Kaixin Xu

Scalable routing protocols for mobile ad hoc networks

The growing interest in mobile ad hoc network techniques has resulted in many routing protocol proposals. Scalability issues in ad hoc networks are attracting increasing attention these days. We survey the routing protocols that address scalability. The routing protocols included in the survey fall into three categories: flat routing protocols; hierarchical routing approaches; GPS augmented geographical routing schemes. The article compares the scalability properties and operational features of the protocols and discusses challenges in future routing protocol designs.

Effectiveness of RTS/CTS handshake in IEEE 802.11 based ad hoc networks

Abstract IEEE 802.11 MAC mainly relies on two techniques to combat interference: physical carrier sensing and RTS/CTS handshake (also known as “virtual carrier sensing”). Ideally, the RTS/CTS handshake can eliminate most interference. However, the effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within transmission range of receivers. In this paper, we prove using analytic models that in ad hoc networks, such an assumption cannot hold due to the fact that power needed for interrupting a packet reception is much lower than that of delivering a packet successfully. Thus, the “virtual carrier sensing” implemented by RTS/CTS handshake cannot prevent all interference as we expect in theory. Physical carrier sensing can complement this in some degree. However, since interference happens at receivers, while physical carrier sensing is detecting transmitters (the same problem causing the hidden terminal situation), physical carrier sensing cannot help much, unless a very large carrier sensing range is adopted, which is limited by the antenna sensitivity. In this paper, we investigate how effective is the RTS/CTS handshake in terms of reducing interference. We show that in some situations, the interference range is much larger than transmission range, where RTS/CTS cannot function well. Two independent solutions are proposed in this paper. One is a simple enhancement to the IEEE 802.11 MAC protocol. The other is to utilize directional antennas. Simulation results verify that the proposed schemes indeed can help IEEE 802.11 resolve most interference caused by large interference range.

An ad hoc network with mobile backbones

A mobile ad hoc network (MANET) is usually assumed to be homogeneous, where each mobile node shares the same radio capacity. However, a homogeneous ad hoc network suffers from poor scalability. Recent research has demonstrated its performance bottleneck both theoretically and through simulation experiments and testbed measurement Building a physically hierarchical ad hoc network is a very promising way to achieve good scalability. In this paper, we present a design methodology to build a hierarchical large-scale ad hoc network using different types of radio capabilities at different layers. In such a structure, nodes are first dynamically grouped into multihop clusters. Each group elects a cluster-head to be a backbone node (BN). Then higher-level links are established to connect the BN into a backbone network. Following this method recursively, a multilevel hierarchical network can be established. Three critical issues are addressed in this paper. We first analyze the optimal number of BN for a layer in theory. Then, we propose a new stable clustering scheme to deploy the BN. Finally LANMAR routing is extended to operate the physical hierarchy efficiently. Simulation results using GloMoSim show that our proposed schemes achieve good performance.

A heterogeneous routing protocol based on a new stable clustering scheme

A mobile ad hoc network (MANET) is usually assumed to be homogeneous, where each mobile node shares the same radio capacity. However, a homogeneous ad hoc network suffers from poor scalability. Research has demonstrated its performance bottleneck both theoretically and through simulation experiments. Building a physically hierarchical ad hoc network is a very promising way to achieve scalability. Routing is critical to operate such a hierarchical structure efficiently. Previous research has been solely focusing on extending popular routing schemes developed for the homogeneous ad hoc network. We propose a new heterogeneous routing protocol specifically designed for the hierarchical ad hoc network. We also present a new active clustering scheme to help build a stable hierarchical structure, which is fundamental to heterogeneous routing protocols. Simulation results using GloMoSim show that our routing and clustering scheme gives good performance. The results also provide some insight into new possible designs of routing protocols and mobile network architectures for MANETs when large scale is desired.

Landmark routing in ad hoc networks with mobile backbones

A mobile ad hoc network is usually assumed to be homogeneous, where each mobile node shares the same radio capacity. However, a homogeneous ad hoc network suffers from poor scalability. Recent research has demonstrated its performance bottleneck through both theoretical analysis and simulation experiments and testbed measurements. This is further exacerbated by heavy routing overhead of ad hoc routing protocols when the network size is large. In this paper, we present a design methodology to build a hierarchical large-scale ad hoc network using different types of radio capabilities at different layers. In such a structure, nodes are first dynamically grouped into multi-hop clusters. Each group elects a cluster-head to be a backbone node (BN). Then higher-level links are established to connect the BNs into a backbone network. Following this method recursively, a multilevel hierarchical network can be established. Three critical issues are addressed in this paper. We first analyze the optimal number of BNs for a layer in theory. Then, we propose a stable and light overhead clustering scheme to deploy the BNs. Finally landmark ad hoc routing (LANMAR) is extended to operate the physical hierarchy efficiently. We show that the hierarchical LANMAR can incorporate and efficiently utilize backbone links to reach remote destinations (thus reducing the hop distance). Simulation results using GloMoSim confirm that our proposed schemes achieve good performance.

TCP performance over multipath routing in mobile ad hoc networks

In this paper, we investigate TCP performance over a multipath routing protocol. Multipath routing can improve the path availability in mobile environment. Thus, it has a great potential to improve TCP performance in ad hoc networks under mobility. Previous research on multipath routing mostly used UDP traffic for performance evaluation. When TCP is used, we find that most times, using multiple paths simultaneously may actually degrade TCP performance. This is partly due to frequent out-of-order packet delivery via different paths. We then test another multipath routing strategy called backup path routing. Under the backup path routing scheme, TCP is able to gain improvements against mobility. We then further study related issues to backup path routing, which can affect TCP performance. Some important discoveries are reported in the paper and simulation results show that by careful selection of the multipath routing strategies, we can improve TCP performance by more than 30% even under very high mobility.

TCP behavior across multihop wireless networks and the wired internet

Emerging wireless ad hoc networks find their most important applications in untethered, mobile, multihop scenarios where there is no wired infrastructure. Yet, when the wired infrastructure (say, the Internet) is within reach, opportunistic connections to Internet sites may be established across the multihop network to transfer files and update databases. These file transfers use TCP for reliability and congestion control. However, recent experiments with ad hoc, multihop 802.11 networks have exposed serious instabilities when TCP connections span both wired and wireless domains. In particular, some TCP connections capture the wireless channel and drive the throughput on other connections virtually to zero. This is most surprising in view of the fact that connections between 802.11 (single hop) wireless LAN stations and the Internet are well behaved. In fact they are routinely used in most Campuses, Businesses and Research Labs. This paper is an experimental study of the unstable behavior of TCP across 802.11 ad hoc networks and the wired Internet. We investigate the fairness issues of multiple TCP flows as well as the coexistence of TCP flows and video streams in the wired/wireless scenario. Detailed analysis of the measurement results is also presented. The paper will prove very valuable to future commercial and military ad hoc networks.

Adaptive bandwidth management and QoS provisioning in large scale ad hoc networks

Quality of service provisioning in wireless ad hoc networks plays an integral part in determining the success of network-centric warfare as envisioned in future military operations. It requires good scalability of the QoS architecture since ad hoc networks in the battlefield tend to be large. Previous work attacking QoS in ad hoc networks seldom considers the scalability issues. In this paper, we propose a scalable QoS architecture for such networks. Our scheme draws upon the positive aspects of both IntServ and DiffServ, and extends upon the scalable LANMAR routing protocol to support QoS. The scheme is also capable of incorporating mobile backbone networks (MBNs) to further improve the scalability. Simulation results show that our proposed QoS architecture can achieve good scalability in terms of large network size and mobility.

Adaptive security for multilevel ad hoc networks

Secure communication is critical in military environments in which the network infrastructure is vulnerable to various attacks and compromises. A conventional centralized solution breaks down when the security servers are destroyed by the enemies. In this paper we design and evaluate a security framework for multilevel ad hoc wireless networks with unmanned aerial vehicles (UAVs). In battlefields, the framework adapts to the contingent damages on the network infrastructure. Depending on the availability of the network infrastructure, our design is composed of two modes. In infrastructure mode, security services, specifically the authentication services, are implemented on UAVs that feature low overhead and flexible managements. When the UAVs fail or are destroyed, our system seamlessly switches to infrastructureless mode, a backup mechanism that maintains comparable security services among the surviving units. In the infrastructureless mode, the security services are localized to each nodes vicinity to comply with the ad hoc communication mechanism in the scenario. We study the instantiation of these two modes and the transitions between them. Our implementation and simulation measurements confirm the effectiveness of our design. Copyright

TCP Unfairness in ad hoc wireless networks and a neighborhood RED solution

Significant TCP unfairness in ad hoc wireless networks has been reported during the past several years. This unfairness results from the nature of the shared wireless medium and location dependency. If we view a node and its interfering nodes to form a “neighborhood”, the aggregate of local queues at these nodes represents the distributed queue for this neighborhood. However, this queue is not a FIFO queue. Flows sharing the queue have different, dynamically changing priorities determined by the topology and traffic patterns. Thus, they get different feedback in terms of packet loss rate and packet delay when congestion occurs. In wired networks, the Randomly Early Detection (RED) scheme was found to improve TCP fairness. In this paper, we show that the RED scheme does not work when running on individual queues in wireless nodes. We then propose a Neighborhood RED (NRED) scheme, which extends the RED concept to the distributed neighborhood queue. Simulation studies confirm that the NRED scheme can improve TCP unfairness substantially in ad hoc networks. Moreover, the NRED scheme acts at the network level, without MAC protocol modifications. This considerably simplifies its deployment.