As described above, in the case of reservation MAC protocols, a network station starts transmission of data segments belonging to a user packet (e.g. IP packet) by using a particularly allocated portion of the transmission resources. After a network station starts transmitting the data segments, it can happen that one or more segments are disturbed. In previous investigations, simple retransmission of the whole packet is applied if at least one segment of the packet is disturbed. However, in communications systems with higher BER, it is more efficient to retransmit smaller data units (Sec. 5.2.1). Therefore, ARQ is applied to retransmit erroneous segments and not the whole packet. In the case of Go-back-N ARQ mechanism, the base station has knowledge of the number of requested segments and can discover if there are some erroneous or missing data segments on the receiving side. In this case, it sends a negative acknowledgment (NAK) to the sending station, including the sequence number of the last received segment. Thus, the sending station has to retransmit only the data segments with the higher sequence number. If the Selective-Reject ARQ mechanism that achieves the best performance from among different ARQ mechanisms is applied, the sending station retransmits only the erroneous data segment. Each of the ARQ variants, described in Sec. 4.3.4, can be applied together with reservation MAC protocols. However, because of the applied per-packet reservation method, the affected station is not able to retransmit all disturbed data segments within the previously reserved transmission turn. It happens because a station receives the right only to send for the requested number of data segments and it is possible that another station will start to send immediately afterwards
