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Ethernet Design Cable Length

In designing Ethernet, we must make sure that the length of cable in the LAN is shorter than the length of the shortest possible message that can be sent. Otherwise, a collision could go undetected.

a. Let's assume that the smallest possible message is 64 bytes (including the 33 byte overhead). If we use 10BaseT, how long (in meters) is a 64-byte message? While electricity in the cable travels a bit slower than the speed of light, once you include delays in the electrical equipment in transmitting and receiving the signal, the effective speed is only about 40 million meters per second. (Hint: first calculate the number of seconds it would take to transmit the message then calculate the number of meters the signal would travel in that time, and you have the total length of the message).

b. If we use 10GbE, how long (in meters) is a 64-byte message?

c. The answer in part b is the maximum distance any single cable could run from a switch to one computer in a switched Ethernet LAN. How would you overcome the problem implied by this?

Solution Preview

Answer is as below and in the attached documents. The contents are same.

a. 10BaseT, so the transmission speed is 10Mbps.

Therefore the time to transfer a 64 bytes is:

(64 bytes * 8 bits/byte)/(10*2^20bits/s) = (2^9 bits)/(10*2^20bits/s)
= 0.1 * 2^(-11)s

So the number of meters the signal would ...

Solution Summary

Calculations for finding the length of a message is provided, as well as a discussion on how to overcome problems with cable length and Ethernet.