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Mohamed G. Gouda						Midterm 2
CS 356: Computer Networks					Spring 2007
Solution
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1. (7 points)

(The solution below corresponds to the interpretation where p can connect
to multiple q[i]'s, but only one connection per q[i]).

process p
const   n
inp     m      : integer
var     conp   : array[0..n-1] of boolean,    {init false}
    	cansnd : array[0..n-1] of 0..m
par 	i      : 0..n-1

begin
	timeout ~conp[i] ^ (#ch.p.q[i]+#ch.q[i].p=0) -> send crqst to q[i]

[] 	rcv crply from q[i] -> conp[i] := true; cansnd[i]:=m;

[]	conp[i] -> if cansnd[i]>0 -> send data to q[i]; cansnd[i]:=cansnd[i]-1
		   [] true -> send drqst to q[i]; cansnd[i] := 0
 		   fi


[] 	rcv drply from q[i] -> conp[i] := false

end

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2. (7 points)

Length of shortest burst: n 
This occurs when exactly n consecutive bits are corrupted, which can be counted
as the first n and the last n, with no uncorrupted bits between them.

Length of the longest burst: 2n-1
The longest possible n-bracketed error can occur when the first n and the last n 
bits of the entire block of m*n and the n parity bits is corrupted, however this
will cause two bits in every column to be corrupted. For the protocol in section 8.1
to work in this case, there must be at least one column with a single error, 
which is guaranteed if the length of the burst is less than 2n, and is 
sufficient to discard the entire block.

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3. (6 points)

process	p
const   w
var     na : array[0..1] of integer, 	  {init. 0}
	ns : array[0..1] of integer, 	  {init. 0}
	i  : integer
par     j  : 0..1

begin

	(ns[0]-na[0]) + (ns[1]-na[1]) < w -> send data(ns[j]) to q[j];
				     	     ns[j] := ns[j] + 1

[]	rcv ack(i) from q[j] -> na[j] := max( na[j], i);

[] 	timeout na[j]<ns[j] ^ (#ch.p.q[j]+#ch.q[j].p=0) ^ ~akn.q[j] -> send data(na[j]) to q[j]

[]	rcv error from q[j] -> skip

end