Assume that propagation speed of electromagnetic waves sent over the medium is   2*10⁸ m/s and the transmission rate of the network is 10Mbps. What is the minimum frame size (in bits) necessary to ensure that CSMA/CD will work properly for this network? Explain your reasoning.

In order for CSMA/CD to work,  a  transmitting host must know about a collision (by hearing it on the link) BEFORE  the transmitting host completes its transmission

Let d = distance between hosts X and Y.

Let R = 2*10⁸ m/s be the propagation speed of the network

• Suppose host X starts transmitting a frame of size F.

• Then, before hearing the first bit of the frame, host Y starts transmitting a frame.

• Since Y transmits BEFORE hearing X this can be at most  d/R time after X starts transmitting.

• X hears the beginning of Y’s transmission d/R time after that. 

Therefore, the MAXIMUM amount of time that can pass from the time that X starts transmitting until X hears the first bit from Y is 2d/R.

 

Therefore we must have F/10Mbps >= 2d/R.

 

d can be 2 X10^3 m  (if X and Y are A and E) so E has to be at least

F  = 10Mbps * (2d/R)  = 200 bits.

 Last revised 05/04/2004