Frame retransmissions considered harmful: Improving spectrum efficiency using micro-ACKs

Retransmissions reduce the efficiency of data communication in wireless networks because of: (i) per-retransmission packet head- ers, (ii) contention overhead on every retransmission, and (iii) re-dundant bits in every retransmission. In fact, every retransmission nearly doubles the time to successfully deliver the packet. To im-prove spectrum efficiency in a lossy environment, we propose a new in-frame retransmission scheme using μACKs. Instead of waiting for the entire transmission to end before sending the ACK, the re-ceiver sends smaller μACKs for every few symbols, on a separate narrow feedback channel. Based on these μACKs, the sender only retransmits the lost symbols after the last data symbol in the frame, thereby adaptively changing the frame size to ensure it is success-fully delivered. We have implemented μACK on the Sora platform. Experiments with our prototype validate the feasibility of symbol-level μACK. By significantly reducing the retransmistion overhead, the sender is able to aggressively use higher data rate for a lossy link. Both improve the overall network efficiency. Our experimen-tal results from a controlled environment and an 9-node software radio testbed show that μACK can have up to 140% throughput gain over 802.11g and up to 60% gain over the best known retrans- mission scheme.