Transmission Delay - Propagation Delay

Transmission Delay - Propagation Delay

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. Indicate the length of the packet by L bits, and indicate the transmission rate of the link from router A to router B by R bits/sec. For instance, 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 (also called the store-and-forward delay, as discussed in "The Network Core") is L/R. This is the amount of time needed to push (that is, transmit) all of the packet's bits into the link. Transmission delays are normally on the order of microseconds to milliseconds in practice.

Propagation Delay

Once a bit is pushed into the link, it needs to propagate to router B. The time needed 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 distance 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 earlier bits of the packet are stored in router B. The entire process then continues with router B now performing the forwarding. In wide-area networks, propagation delays are on the order of milliseconds.


transmission delay, propagation delay, routers

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