Specifications

8-1
Chapter 8
Propagation Delay
From the preceding, Chapter 7, Ethernet Network Design , we have seen how it is possible to build a
maximum size network using each of the available media. When designing any Ethernet network, it is
wise to calculate the maximum round trip propagation delay for the proposed design. The maximum
allowable round-trip propagation delay of 51.2 µs, governed by the minimum frame size of 64 bytes, is
a very important consideration when it comes to accurate collision detection and data integrity.
Everything that lies in the signal path will contribute to the overall propagation delay. The items that add
delay are transceivers, repeaters, active hubs, passive hubs and cables. Bridges, which effectively
reset propagation delay, are not considered in the calculation of propagation delay. They will be
discussed in Chapter 9, Propagation Delay .
Calculating the Delay
As you design your network, you should sketch the overall topology to
determine rough equipment locations, cable segment loading and
maximum repeater hops. Calculating the overall round trip network
propagation delay is an easy task that should only take a few minutes. The
most difficult part, by far, is finding the proper delay times for individual
pieces of hardware. These numbers should be obtainable from the original
hardware manufacturer.
The total propagation delay is found by simply adding up the delays of
the individual components and cabling in the longest signal path and
multiplying the result by 2 to get the round trip delay time. The longest
signal path is found by identifying the longest path between any two
network devices, even though the two devices may not directly
communicate with each other. Examples of network devices that could be
considered an end point on the network would be PCs, workstations, file
servers, bridges, routers, gateways, and printers.