Low Power MAC with Anycasting

Packet retransmissions, idle listening, and overhearing, are the three main sources of energy wastage in the MAC Layer. The problem of excessive retransmissions can be avoided by observing and predicting the channel quality, and avoiding periods with poor channel quality. CSMA relies on carrier sensing to decide if retransmissions should be performed immediately. However, in cases where the poor channel quality persists, or packet losses are due to interference that is undetectable by carrier sensing, the channel assessment alone is not a good indicator of the chance of a successful transmission. To schedule retransmissions at appropriate moments, we have developed a new technique called transmission pushbacks, to reduce such losses by delaying retransmissions. This technique overcomes periods of poor channel quality while ensuring a throughput matching the incoming packet rate. In order to determine the optimal pushback period, we devise an adaptive channel prediction technique based on estimating the parameters of a simple hidden Markov model (HMM) which represents the channel. The parameters of the HMM are dynamically updated based solely on the ACK sequence for the previous packet transmissions. By considering both the packet incoming rate and the packet loss pattern, the appropriate pushback period is calculated and applied for future retransmissions.

To deal with idle listening and packet overhearing, we have developed a protocol for duty-cycled networks, called CMAC (Convergent MAC), that primarily uses anycasting to forward packets. CMAC can work at very low duty cycles (0.1%), and it requires no synchronization when there is no traffic. When carrying traffic, CMAC first uses anycasting to wake up forwarding nodes, and then converges gradually from route-suboptimal anycast to route-optimal unicast. Experiments on the Kansei testbed, and ns-2 simulations show that CMAC significantly outperforms other duty cycling protocols in terms of latency, throughput, and energy efficiency. CMAC is currently being ported to the TelosB platform.

More recently, we have investigated the problem of selection of the forwarding set of nodes to optimize the end-to-end delay for all nodes. Further, in our experiments with anycast based MAC protocols, we have made some key observations. Unreliability in the reverse link from the potential forwarders to the sender can result in packet duplication. Each ACK must also be received by other potential forwarders to suppress duplicate transmissions. However, the link between the potential forwarders may not be reliable. These issues call for further investigation.

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