Modeling Per-Flow Throughput and Capturing Starvation in CSMA Multi-Hop Wireless Networks

Abstract

Abstract-Multi-hop wireless networks employing random access protocols have been shown to incur large discrepancies in the throughputs achieved by the flows sharing the network. Indeed, flow throughputs can span orders of magnitude from near starvation to many times greater than the mean. In this paper, we address the foundations of this disparity. We show that the fundamental cause is not merely differences in the number of contending neighbors, but a generic coordination problem of CSMA-based random access in a multi-hop environment. We develop a new analytical model that incorporates this lack of coordination, identifies dominating and starving flows and accurately predicts per-flow throughput in a large-scale network. We then propose metrics that quantify throughput imbalances due to the MAC protocol operation. Our model and metrics provide a deeper understanding of the behavior of CSMA protocols in arbitrary topologies and can aid the design of effective protocol solutions to the starvation problem.

Insensitivity and stability of random-access networks

“…Canceling the remaining terms we get (8), so π PH indeed satisfies (12). Substituting π PH into (13), and canceling common terms yields…”

QoS provisioning for large-scale multi-ap WLANs

“…Many studies have already identified the channel unfair share and the flow starvation problem in 802.11-based wireless ad hoc networks [37,13,16,20]. There have also been numerous efforts to mitigate the inter-cell interference problem, including channel assignment [17,28,34], power control (or carrier sense control) [8,26,36], and association control [9,10].…”

“…In particular, it has been shown that both EDCA and DCF do not utilize their maximum potential capacity, and their throughput performance and degree of fairness degrade significantly as the number of contending stations increases [21]. A more general problem with EDCA is that it inherently involves the unfairness problem stemming from the CSMA-based medium access rule [23,16].…”

“…Despite their potential merits, however, the high-density WLANs suffer from severe performance degradation and unfair throughput imbalance due to significant channel contention among cochannel neighboring cells [7,19,34]. The performance imbalance problem is mainly due to the fundamental limitation of CSMA-based random access mechanism, which gives preference in media access to some links over others depending on their spatial locations [23,16]. Although the inter-cell channel contention and interference can be reduced by allocating orthogonal channels to neighbor cells [19,28,22,17,34], only 3 orthogonal channels are available in the 2.4 GHz band, which is not enough to eliminate the co-channel interference.…”

“…However, the EDCA is inherently based on the CSMA random access protocol and thus can easily suffer from the severe unfairness among neighboring cells. For example, our simulation result shows that the QoS guarantee for flows in a cell -whose spatial location is disadvantageous than neighboring cells in terms of channel sensing -is hardly achieved by the EDCA due to lack of transmission opportunities even when its intra-cell channel contention is moderate [37,13,16,20].…”