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Abstract not found

Routing protocols for wireless ad hoc networks have traditionally focused on finding paths with minimum hop count. However, such paths can include slow or lossy links, leading to poor throughput. A routing algorithm can select better paths by explicitly taking the quality of the wireless links into account. In this paper, we conduct a detailed, empirical evaluation of the performance of three link-quality metrics— ETX, per-hop RTT, and per-hop packet pair—and compare them against minimum hop count. We study these metrics using a DSR-based routing protocol running in a wireless testbed. We find that the ETX metric has the best performance when all nodes are stationary. We also find that the per-hop RTT and per-hop packet-pair metrics perform poorly due to self-interference. Interestingly, the hop-count metric outperforms all of the link-quality metrics in a scenario where the sender is mobile.

Opportunistic routing is a recent technique that achieves high throughput in the face of lossy wireless links. The current opportunistic routing protocol, ExOR, ties the MAC with routing, imposing a strict schedule on routers ’ access to the medium. Although the scheduler delivers opportunistic gains, it misses some of the inherent features of the 802.11 MAC. For example, it prevents spatial reuse and thus may underutilize the wireless medium. It also eliminates the layering abstraction, making the protocol less amenable to extensions to alternate traffic types such as multicast. This paper presents MORE, a MAC-independent opportunistic routing protocol. MORE randomly mixes packets before forwarding them. This randomness ensures that routers that hear the same transmission do not forward the same packets. Thus, MORE needs no special scheduler to coordinate routers and can run directly on top of 802.11. Experimental results from a 20-node wireless testbed show that MORE’s median unicast throughput is 22 % higher than ExOR, and the gains rise to 45 % over ExOR when there is a chance of spatial reuse. For multicast, MORE’s gains increase with the number of destinations, and are 35-200 % greater than ExOR.

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Wireless mesh networks (WMNs) have been proposed as a solution for ubiquitous last-mile broadband access. A critical limiting factor for many WMN protocols in realizing their throughput potential is the interference between nodes in the WMN. Understanding and characterizing such interference is important for a variety of purposes such as channel assignment, route selection, and fair scheduling. Instead of using ad hoc heuristics, a recent study proposed characterizing interference in a WMN by measuring two-way interference, i.e., interference between each pair of communicating links. In this paper, we study the extent of multi-way interference, i.e., the interference caused by multiple transmitters to a communicating link. We find through simulations and through measurements of a 32-node wireless testbed that even if these transmitters individually do not interfere significantly with a given communicating link, simultaneous transmissions of them have the potential to significantly affect the throughput of the communicating link. This implies that pairwise interference measurements may be optimistic when used to drive protocols in wireless mesh networks. Encouragingly, we find that this phenomenon, although significant when it occurs, is not widespread. In particular, multi-way interference caused significant additional throughput degradation compared to pairwise interference to a small fraction of the links in the testbed over our measurement period. In addition, we find that there is a strong correlation between the impact of multi-way interference and the quality of the link under consideration. We conclude with recommendations on how protocols should take multi-way interference into account.

Abstract — Despite the increasing number of WiFi-based Long Distance (WiLD) network deployments, there is a lack of understanding of how WiLD networks perform in practice. In this paper, we perform a systematic study to investigate the commonly cited sources of packet loss induced by the wireless channel and by the 802.11 MAC protocol. The channel induced losses that we study are external WiFi, non-WiFi and multipath interference. The protocol induced losses that we study are protocol timeouts and the breakdown of CSMA over WiLD links. Our results are based on measurements performed on two real-world WiLD deployments and a wireless channel emulator. The two deployments allow us to compare measurements across rural and urban settings. The channel emulator allows us to study each source of packet loss in isolation in a controlled environment. Based on our experiments we observe that the presence of external WiFi interference leads to significant amount of packet loss in WiLD links. In addition to identifying the sources of packet loss, we analyze the loss variability across time. We also explore the solution space and propose a range of MAC and network layer adaptation algorithms to mitigate the channel and protocol induced losses. The key lessons from this study were also used in the design of a TDMA based MAC protocol for high performance long distance multihop wireless networks [12]. I.

In this paper, we consider the problem of mitigating interference and improving network capacity in wireless mesh networks from the angle of temporal-spatial diversity. In a nutshell, while the achievable throughput on a multihop wireless path is limited by intra-flow interference, the overall capacity of a multihop wireless network can be increased by exploiting temporal-spatial diversity of concurrent transmissions that exist among a number of wireless links. Connections that are routed along multihop wireless paths can be scheduled to take place simultaneously if their transmissions do not interfere with each other (significantly). To make a case of exploiting the temporal-spatial diversity to improve network capacity, we focus on transporting downstream traffic at gateway nodes with Internet access. We propose to construct, based on measurements of received signal strengths, a virtual coordinate system that is used to determine the sets of paths along which transmissions can take place with the least inter-flow interference. Based on the sets of non-interfering paths, the gateway node then determines the order with which a gateway node schedules frames of different connections to be transmitted. Through extensive simulation (with real-life measurement traces on an operational, city-wide wireless community network), we show that the downstream throughput of a gateway node in a wireless mesh network can be improved by 10-35 % under a variety of network topologies and traffic distributions. This, coupled with the fact that the proposed approach requires only minor code change in the gateway nodes and does not require any additional hardware, makes it a viable option to improving network capacity in existing wireless mesh networks.

Abstract—The use of random linear network coding (NC) has significantly simplified the design of opportunistic routing (OR) protocols by removing the need of coordination among forwarding nodes for avoiding duplicate transmissions. However, NC-based OR protocols face a new challenge: How many coded packets should each forwarder transmit? To avoid the overhead of feedback exchange, most practical existing NC-based OR protocols compute offline the expected number of transmissions for each forwarder using heuristics based on periodic measurements of the average link loss rates and the ETX metric. Although attractive due to their minimal coordination overhead, these approaches may suffer significant performance degradation in dynamic wireless environments with continuously changing levels of channel gains, interference, and background traffic. In this paper, we propose CCACK, a new efficient NCbased OR protocol. CCACK exploits a novel Cumulative Coded ACKnowledgment scheme that allows nodes to acknowledge network coded traffic to their upstream nodes in a simple way, oblivious to loss rates, and with practically zero overhead. In addition, the cumulative coded acknowledgment scheme in CCACK enables an efficient credit-based, rate control algorithm. Our evaluation shows that, compared to MORE, a state-of-theart NC-based OR protocol, CCACK improves both throughput and fairness, by up to 20x and 124%, respectively, with average improvements of 45 % and 8.8%, respectively. I.

Ad hoc networks have been proposed for a variety of applications where support for real-time multimedia services will be necessary. This requires that the network is able to offer quality of service (QoS) appropriate for the latency and jitter bounds of the real-time application constraints. In this paper, we analyze the primary challenges of realizing QoS in large scale mobile ad hoc networks and propose a QoS framework for real-time traffic support. Specifically, our proposed QoS framework first utilizes a call setup protocol at the IP layer to discover paths for real-time flows, as well as to perform admission control by accurate service quality prediction. We then use a prioritized MAC protocol to provide priority access for flows with real-time constraints to reduce interference from unregulated non-real-time traffic. We foresee the utility of our proposed solution in large-scale ad hoc networks, such as campus or community-wide wireless networks. In these environments, fixed wireless routers may further be leveraged to achieve better service quality when node movement is significant. Through experimental results, we demonstrate the utility and efficiency of our approach. 1