Improving throughput in multihop wireless networks

One of the main characteristics of wireless ad hoc networks is their node-centric broadcast nature of communication, leading to interferences and spatial contention between adjacent wireless links. Due to such interferences, pessimistic concerns have been recently raised with respect to the decreasing network capacity in wireless ad hoc networks when the number of nodes scales to several orders of magnitude higher. Such studies assume uniformly distributed nodes in the network and randomized traffic patterns. In this paper, we argue that in all cases of end-to-end data communications-including one-to-k unicast and multicast data dissemination as well as k-to-one data aggregation-the maximum achievable end-to-end data throughput (measured on the sources) heavily depends on the strategy of arranging the topology of transmission between sources and destinations, as well as possible per-node operations such as coding. An optimal strategy achieves better end-to-end throughput than an arbitrary one. We present theoretical studies and critical insights with respect to how these strategies may be designed so that end-to-end throughput may be increased. After all, under all circumstances-in either a lightly loaded or a congested network-increasing end-to-end throughput from its baseline is always beneficial to applications using ad hoc networks to communicate.     

An energy-efficient MAC protocol based on IEEE 802.11 in wireless ad hoc networks

Energy efficiency is a measure of the performance of IEEE 802.11 wireless multihop ad hoc networks. The IEEE 802.11 standard, currently used in wireless multihop ad hoc networks, wastes bandwidth capacity and energy resources because of many collisions. Therefore, controlling the contention window size at a given node will increase not only the operating life of the battery but also the overall system capacity. It is essential to develop effective backoff schemes for saving power in IEEE 802.11 wireless multihop ad hoc networks. In this paper, we propose an energy-efficient backoff scheme and evaluate its performance in an ad hoc network. Our contention window mechanism devised by us grants a node access to a channel on the basis of the node’s percentage of residual energy. We use both an analytical model and simulation experiments to evaluate the effective performance of our scheme in an ad hoc network. Our extensive ns-2-based simulation results have shown that the proposed scheme provides excellent performance in terms of energy goodput, end-to-end goodput, and packet delivery ratio, as well as the end-to-end delay.     

Evaluating the communication performance of an ad hoc wirelessnetwork

Adaptive and self-organizing wireless networks are gaining in popularity. Several media access and routing protocols were proposed for such networks and the performance of such protocols were evaluated based on simulations. In this paper, we evaluate the practicality of realizing an ad hoc wireless network and investigate on performance issues. Several mobile computers were enhanced with ad hoc routing capability and were deployed in an outdoor environment and communication performance associated with ad hoc communications were evaluated. These computers periodically send beacons to their neighbors to declare their presence. We examined the impact of varying packet size, beaconing interval, and route hop count on route discovery time, communication throughput, end-to-end delay, and packet loss. We had also performed mobility experiments and evaluated the route reconstruction time incurred. File transfer times associated with sending information reliably (via TCP) over multihop wireless links are also presented. The experimental results obtained revealed that it is feasible to augment existing wireless computers with ad hoc networking capability. End-to-end performance in ad hoc routes are less affected by beaconing intervals than packet size or route length. Similarly, communication throughput is more dependent on packet size and route length with the exception at very high beaconing frequencies. Packet loss, on the other hand, is not significantly affected by packet size, route length or beaconing frequency. Finally, route discovery time in ad hoc wireless networks are more dependent on channel conditions and route length than variations in beaconing intervals     

Towards an end-to-end delay analysis of wireless multihop networks

In wireless multihop networks, end-to-end (e2e) delay is a critical parameter for quality of service (QoS) guarantees. We employ discrete-time queueing theory to analyze the end-to-end (e2e) delay of wireless multihop networks for two MAC schemes, m-phase TDMA and slotted ALOHA. In one-dimensional (1-D) networks, due to the lack of sufficient multiplexing and splitting, a space–time correlation structure exists, the nodes are spatially correlated with each other, and the e2e performance cannot be analyzed as in general two-dimensional networks by assuming all nodes independent of each other. This paper studies an 1-D network fed with a single flow, an extreme scenario in which there is no multiplexing and splitting. A decomposition approach is used to decouple the whole network into isolated nodes. Each node is modeled as a GI/Geo/1 queueing system. First, we derive the complete per-node delay distribution and departure characterization, accounting for both the queueing delay and access delay. Second, based on the departure process approximation, we define a parameter to measure the spatial correlation and its influence on the e2e delay variance. Our study shows that traffic burstiness of the source flow and MAC together determines the sign of the correlation.     

Capacity and delay of hybrid wireless broadband access networks

An optical network is too costly to act as a broadband access network. On the other hand, a pure wireless ad hoc network with n nodes and total bandwidth of W bits per second cannot provide satisfactory broadband services since the pernode throughput diminishes as the number of users goes large. In this paper, we propose a hybrid wireless network, which is an integrated wireless and optical network, as the broadband access network. Specifically, we assume a hybrid wireless network consisting of n randomly distributed normal nodes, and m regularly placed base stations connected via an optical network. A source node transmits to its destination only with the help of normal nodes, i.e., in the ad hoc mode, if the destination can be reached within L (L /spl geq/ 1) hops from the source. Otherwise, the transmission will be carried out in the infrastructure mode, i.e., with the help of base stations. Two transmission modes share the same bandwidth of W bits/sec. We first study the throughput capacity of such a hybrid wireless network, and observe that the throughput capacity greatly depends on the maximum hop count L and the number of base stations m. We show that the throughput capacity of a hybrid wireless network can scale linearly with n only if m = Ω(n), and when we assign all the bandwidth to the infrastructure mode traffics. We then investigate the delay in hybrid wireless networks. We find that the average packet delay can be maintained as low as Θ(1) even when the per-node throughput capacity is Θ(W).     

A hotspot mitigation protocol for ad hoc networks

Hotspots represent transient but highly congested regions in wireless ad hoc networks that result in increased packet loss, end-to-end delay, and out-of-order packets delivery. We present a simple, effective, and scalable hotspot mitigation protocol (HMP) where mobile nodes independently monitor local buffer occupancy, packet loss, and MAC contention and delay conditions, and take local actions in response to the emergence of hotspots, such as, suppressing new route requests and rate controlling TCP flows. We use analysis, simulation, and experimental results from a wireless testbed to demonstrate the effectiveness of HMP in mobile ad hoc networks. 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 the network connectivity preventing premature network partitions. We present analysis of hotspots, and the detailed design of HMP. We evaluate the protocol’s ability to effectively mitigate hotspots in mobile ad hoc networks that are based on on-demand and proactive routing protocols.     

Optimal resource allocation in wireless ad hoc networks: a price-based approach

The shared-medium multihop nature of wireless ad hoc networks poses fundamental challenges to the design of effective resource allocation algorithms that are optimal with respect to resource utilization and fair across different network flows. None of the existing resource allocation algorithms in wireless ad hoc networks have realistically considered end-to-end flows spanning multiple hops. Moreover, strategies proposed in wireline networks are not applicable in the context of wireless ad hoc networks, due to their unique characteristics of location-dependent contention. In this paper, we propose a new price-based resource allocation framework in wireless ad hoc networks to achieve optimal resource utilization and fairness among competing end-to-end flows. We build our pricing framework on the notion of maximal cliques in wireless ad hoc networks, as compared to individual links in traditional wide-area wireline networks. Based on such a price-based theoretical framework, we present a two-tier iterative algorithm. Distributed across wireless nodes, the algorithm converges to a global network optimum with respect to resource allocations. We further improve the algorithm toward asynchronous network settings and prove its convergence. Extensive simulations under a variety of network environments have been conducted to validate our theoretical claims.     

Self-coordinating localized fair queueing in wireless ad hoc networks

Distributed fair queueing in a multihop, wireless ad hoc network is challenging for several reasons. First, the wireless channel is shared among multiple contending nodes in a spatial locality. Location-dependent channel contention complicates the fairness notion. Second, the sender of a flow does not have explicit information regarding the contending flows originated from other nodes. Fair queueing over ad hoc networks is a distributed scheduling problem by nature. Finally, the wireless channel capacity is a scarce resource. Spatial channel reuse, i.e., simultaneous transmissions of flows that do not interfere with each other, should be encouraged whenever possible. In this paper, we reexamine the fairness notion in an ad hoc network using a graph-theoretic formulation and extract the fairness requirements that an ad hoc fair queueing algorithm should possess. To meet these requirements, we propose maximize-local-minimum fair queueing (MLM-FQ), a novel distributed packet scheduling algorithm where local schedulers self-coordinate their scheduling decisions and collectively achieve fair bandwidth sharing. We then propose enhanced MLM-FQ (EMLM-FQ) to further improve the spatial channel reuse and limit the impact of inaccurate scheduling information resulted from collisions. EMLM-FQ achieves statistical short-term throughput and delay bounds over the shared wireless channel. Analysis and extensive simulations confirm the effectiveness and efficiency of our self-coordinating localized design in providing global fair channel access in wireless ad hoc networks.     

Capacity of interference-limited ad hoc networks with infrastructure support

In this letter, we consider the capacity of ad hoc networks with infrastructure support. Although Grossglauser-Tse mobile network model enables /spl Theta/(1) per-node throughput scaling, the mobility assumption may be too unrealistic to be accepted in some practical situations. One of the key observations we acquired is that the infrastructure support plays the same role played by the mobility in the Grossglauser-Tse model. We show that nodes can utilize the randomly located infrastructure support instead of mobility when nodes are nearly static. In this case, we show that the per-node throughput of /spl Theta/(1) is still achievable when the number of access points grows linearly with respect to the number of nodes.     

Aggregated traffic flow weight controlled hierarchical MAC protocol for wireless sensor networks

It has been discussed in the literature that the medium-access control (MAC) protocols, which schedule periodic sleep–active states of sensor nodes, can increase the longevity of sensor networks. However, these protocols suffer from very low end-to-end throughput and increased end-to-end packet delay. How to design an energy-efficient MAC protocol that greatly minimizes the packet delay while maximizing the achievable data delivery rate, however, remains unanswered. In this paper, motivated by the many-to-one multihop traffic pattern of sensor networks and the heterogeneity in required data packet rates of different events, we propose an aggregated traffic flow weight controlled hierarchical MAC protocol (ATW-HMAC). We find that ATW-HMAC significantly decreases the packet losses due to collisions and buffer drops (i.e., mitigates the congestion), which helps to improve network throughput, energy efficiency, and end-to-end packet delay. ATW-HMAC is designed to work with both single-path and multipath routing. Our analytical analysis shows that ATW-HMAC provides weighted fair rate allocation and energy efficiency. The results of our extensive simulation, done in ns-2.30, show that ATW-HMAC outperforms S-MAC; traffic-adaptive medium access; and SC-HMAC.     

Efficient on-demand routing for mobile ad hoc wireless access networks

In this paper, we consider a mobile ad hoc wireless access network in which mobile nodes can access the Internet via one or more stationary gateway nodes. Mobile nodes outside the transmission range of the gateway can continue to communicate with the gateway via their neighboring nodes over multihop paths. On-demand routing schemes are appealing because of their low routing overhead in bandwidth restricted mobile ad hoc networks, however, their routing control overhead increases exponentially with node density in a given geographic area. To control the overhead of on-demand routing without sacrificing performance, we present a novel extension of the ad hoc on-demand distance vector (AODV) routing protocol, called LB-AODV, which incorporates the concept of load-balancing (LB). Simulation results show that as traffic increases, our proposed LB-AODV routing protocol has a significantly higher packet delivery fraction, a lower end-to-end delay and a reduced routing overhead when compared with both AODV and gossip-based routing protocols.     

Distributed Flow Control and Medium Access in Multihop Ad Hoc Networks

Recent studies have shown that the performance of wireless multihop ad hoc networks is very poor. In this paper, we first demonstrate that one important reason of the poor performance is the close coupling between medium contention and network congestion. Therefore, we present a framework of distributed flow control and medium access control to address both medium contention and network congestion. The proposed scheme utilizes the MAC layer control frames to efficiently conduct the network layer's flow control function and only allows the upstream nodes to forward enough packets to make it possible for the downstream nodes to fully utilize the shared channel but never introduce severe MAC collisions and network congestion. Extensive simulations illustrate that the proposed scheme well controls congestion and greatly alleviates medium collisions. It achieves up to 12 times the end-to-end throughput of IEEE 802.11, maintains a short delay and a low control overhead, and improves the fairness regardless of the hop count and the traffic load     

QoS guarantee and provisioning for realtime digital video over mobile ad hoc cdma networks with cross-layer design

In this article we investigate the trade-offs and the constraints for multimedia over mobile ad hoc CDMA networks, and propose a cross-layer distributed power control and scheduling protocol to resolve those trade-offs and constraints in order to provide high-quality video over wireless ad hoc CDMA networks. In particular, a distributed power control and scheduling protocol is proposed to control the incurred delay of video streaming over multihop wireless ad hoc networks, as well as the multiple access interference (MAI). We also investigate the impacts of Doppler spread and noisy channel estimates upon the end-to-end video quality, and provide a relatively robust system which employs a combination of power control and coding/interleaving to combat the effects of Doppler spread by exploiting the increased time diversity when the Doppler spread becomes large. Thus, more robust end-to-end video quality can be achieved over a wide range of channel conditions     

Timeout Period Analysis to Detect Black Hole Attack in Multihop Wireless Ad Hoc Networks

A novel approach is proposed to estimate the timeout period used in wireless ad hoc networks in order to detect misbehaving nodes that make black hole attacks. Timeout period is an acceptable time frame for a node to forward a packet and is used to judge if the node is behaving properly. To avoid misjudgment and false alarms, the accuracy of the estimate of the timeout period is very important. Our method is based on the IEEE 802.11 MAC protocol and Dynamic Source Routing protocol. The main contribution of this paper is proposing a queuing analysis to calculate the mean and maximum delay per hop implementing the 95-percentile of medium access waiting time. In addition, a new technique is introduced that can be applied to determine the mean number of hops in flooding-based ad hoc networks, taking into account edge effects. For each proposed model, analytical results are compared with results obtained from simulations and the validity of the models is confirmed by observing the close relationship between the results.     

Improving Spatial Reuse in Multihop Wireless Networks - A Survey

In multihop wireless ad-hoc networks, the medium access control (MAC) protocol plays a key role in coordinating the access to the shared medium among wireless nodes. Currently, the distributed coordination function (DCF) of the IEEE 802.11 is the dominant MAC protocol for both wireless LANs and wireless multihop ad hoc environment due to its simple implementation and distributed nature. The current access method of the IEEE 802.11 does not make efficient use of the shared channel due to its conservative approach in assessing the level of interference; this in turn affects the spatial reuse of the limited radio resources and highly affect the achieved throughput of a multihop wireless network. This paper surveys various methods that have been proposed in order to enhance the channel utilization by improving the spatial reuse.     

Multipath multihop routing analysis in mobile ad hoc networks

This paper proposes an analytical modeling framework to investigate multipath routing in multihop mobile ad hoc networks. In this paper, a more generalized system has been considered and mathematically analyzed to observe some of the related performance measures of the ad hoc network. Each node in the network is assumed to have finite buffer. The single-path model is approximated to be a multi-node M/M/1/B tandem network, and the multi-path model as a set of multiple parallel paths. This proposed model allows us to investigate issues such as end-to-end delivery delay, throughput and routing reliability in mobile ad hoc networks. Theoretical results have been verified by numerical results. An optimal path selection strategy has been proposed to select a minimized delay path among the available multiple paths between source-destination pair.     

Does the IEEE 802.11 MAC protocol work well in multihop wireless adhoc networks?

The IEEE 802.11 MAC protocol is the standard for wireless LANs; it is widely used in testbeds and simulations for wireless multihop ad hoc networks. However, this protocol was not designed for multihop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multihop connectivity is one of the most prominent features. In this article we focus on the following question: can the IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in an IEEE 802.11-based multihop network and revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multihop ad hoc networks. We thus doubt whether the WaveLAN-based system is workable as a mobile ad hoc testbed     

Capacity regions for wireless ad hoc networks

We define and study capacity regions for wireless ad hoc networks with an arbitrary number of nodes and topology. These regions describe the set of achievable rate combinations between all source-destination pairs in the network under various transmission strategies, such as variable-rate transmission, single-hop or multihop routing, power control, and successive interference cancellation (SIC). Multihop cellular networks and networks with energy constraints are studied as special cases. With slight modifications, the developed formulation can handle node mobility and time-varying flat-fading channels. Numerical results indicate that multihop routing, the ability for concurrent transmissions, and SIC significantly increase the capacity of ad hoc and multihop cellular networks. On the other hand, gains from power control are significant only when variable-rate transmission is not used. Also, time-varying flat-fading and node mobility actually improve the capacity. Finally, multihop routing greatly improves the performance of energy-constraint networks.     

Enabling Real-Time H.264 Video Services over Wireless Ad Hoc Networks Using Joint Admission and Transmission Power Control

In a wireless ad hoc network, packets are sent from node-to-node in a multihop fashion until they reach the destination. In this paper we investigate the capacity of a wireless ad hoc network in supporting packet video transport. The ad hoc network consists of n homogeneous video users with each of them also serving as a relay node for other users. We investigate how the time delay affects the video throughput in such an ad hoc network and how to provide a time-delay bounded packet video delivery service over such a network. The analytical results indicate that appropriate joint admission and power control have to be employed in order to efficiently utilize the network capacity while operating under the delay constraint as the distance between source and destination changes.     

Scalability and Performance Evaluation of Hierarchical Hybrid Wireless Networks

This paper considers the problem of scaling ad hoc wireless networks now being applied to urban mesh and sensor network scenarios. Previous results have shown that the inherent scaling problems of a multihop ldquoflatrdquo ad hoc wireless network can be improved by a ldquohybrid networkrdquo with an appropriate proportion of radio nodes with wired network connections. In this work, we generalize the system model to a hierarchical hybrid wireless network with three tiers of radio nodes: low-power end-user mobile nodes (MNs) at the lowest tier, higher power radio forwarding nodes (FNs) that support multihop routing at intermediate level, and wired access points (APs) at the highest level. Scalability properties of the proposed three-tier hierarchical hybrid wireless network are analyzed, leading to an identification of the proportion of FNs and APs as well as transmission range required for linear increase in end-user throughput. In particular, it is shown analytically that in a three-tier hierarchical network with nA APs, nF FNs, and nM MNs, the low-tier capacity increases linearly with nF, and the high-tier capacity increases linearly with nA when nA = Omega(radic{nF}) and n _A = O(nF). This analytical result is validated via ns-2 simulations for an example dense network scenario, and the model is used to study scaling behavior and performance as a function of key parameters such as AP and FN node densities for different traffic patterns and bandwidth allocation at each tier of the network.