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3.11] What is propagation delay? (Ethernet Physical Layer)




Description

This article is from the Ethernet FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.

3.11] What is propagation delay? (Ethernet Physical Layer)

The propagation speed of a medium refers to the speed that the data
travels through that medium. Propagation delays differ between
mediums, which affect the maximum possible length of the Ethernet
topology running on that medium.

In the following table, c refers to the speed of light in a vacuum,
or 300,000 kilometers per second.

        Medium        Propagation Speed
        ------        -----------------
        Thick Coax    .77c (231,000 km/sec)
        Thin Coax     .65c (195,000 km/sec)
        Twisted Pair  .59c (177,000 km/sec)
        Fiber         .66c (198,000 km/sec)
        AUI Cable     .65c (195,000 km/sec)

From these values, the size of a bit on 10BaseT can be calculated.
10BaseT is twisted pair, which has a propagation delay of 177,000
km/sec. 177,000 km/sec divided by 10 million bits per second is
17.7 meters, or the size of a single bit on a 10BaseT network.

The maximum propagation delay through the network can be calculated
by dividing the maximum length by the speed. For 10Base2 thin coax
network, this is 185 meters divided by 195,000 km/sec, or 950
nanoseconds. If the actual propagation delay from one end of the
network to the other is greater than 950 nanoseconds, late
collisions may occur. See section [5.4] for more information on
late collisions.

 

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