I. Designing a New Ethernet One important issue in designing Ethernet lies in making sure that if a computer transmits a frame, any other computer that attempts to transmit at the same time will be able to hear the incoming frame before it stops transmitting, or else a collision might go unnoticed. For example, assume that we are on earth and send an Ethernet frame over a very long piece of category 5 wire to the moon. If a computer on the moon starts transmitting at the same time as we do on earth and finishes transmitting before our frame arrives at the moon, there will be a collision, but neither computer will detect it; the frame will be garbled, but no one will know why. So, in designing Ethernet, we must make sure that the length of cable in the LAN is shorter than the length of the shortest possible frame 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 10Base-T, 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 frame then calculate the number of meters the signal would travel in that time, and you have the total length of the frame.)

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

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?