E-MAC: an elastic MAC layer for IEEE 802.11 networks

We present a system for real-time traffic support in infrastructure and ad-hoc IEEE 802.11 networks. The proposed Elastic MAC (E-MAC) protocol provides a distributed transmission schedule for stations with real-time traffic requirements, while allowing a seamless coexistence with standard IEEE 802.11 clients, protecting best-effort 802.11 traffic from starvation by means of admission control policies. Our scheduling decisions are based on an “elastic” Transmission Opportunity (TXOP) assignment which allows for efficient wireless resource usage: whenever a real-time station does not use the assigned TXOP, the other real-time stations can take over the unused access opportunity, thus preventing the well known inefficiencies of static TDMA schemes. Unlike other TDMA-based solutions for 802.11, E-MAC does not require a tight synchronization among the participating clients, thus allowing its implementation on commodity WLAN hardware via minor software changes at the client side, and no changes at the access points. We studied the performance of our mechanism via ns-2 simulations and a mathematical model, showing that it outperforms IEEE 802.11e in terms of throughput, delay, and jitter. We finally provide a proof of concept through the results obtained in a real testbed where we implemented the E-MAC protocol.