Coupled 802.11 Flows in Urban Channels: Model and Experimental Evaluation

Contending flows in multihop 802.11 wireless networks compete with two fundamental asymmetries: 1) channel asymmetry, in which one flow has a stronger signal, potentially yielding physical layer capture; and 2) topological asymmetry, in which one flow has increased channel state information, potentially yielding an advantage in winning access to the channel. Prior work has considered these asymmetries independently with a highly simplified view of the other. However, in this paper, we perform thousands of measurements on coupled flows in urban environments and build a simple yet accurate model that jointly considers information and channel asymmetries. We show that if these two asymmetries are not considered jointly, throughput predictions of even two coupled flows are vastly distorted from reality when traffic characteristics are only slightly altered (e.g., changes to modulation rate, packet size, or access mechanism). These performance modes are sensitive not only to small changes in system properties, but also small-scale link fluctuations that are common in an urban mesh network. We analyze all possible capture relationships for two-flow subtopologies and show that capture of the reverse traffic can allow a previously starving flow to compete fairly. Finally, we show how to extend and apply the model in domains such as modulation rate adaptation and understanding the interaction of control and data traffic.

I. Introduction

In urban environments, IEEE 802.11 nodes interact in many ways, e.g., within and among paths in a multihop network and among deployments from different domains. Competing transmitters rarely have equal link quality to a given receiver, i.e., channel asymmetries are prevalent, especially in urban channels. When packets overlap in time, even slight link quality differences have been shown to cause physical layer capture such that the packet sent over the higher-quality link is received correctly but the packet sent over the weaker link is dropped [1]. Moreover, transmitters or receivers of competing flows often have unequal channel state information, a situation termed information asymmetry. in such cases, a topological asymmetry results in a hidden node having inferior channel availability information (see definition below and Fig. 5), forcing the hidden node to contend at random times guided by binary exponential backoff rather than at “idle times” driven by carrier sense. However, while the effects of information asymmetry and channel asymmetries are understood in isolation ([2]–[5] and [6]–[8], respectively), their interdependencies have thus far been ignored.