*Comparing Transmission and Propagation Delay*

Newcomers to the field of computer networking often have difficulty understanding the difference between transmission delay and propagation delay. The difference is slight but important. The transmission delay is the amount of time needed 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 concepts of transmission and propagation delay. Consider a highway that has a tollbooth every 100 kilometers, as shown in the following figure. 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/hour (that is, when a car leaves a tollbooth, it immediately accelerates to 100 km/hour 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 services (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 at 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 needed for the tollbooth to push the entire caravan onto the highway is (10 cars)/(5 cars/minute) = 2 minutes. This time is similar 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/hour) = 1 hour. This time is similar 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 instance, suppose now that the cars travel at the rate of 1,000 km/hour and the tollbooth services cars at the rate of one car per minute. Then the traveling delay between two tollbooths 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 d

_{proc}, d

_{queue}, d

_{trans}, and d

_{prop}denote the processing, queuing, transmission, and propagation delays, then the total nodal delay is given by

d

_{nodal}= d

_{proc}+ d

_{queue}+ d

_{trans}+ d

_{prop}

The contribution of these delay elements can vary considerably. For instance, d

_{prop}can be insignificant (for example, a couple of microseconds) for a link connecting two routers on the same university campus; however, d

_{prop}is hundreds of milliseconds for two routers interconnected by a geostationary satellite link, and can be the dominant term in d

_{nodal}. Similarly, d

_{trans}can range from insignificant to significant. Its contribution is normally insignificant for transmission rates of 10 Mbps and higher (for instance, for LANs); however, it can be hundreds of milliseconds for large Internet packets sent over low-speed dial-up modem links. The processing delay, d

_{proc}is often insignificant; however, it strongly affects a routers maximum throughput, which is the maximum rate at which a router can forward packets.