The GoBackN sliding window protocol has been edited such that it gives explicit acknowledgments (colour green in the simulation) whenever there is an unbalance between the traffic from Protocol A to B and vice versa. Later on negative acknowledgments were also added, in case a frame has checksum errors, and thus it will prompt the protocol to resend that frame. The acknowledgements are usually sent by piggybacking, and the explicit acknowledgments come into play only when one of the protocols doesn't have a frame/packet to send with which it can also send the acknowledgment.