Out-of-sequence acknowledgment cannot be used for selective-reject ARQ. That is, if frame i is rejected by station X, all subsequent I-frames and RR frames sent by X must have N1R2 = i until frame i is successfully received, even if other frames with N1S2 7 i are successfully received in the meantime. One possible refinement is the following: N1R2 = j in an I-frame or an RR frame is interpreted to mean that frame j - 1 and all preceding frames are accepted except for those that have been explicitly rejected using an SREJ frame. Comment on any possible drawback to this scheme.