An analytical comparison of the Block NAK protocol and the IEEE 802.11aa feedback policy for a reliable multicast transport in the WLAN

The IEEE 802.11 standard does not define any feedback policy for the multicast transport. Hence missing multicast packets are not retransmitted and are definitely lost. In this paper we define the Block Negative-Ack (BNAK) recovery policy. Using our protocol, the multicast source transmits a block of multicast packets followed by a Block NAK Request (BNR). Upon reception of a BNR, a multicast member generates a Block NAK Response (BNAK) only if it missed some packets. A BNAK is transmitted after channel contention in order to avoid any eventual collision with other feedbacks, and is acknowledged. Under the assumption that the packet error rate (PER) of the wireless medium is low, a very limited number of BNAK will be generated and the network bandwidth will be saved. To measure the bandwidth cost of the Block NAK policy and to demonstrate its utility, we define an analytical model of our proposal and of the GCR-Block-Ack policy defined by the IEEE 802.11aa. The obtained results show that our protocol has a very high efficiency when the PER is limited, and outperforms the IEEE 802.11aa significantly.