In a Multi-hop Wireless Networks (MHWN), packets are routed between source and destination using a chain of intermediate nodes; chains are a fundamental communication structure in MHWNs whose behavior must be understood to enable building effective protocols. The behavior of chains is determined by a number of complex and interdependent processes that arise as the sources of different chain hops compete to transmit their packets on the shared medium. In this paper, we show that MAC level interactions play the primary role in determining the behavior of chains. We evaluate the types of chains that occur based on the MAC interactions between different links using realistic propagation and packet forwarding models. We discover that the presence of destructive interactions, due to different forms of hidden terminals, does not impact the throughput of an isolated chain significantly. However, due to the increased number of retransmissions required, the amount of bandwidth consumed is significantly higher in chains exhibiting destructive interactions, substantially influencing the overall network performance. These results are validated by testbed experiments. We finally study how different types of chains interfere with each other and discover that well behaved chains in terms of self-interference are more resilient to interference from other chains.
Deep Dive into How do Wireless Chains Behave? The Impact of MAC Interactions.
In a Multi-hop Wireless Networks (MHWN), packets are routed between source and destination using a chain of intermediate nodes; chains are a fundamental communication structure in MHWNs whose behavior must be understood to enable building effective protocols. The behavior of chains is determined by a number of complex and interdependent processes that arise as the sources of different chain hops compete to transmit their packets on the shared medium. In this paper, we show that MAC level interactions play the primary role in determining the behavior of chains. We evaluate the types of chains that occur based on the MAC interactions between different links using realistic propagation and packet forwarding models. We discover that the presence of destructive interactions, due to different forms of hidden terminals, does not impact the throughput of an isolated chain significantly. However, due to the increased number of retransmissions required, the amount of bandwidth consumed is signif
Multi-Hop Wireless Networks (MHWNs), which include mesh, sensor, and ad hoc networks, are forecast to play an important role in an Internet that will grow increasingly wireless at the edge. MHWNs reduce infrastructure requirements by having wireless nodes relay traffic towards access points; they are attractive whenever infrastructure is unavailable or costly, or quick deployment is desired [1]- [3]. The complex and dynamic nature of wireless propagation, interference, and user mobility make developing effective networking protocols for MHWNs a significant challenge.
In an MHWN, packets are forwarded from source to destination using a chain of nodes. Starting from the source, a node forwards packets to the next node in the chain forming a path towards the destination. Chains represent a fundamental communication structure in MHWNs, and understanding their behavior is critical to designing effective protocols. In particular, routing protocols must discover efficient chains that can then be used for communication. Early routing protocols used path length to discriminate between chains, favoring the shortest available path [4]- [7]. Recently, individual link qualities have been taken into account in evaluating path quality [8]. However, the behavior of a chain is a complex process that cannot be accurately characterized by looking at the individual links without consideration to how they interact with each other.
Several studies have examined the behavior of chains. Li et al. study the performance of chains as the number of hops are increased [9]. They also study the effect of cross-interference between chains. Xu and Sadaawi analyze TCP instability due to chain self-interference and discover short-term and long-term unfairness issues in cross-chain interactions [10]. Ping et al. present the effect of traffic on routing instability, packet drops, and unfairness due to self interference within chains [11]. These analyses significantly differ from ours in a number of ways, including the fact that they do not consider the detailed processes, such as the impact of the MAC level interactions, on the performance of chains. We review these and other related works in Section II.
Several complex and inter-dependent processes combine to determine the behavior of chains. In particular, the performance of chains is affected by self-interference among the different hops of chain as they compete to transmit on shared wireless medium. This interference not only reduces the available transmission time at each hop, but also causes packet collisions due to a variety of MAC level interactions that occur in different chains. Moreover, nodes in the middle of the chain experience higher interference than nodes at the edge because they are in interference range with more nodes in the chain; this is a process we call contention unfairness.
Among the different processes that impact chain performance, MAC level interactions play a central role. They also significantly moderate the effect of the other processes. In order to better understand chain behavior, we first analyze the types of interactions that occur most frequently in four-hop chains.We later explore generalizing these results. Although there is a large number of potential interaction configurations that may arise in chains, we discover that only a small number of them occur in practice due to chain geometry restrictions. Specifically, we set up forwarding rules to produce chains in a way representative of how routing protocols work. We use a Signal to Interference and Noise Ratio (SINR) model for packet reception which allows us to account for the effect of capture.
We then evaluate, in Section IV, the effects of the interference interactions within a chain on its overall performance.
We first use simulation to determine the throughput and the number of packet drops for different types of chains. Afterwards, we validate our simulation results by comparing them against results obtained from an experimental testbed.
The next contribution of the paper, discussed in Section V, is to develop an approach for estimating the performance of general n-hop chains. Specifically, we observe that the best chains are those where senders are in carrier sense range with each other, allowing the MAC protocol to effectively arbitrate the medium. In those chains where this is not the case, the presence of a hidden terminal has a higher impact than a hidden terminal with capture. Finally, the location of the hidden terminal is also a factor in determining the performance; the earlier the hidden terminal, the worse its impact.
After characterizing how a single chain self-interferes in isolation, we look at the problem of how multiple chains interfere with each other. In a general MHWNs, multiple connections are active simultaneously, interfering with each other. In Section VI, we evaluate cross-chain interactions and study their effects on the performance of chains. Again, we discover tha
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