A Smart Backoff Scheme for Multicast Flows in Wireless LAN

In IEEE 802.11 wireless protocols, a multicast sender basically adopts a simple broadcasting mechanism to transmit multicast frames. As such, the multicast frames are locally transmitted at a low, fixed rate without any backoff process on the sender’s side or any feedback requirements on the receiver side. This simplicity reveals an unfairly shared channel access between unreliable multicast flows and reliable unicast flows. In this paper, we propose a novel backoff scheme for multicast frame retransmission. The proposed scheme has been strategically designed after considering the sources of multicast frame errors. We define two different types of NAK messages that include a NAK for a single frame error and a cumulative NAK for multiple continuous erroneous frames. For frame retransmission, the multicast sender is required to adaptively adjust its contention window size based on the NAKs sent by its local receiver. With the proposed scheme, we ultimately provide reliable local multicast transmission and a relatively fair channel sharing with low-priority unicast flows.