Exploiting path diversity in the link layer in wireless ad hoc networks

We develop an anycast mechanism at the link layer for wireless ad hoc networks. The goal is to exploit path diversity in the link layer by choosing the best next hop to forward packets when multiple next hop choices are available. Such choices can come from a multipath routing protocol, for example. This technique can reduce transmission retries and packet drop probabilities in the face of channel fading. We develop an anycast extension of the IEEE 802.11 MAC layer based on this idea. We implement the protocol in an experimental proof-of-concept testbed using the Berkeley motes platform and S-MAC protocol stack. We also implement it in the popular ns-2 simulator and experiment with the AOMDV multipath routing protocol and Ricean fading channels. We show that anycast performs significantly better than 802.11 in terms of packet delivery, particularly when the path length or effect of fading is large. Further we experiment with anycast in networks that use multiple channels and those that use directional antennas for transmission. In these networks, deafness and hidden terminal problems are the main source of packet loss. We implemented anycast as extension of 802.11 like protocols that were proposed for these special networks. We are able to show that anycast is capable of enhancing the performance of these protocols by simply making use of the path diversity whenever it is available.