(ii) 2^(n-1)

For Selective Repeat with n-bit sequence numbers (giving 2^n distinct sequence numbers in total), the maximum window size is limited to half of the total sequence number space: 2^(n-1).

Why it must be half (not 2^n − 1 like Go-Back-N): Selective Repeat receivers must distinguish between a retransmitted OLD frame and a NEW frame carrying the same sequence number (after wraparound). If sender and receiver windows were each allowed to be larger than half the sequence space, their windows could overlap in a way that makes an old, already-received frame indistinguishable from a new one with the same sequence number — causing the receiver to accept a duplicate as if it were new data. Capping both windows at 2^(n-1) guarantees the sender's and receiver's windows never overlap incorrectly.