Packet aggregation in multi-rate wireless LANs

In CSMA networks, there is significant overhead associated with packet transmission, including header and contention overhead. For applications where packets are small, such as Voice over IP (VoIP), these overheads mean that a majority of the transmission time is wasted. Packet aggregation is a technique to amortize the per-transmission overhead over multiple aggregated packets. However, existing heuristics are limited, often not considering multi-rate wireless MAC, or operation in a Wireless LAN (WLAN) environment. In this paper we formulate the problem of optimal aggregation for a multi-rate CSMA MAC protocol and show that it is NP-hard. We then propose two heuristics that solve the aggregation problem for multi-rate WLANs. The first, which we call Data Rate based Aggregation protocol (DRA), divides packets in the MAC queue into groups based on their data rate. DRA then aggregates packets in the same group and broadcasts the aggregated frame at the data rate of that group. DRA substantially increases throughput compared to state of the art aggregation protocols; in certain cases achieving up to a 200% increase in the number of VoIP calls supported by a single 802.11g AP. The second heuristic, which we call Data Rate based Aggregation with Selective Demotion (DRA-SD), enables cross data rate aggregation. Through preliminary evaluation, we show that selectively demoting packets can further improve performance.