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Transmission scheduling to improve the capacity in wireless mesh networks (WMNs) is challenging. One promising candidate solution is backpressure scheduling, which was proved to provide throughput optimality and queue stability in theory. Additionally, several recent practical systems that implement approximations of backpressure scheduling show performance benefits in WMNs based on IEEE 802.11, which is the most widely adopted MAC protocol. However, a detailed analysis of the queue stability is still missing for practical systems. In this paper, we experimentally show that applying backpressure scheduling over IEEE 802.11-based WMNs presents significant queue instability due to the delayed interaction between MAC and network layers. To understand how and when queue instability occurs, we present weighted backpressure scheduling (WBS), which uses two additional weight factors β and γ for the queue lengths of a node and its next-hop node when computing the backpressure value. By using WBS, we experimentally investigate the interactions between the components of backpressure scheduling implementations, i.e., packet scheduling and link scheduling, and show that, usually, β > γ leads to queue stabilization.

Transposon mutagenesis was used to isolate nonpathogenic mutants of Xanthomonas campestris pv. glycines 8ra, which causes bacterial pustule disease in soybean. A 6.1-kb DNA region in which a mutation gave loss of pathogenicity was isolated and found to carry six open reading frames (ORFs). Four ORFs had homology with hrcU, hrcV, hrcR, and hrcS genes of Ralstonia solanacearum and X. campestris pv. vesicatoria. One nonpathogenic mutant, X. campestris pv. glycines H80, lost pathogenicity on soybean but was able to elicit the hypersensitive response (HR) on nonhost pepper and tomato plants. This mutant still multiplied as well as the wild type in the leaves or cotyledons of soybean. Although the DNA and amino acid sequences showed high homology with known hrp genes, the hrcU-homolog ORF is not required for HR induction on nonhost plants, pepper and tomato, or for the multiplication of bacteria in the host plant. This gene was only required for the pathogenic symptoms of X. campestris pv. glycines 8ra on soybean.

Achieving efficient bandwidth utilization in wireless networks requires solving two important problems: (1) which packets to send (i.e., packet scheduling) and (2) which links to concurrently activate (i.e., link scheduling). To address these scheduling problems, many algorithms have been proposed and their throughput optimality and stability are proven in theory. One of the most well-known scheduling algorithms is backpressure scheduling which performs both link and packet scheduling assuming a TDMA (Time Division Multiple Access) MAC (Medium Access Control) layer. However, there has been limited work on realizing backpressure scheduling with a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) MAC layer (e.g., IEEE 802.11). In IEEE 802.11 networks, it is expected that the throughput optimality will not be achieved. In this paper, we investigate the extent of this throughput gap between theoretical TDMA-based backpressure scheduling and an approximation of it for IEEE 802.11 WMNs (Wireless Mesh Networks). Through extensive testbed measurements, we verify that there is indeed a non-negligible throughput gap. We present two main reasons behind this gap: Control inaccuracy that results from approximation of link scheduling and information inaccuracy that results from late or incorrect information, for instance, about queue lengths or network topology. Our results show that losses by MAC-layer collisions and backoff, which mainly occur due to control inaccuracy plays a major role for the throughput gap. On the other hand, while losses by queue drops, typically due to information inaccuracy, do occur, their effect can be tolerated. Nevertheless, both types of inaccuracies need to be mitigated in order to improve throughput.

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