a) Why does Ethernet need a preamble before each frame? b) Assume an Ethernet co

Question

 a) Why does Ethernet need a preamble before each frame?  b) Assume an Ethernet consists of five hosts, A, B, C, D, and E, interconnected via a hub (like a star, each host directly connected
 to the hub). Assume A and B transmit at the same time, and there is a collision. Will the jamming signal of A propagate to C, D, 
and E? Briefly explain why.  c) Same as b) above, except that we are using a bridge (switch) rather than a hub, and the bridge has full knowledge of the 
location of each host.  d) Assume that after a collision, we always choose to wait a random number of time slots where the random number is in the
 range [0, 512]. Is this reasonable? Why yes or why no? 

Explanation / Answer

Preamble is of 8 bytes.It contains bit pattern 10101010.
The manchester encoding of this pattern produces a 10 MHz square wave for 6.4 microsec to allow the receiver's clock to synchronize with the sender's.
They are required to stay synchronized for the rest of the frame,using the Manchester encoding to keep track of the bit boundaries.

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