Go-back n uses concept of pipelining, so efficiency of go-back n is given by
Efficiency = N/ (1 + 2a)
Here N = sender window size. a = Tp /Tt where Tp = propagation delay and Tt = transmission time.
Now to get 100% efficiency, N = 1+2a.
My doubt is can we have window size greater than 1+2a ? If we have window size greater than 1 + 2a then efficiency exceeds 100%. Is this possible? According to the formula it doesn't seem to be possible but I think we can increase the window size beyond 1 + 2a