 cars, travelling together as a caravan, follow each other in a fixed order. You can think of each car as a bit and the caravan as a packet. Also suppose that each tollbooth service (that is, transmits) a car at a rate of one car per 12 seconds, and that it is late at night so that the caravan’s cars are the only cars on the highway. Finally, suppose that whenever the first car of the caravan arrives on a tollbooth, it waits at the entrance until the other nine cars have arrived and lined up behind it. (Thus the entire caravan must be stored at the tollbooth before it can begin to be forwarded). The time required for the tollbooth to push the entire caravan onto the highway is (10 cars)/(5 cars/minute) = 2 minutes. This time is analogous to the transmission delay in a router. The time required for a car to travel from the exit of one tollbooth to the next tollbooth is 100 km/(100 km/her) = 1 hr. This time is analogous to propagation delay. Therefore, the time from when the caravan is stored in front of a tollbooth until the caravan is stored in front of the next tollbooth is the sum of transmission delay and propagation delay – in this example , 62 minutes.
Let’s explore this analogy a bit more. What would happen if the tollbooth service time for a caravan were greater than the time for a car to travel between tollbooths? For example, suppose now that the cars travel at the rate of 1,000 km/hr and tollbooth services cars at the rate of one car per minute. Then the travelling delay between two tollbooth is 6 minutes and the time to serve a caravan is 10 minutes. In this case, the first few cars in the caravan will arrive at the second tollbooth before the last cars in the caravan leave the first tollbooth. This situation also arises in packet-switched networks – the first bits in a packet can arrive at a router while many of the remaining bits in the packet are still waiting to be transmitted by the preceding router.
If we let dproc , dqueuq , dtran , and dprop denote the processing, queuing, transmission, and propagation delays, then the nodal delay is given by
dnodal = dproc + dqueuq + dtran + dprop
The contribution of these delay components can vary significantly. For example, dprop can be negligible (for example , a couple of microseconds) for a link connecting two routers on the same university campus; however, dprop is hundreds of milliseconds for two routers interconnected by a geostationary satellite link, and can be the dominant term in dnodal . Similarly, dtrans can range from negligible to significant. Its contribution is typically negligible for transmission rates of 10 Mbps and higher (for example, for LANs); however , it can be hundreds of milliseconds for large packets sent over low-speed dial-up modem links. The processing delay, dproc , is often negligible ; however, it strongly influences a router’s maximum throughput, which is the maximum rate at which a router can forward packets.