Introduction

Our work deals with issues related to the way in which 802.11 WLANs resolve contention for the channel in non-cooperative environments, such as public hot-spots or private enterprises that are physically close to each other (e.g. neighboring office suites in a commercial building or neighboring residences). In such environments, multiple nodes may compete for channel access in a rational but non-cooperative manner. That is each competing node will maximize its utility regardless of what other nodes achieve.

As the deployment of WLANs continues to grow rapidly, we believe that neighboring wireless networks that are managed by different administrative authorities will often need to share a common channel due to scarcity of available spectrum. For example, the bandwidth of the 2.4GHz band used by 802.11 is wide enough for 3 orthogonal 802.11b or 802.11g channels, i.e., a maximum of 3 neighboring nodes can simultaneously transmit data with little interference.

In indoor environments, the channel loss rates of nodes vary widely: even receiving nodes that are equi-distant from a common sender experience differing channel conditions. When the average signal strength at the receiver is lower than the threshold required for successful frame reception, the sender can unilaterally elect to use an alternative coding scheme that exploits the trade-off between data rate and BER [1]. Transmitting at a lower data rate by using a more resilient modulation scheme leads to higher frame transmission time but reduces the frame loss rate.

Each competing 802.11 node can use any 802.11-compliant strategy to maximize its achieved throughput. An 802.11 node can determine, for each frame transmission, the frame size and the data transmission rate. If each competing node uses the most efficient transmission strategy, i.e., the strategy that yields the highest achievable throughput when the node alone occupies the channel, the resulting aggregate throughput will be optimal.

However, in the presence of competition, a rational node may not use its most efficient transmission rate. The root cause of this behavior is the mechanism used by the 802.11's MAC protocol DCF (for Distributed Coordinating Function) to dictate how the medium is shared. DCF is designed to give an approximately equal probability of channel access (measured in number of frame transmissions opportunities) to each competing node with similar loss characteristics. That is to say, over any period lasting hundreds of milliseconds, each node will be able to transmit an equal number of frames, irrespective of the amount of channel time required to transmit the frame.

A rational node will attempt to maximize its throughput by maximizing the product of its share of channel occupancy time and its achievable throughput per unit of occupancy. Under DCF, the share of channel occupancy time a node obtains depends on the data rates used by it and its competitors. By intentionally transmitting at a lower data rate, a node may achieve a higher channel time share than it would by transmitting at a higher and more efficient data rate. This effect combined with the reduction in the node's frame loss rate may lead to higher achieved throughput for that node. However, this is done at the cost of overall efficiency. The aggregate throughput lost by other nodes will exceed the throughput gained by the node using a non-optimal transmission rate.

In this paper [2], we:

• Develop a game theoretic model for rational wireless nodes competing for the shared channel resource,
• Show analytically using our model and through simulation that under certain conditions both DCF and its enhanced cousin EDCF (for Enhanced Distributed Coordinating Function), which is being drafted as part of 802.11e, can lead competing rational nodes to undesirable equilibriums, in which the shared wireless channel is inefficiently used, and
• Show that by guaranteeing the allocation of long-term shares of channel time to competing nodes with respect to a desired fairness constraint, the MAC protocol can force rational nodes to efficiently use the shared medium, thereby improving the combined achieved throughputs of all competing nodes.

References

[1] Holland. A Rate-adaptive {MAC} Protocol for Multi-hop Wireless Networks. In The Proceedings of the Seventh Annual Int. Conf. on Mobile Computing and Networking (Mobicom), pp. 236--251, Rome, Italy, July 2001.

[2] Tan and Guttag. The 802.11 MAC Protocol Leads to Inefficient Equilibria. In The Proceedings of INFOCOM , Miami, Florida, March 2005.