Hotspots are often created in regions of mobile ad hoc networks (MANETs) where flows converge and intersect with each other. We define hotspots as nodes that experience flash congestion conditions or excessive contention over longer time-scales (e.g., order of seconds). Under such conditions nodes typically consume more resources (e.g., energy) and attempt to receive, process, and forward packets but the performance of the packet forwarding and signaling functions is considerably diminished and limited during hotspot periods. This is the result of excessive contention of the shared media wireless access, and due to flash loading at hotspot nodes, and importantly, at neighboring nodes that are in the region of hotspots. Hotspots are often transient in nature because the mobility of nodes in the network continuously creates, removes, and to some degree, migrates hotspots because node mobility changes the network topology and causes flows to be rerouted. Hotspots are characterized by excessive contention, congestion, and resource exhaustion in these networks. In other words, hotspots appear when excessive contention exists, prompting congestion when insufficient resources are available to handle the increased traffic load.
Hotspots are intrinsic to many on-demand MANET routing protocols because most on-demand routing protocols utilize shortest path (or hop count) as their primary route creation metric. Most on-demand routing protocols allow an intermediate node to reply to a route query using cached route information, causing traffic to concentrate at certain nodes. We observe from our analysis of hotspots presented in this paper that although many on-demand routing protocols prove to be effective in routing packets in these networks they also have a propensity to create hotspots. Other researchers have also made such observations. We also observe that hotspot nodes consume a disproportionate amount of resources (e.g., energy).
In this paper, we present a simple, effective, and scalable hotspot mitigation protocol (HMP), which seamlessly operates with existing on-demand (e.g., AODV and DSR) and proactive (e.g., DSDV and OLSR) ad hoc routing protocols. HMP balances resource consumption among neighboring nodes and improves end-to-end throughput, delay, and packet loss. Our results indicate that HMP can also improve network connectivity preventing premature network partitions. Ideally, establishing routes through non-congested areas of the network and rerouting active flows away from congested areas to non-congested areas would be the best approach to hotspot mitigation. However, this requires extensive collaboration between nodes to establish load-aware routes and sophisticated algorithms to update time-varying loading conditions. Such an approach is unscalable and not practical in mobile ad hoc networks.
HMP represents a fully distributed and scalable protocol where nodes independently monitor local conditions and take local actions:
• to declare a node to be a hotspot if a combination of MAC contention/delays, packet loss, buffer occupancy, and remaining energy reserves exceed certain predefined system thresholds;
• to suppress new route requests at hotspots to ensure that routed traffic does not compound congestion problems; and
• to throttle traffic locally at hotspots to force TCP flows to slow down.
HMP also seeks to decrease the energy consumption of nodes in ad hoc networks via use of these mechanisms.
This paper is structured as follows. In Section 2, we first analyze the behavior of hotspots using existing on-demand MANET routing protocols. Observations from this evaluation indicate that hotspots are evident even under relatively lightly loaded conditions in ad hoc networks, motivating the need for hotspot mitigation protocols. Related work is discussed in Section 3, followed by the design of the protocol in Section 4. We present a detailed analysis of HMP in Section 5 using both on-demand and proactive routing protocols and discuss results from the implementation of HMP in a wireless testbed in Section 6. Finally, in Section 7, we present some concluding remarks.
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