Introduction to the concept about Ethernet design limitation (see also Ch7 Minic

Question

Introduction to the concept about Ethernet design limitation (see also Ch7 Minicase I.):

One important issue in designing shared 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 distance (maybe 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 distance a shortest possible frame travels. Otherwise, a collision could go undetected.

Consider this Analogy/Example:

First: I am dropping 8 quarters (coins) on a conveyor belt every minute. How long will it take to drop $10?   8 quarters is $2, so $10/$2= 5 minutes.

Then:  The conveyor belt is advancing 2 meters per minute. How far it will advance in 5 minutes? 5 [minutes] * 2 [meters/minute] = 10 meters. The distance of the $10 in quarters on the conveyor belt will be 10 meters.

Assumptions for Question:

The effective speed is 40 million meters per second (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 speed is much slower)

The frame size is 64 bytes (the smallest possible Ethernet frame size, including the 33 byte overhead).

The Question:   What distance in meters will this message travel (the length of the message in meters) over the medium when using Ethernet protocol with speed 10 Mbps?

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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.

The distance a 64 byte frame will travel if using 1GbE is ______ meters.

QUESTION 3

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?

In other words, if the cable is longer than the calculated maximum distance in the answers above, what would be the possible solutions to still make it work?

(Select 2 answers; partial credit)

Use shared, not switched Ethernet, or

Decrease the circuit speed, or

Decrease the message length, or

Use different cable, or

Increase the circuit speed, or

Increase the message length, or

Use shared, not switched Ethernet, or

Decrease the circuit speed, or

Decrease the message length, or

Use different cable, or

Increase the circuit speed, or

Increase the message length, or

Explanation / Answer

firstly, to transmit 64 bytes frame along the medium 10MBPS it takes 6 micro seconds ..

     10*1024*1024 bytes ----- 1sec

      64 bytes                    ----- ?sec

Now to the distance travelled whose speed is 44 million meters per second is 44 meters

      44 * 10^5 meters ------ 1sec

      ?           meters ------- 6 micro seconds

So it travels 44 meters to switch to other system

INCREASE THE CIRCUIT SPEED AND USE DIFFERENT CABLE are the two possible solutions....

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