6. Consider the Go-Back-N ARQ scheme discussed in class. Suppose a window size W

Question

6. Consider the Go-Back-N ARQ scheme discussed in class. Suppose a window size Ws is chosen to be 7 so that the ARQ scheme is fully efficient when there are no errors. Let the L2 frame size be 256 bytes. Assume negligible contribution from overheads. a. What should be the bit error probability as seen by Layer 2 so that the efficiency of this Go-Back-N mechanism is at least 90%. b. Does increasing L2 frame size and keeping the same window size W increase efficiency or decrease the efficiency? Please show your work. 15 points

Explanation / Answer

Efficiency of Go-Back- N method

Efficiency = 1/E[x] ....(1)

Where E[x] = 1 + {(N*p)/(1-p)} .... (2)

Here p : Probability of Error and N = [S/Dtp]

Given Efficiency is 90% ; from equation (1)

==> 0.90 =1/E[x]

E[x] = 1.1111

Substitute in Equation (2) taking N= 7 is more ideal way

1.111 = 1+ {7*p/(1-p)}

==> p = 0.01562

Hence, probability of error is 1.56 %

(b) In sliding window protocols

Link Utilizatio is U = 1/{2(alpha) +1}

Where Alpha = Tprogation / Tfram

Increasing the frame size in Go-back- N ARQ leads to ineeficiency as N numbers of frames needs to be retransmitted if error occurs.

  

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