Network flows running on a wireless mesh network (WMN) may suffer from partial failures in the form of serious throughput degradation, sometimes to the extent of starvation, because of weaknesses in the underlying MAC protocol, dissimilar physical transmission rates or different degrees of local congestion. Most existing WMN transport protocols fail to take these factors into account. This paper describes the design, implementation and evaluation of a coordinated congestion control (C3L) algorithm that guarantees fair resource allocation under adverse scenarios and thus provides end-to-end max-min fairness among competing flows. The C3L algorithm features an advanced topology discovery mechanism that detects the inhibition of wireless communication links, and a general collision domain capacity re-estimation mechanism that effectively addresses such inhibition. A comprehensive ns-2-based simulation study as well as empirical measurements taken from an IEEE 802.11a-based multi-hop wireless testbed demonstrate that the C3L algorithm greatly improves inter-flow fairness, eliminates the starvation problem, and at the same time maintains high radio resource utilization efficiency.

Economies of scale make IEEE 802.11 an attractive technology for building wireless mesh networks (WMNs). However, the IEEE 802.11 protocol exhibits serious link-layer unfairness when used in multi-hop networks. Existing fairness solutions either do not address this problem, or require proprietary MAC protocol to provide fairness. In this paper, we argue that an ideal transport protocol should be able to achieve fairness even on top of an unfair MAC layer such as 802.11. Towards this end, we propose a co-ordinatedcongestioncontrolalgorithm that performs global bandwidth allocation and provides end-to-end flow-level max-min fairness despite weaknesses in the MAC layer. The proposed algorithm features an advanced topology discovery mechanism that detects the inhibition of wireless communication links, and a general collision domain capacity re-estimation mechanism that effectively addresses such inhibition. Through an ns-2-based simulation study we demonstrate that the proposed algorithm substantially improves the fairness across flows, eliminates starvation problem, and simultaneously maintains a high overall network throughput.

An effective transport protocol for a wireless mesh network (WMN) must fairly and efficiently allocate the limited network resources among multiple flows sharing the network while minimizing the performance overhead it incurs. While many transport protocols have been proposed specifically for multi-hop wireless networks, most of them refrain from keeping state in the intermediate network nodes. In this paper, we focus on the other extreme of the design space:statefultransportprotocol, and study the research question of how much performance improvement is possible if intermediate network nodes could maintain as much state as needed. We present the design of a stateful transport protocol, namedlink-awarereliabletransportprotocol(LRTP), and examine how LRTP can fairly and efficiently allocate the network resources by accurately estimating the sending rate of each flow traversing the network using information about effective physical link capacity and the number of sharing flows. LRTP reduces the performance overhead associated with reliable packet delivery by leveraging the link-layer retransmission mechanism to eliminate per-packet end-to-end acknowledgments and unnecessary packet transmissions. Experiments conducted on anIEEE802.1la-basedmulti-channelwirelessmeshnetworktestbedas well asns-2simulationsdemonstrate that LRTP can achieve significant improvements in both overall network throughput and inter-flow fairness, especially on wireless networks with channel errors, when compared with the de facto Internet transport protocol TCP, and state-of-the-art MANET transport protocols such as ATP.

Viking [13] is a multi-spanning-tree Ethernet architecture that is designed to leverage commodity Ethernet switches to support Metro-Ethernet services. In particular, it exploits VLAN switching to provide network-wide load balancing across a metro-area network. However, Viking assumes the traffic matrix is fixed; as time goes by, the input load may gradually deviate from the assumed traffic matrix because of addition/deletion of subscribers and/or changes in bandwidth requirements from existing subscribers. Therefore, dynamic resource management is required to accommodate such traffic load fluctuations. This paper describes the design, implementation and evaluation of an autonomic network resource management system called Viking2 that enables a Metro-Ethernet network to dynamically self-reconfigure itself, with minimal human intervention, to adapt to changing traffic loads. In particular, whenever possible Viking2 uses a local repair approach to eliminate congestion while minimizing the number of flows affected. If local repair cannot fix a congestion problem, Viking2 resorts to a global reconfiguration approach that recomputes a new path for every flow and deploys the new routing state in a way that reduces the disruption during the period of transition to the minimum. Simulation results show that compared with the original Viking, Viking2 can indeed significantly improve a network's ability to dynamically adapt itself to changing traffic loads, to minimize the degree of congestion during normal network operation, and to increase the total effective network capacity.

Abstract Wi‐Fi based hotspots offer mobile users broadband wireless Internet connectivity in public work spaces and corporate/university campuses. Despite the aggressive deployment of these hotspots in recent years, high‐speed wireless Internet access remains restricted to small geographical areas due to the limited physical coverage of wireless LANs. On the other hand, despite their lower throughput, cellular networks have a significantly wider coverage and are thus much more available. Recognizing that 2.5G or 3G cellular networks can effectively complement wireless LANs, we set out to develop a vertical handoff system that allows mobile users to seamlessly fall back to such cellular networks as the general packet radio service (GPRS) or 3G whenever wireless LAN connectivity is not available. The resulting handoff mechanism allows a network connection of a mobile node to operate over multiple wireless access networks in a way that is transparent to end user applications. In this paper, we present the design, implementation, and evaluation of a fully operational vertical handoff system, called OmniCon, which enables mobile nodes to automatically switch between wireless LAN and GPRS, based on wireless LAN availability, by introducing a simple extension to the existing Mobile IP implementation. We discuss the design issues in the proposed vertical handoff system for heterogeneous networks, including connection setup problems due to network address translation, and the disparity in link characteristics between wireless LANs and GPRS. A detailed performance evaluation study of the OmniCon prototype demonstrates its ability to migrate active network connections between these two wireless technologies with low handoff latency and close to zero packet loss. Copyright © 2006 John Wiley & Sons, Ltd.

Limited fidelity of software-based wireless network simulations has prompted many researchers to build testbeds for developing and evaluating their wireless protocols and mobile applications. Since most testbeds are tailored to the needs of specific research projects, they cannot be easily reused for other research projects that may have different requirements on physical topology, radio channel characteristics or mobility pattern. In this paper, we describe the design, implementation and evaluation of MiNT-m, an experimentation platform devised specifically to support arbitrary experiments for mobile multi-hop wireless network protocols. In addition to inheriting the miniaturization feature from its predecessor MiNT [9], MiNT-m enables flexible testbed reconfiguration on an experiment-by-experiment basis by putting each testbed node on a centrally controlled untethered mobile robot. To support mobility and reconfiguration of testbed nodes, MiNT-m includes a scalable mobile robot navigation control subsystem, which in turn consists of a vision-based robot positioning module and a collision avoidance-based trajectory planning module. Further, MiNT-m provides a comprehensive network/experiment management subsystem that affords a user full interactive control over the testbed as well as real-time visualization of the testbed activities. Finally, because MiNT-m is designed to be a shared research infrastructure that supports 24x7 operation, it incorporates a novel automatic battery recharging capability that enables testbed robots to operate without human intervention for weeks.

A pass-through server such as an NFS server backed by an iSCSI[1] storage server only passes data between the storage server and NFS clients. Ideally it should require at most one data copying operation on sending or receiving, as in normal IP routers. In practice, pass-through servers actually incur multiple data copying operations because they are implemented using a layered architecture where each layer has its own internal data representation. This paper describes the design, implementation and evaluation of a novel network-centric buffer caching scheme called NCache that minimizes data copying overhead in pass-through servers without requiring significant changes to their existing implementation. By organizing the data being passed or cached in a network friendly format, NCache is able to eliminate all unnecessary data copying. The key innovation in NCache is that it exploits the fact that pass-through servers do not interpret data, by replacing physical copying with logical copying in a way transparent to existing software. This transparency enables NCache to be easily portable to many operating systems. We have successfully built a Linux-based NCache prototype that can be applied to in-kernel NFS and static Web servers. Empirical performance measurements collected from this prototype show that by reducing the CPU load associated with data copying, NCache is able to provide up to 92% improvement in throughput for NFS server and up to 47% for Web server.

Most mobile wireless networking research today relies on simulations. However, fidelity of simulation results has always been a concern, especially when the protocols being studied are affected by the propagation and interference characteristics of the radio channels. Inherent difficulty in faithfully modeling the wireless channel characteristics has encouraged several researchers to build wireless network testbeds. A full-fledged wireless testbed is spread over a large physical space because of the wide coverage area of radio signals. This makes a large-scale testbed difficult and expensive to set up, configure, and manage. This paper describes a miniaturized 802.11b-based, multi-hop wireless network testbed called MiNT. MiNT occupies a significantly small space, and dramatically reduces the efforts required in setting up a multi-hop wireless network used for wireless application/protocol testing and evaluation. MiNT is also a hybrid simulation platform that can execute ns-2 simulation scripts with the link, MAC and physical layer in the simulator replaced by real hardware. We demonstrate the fidelity of MiNT by comparing experimental results on it with similar experiments conducted on a non-miniaturized testbed. We also compare the results of experiments conducted using hybrid simulation on MiNT with those obtained using pure simulation. Finally, using a case study we show the usefulness of MiNT in wireless application testing and evaluation.

The ability to deliver digital video over wireless networks is an enabling technology for many useful applications, ranging from home entertainment and security monitoring to enterprise messaging and military reconnaissance, and thus represents the holy grail of wireless technology development. We describe a wireless video delivery system, WiVision, which uses IEEE 802.11 wireless LANs as the last mile for both real-time video distribution and on-demand video playback. WiVision can air both live events, such as on-campus seminars and sports activities, and prestored video streams, such as course lectures and financial analysis sessions, to mobile users, who can tune in to selected channels of their choice from their laptops or PDAs. An innovative feature of WiVision is the support for random video access based on keyword-search, where keywords are extracted from the closed-caption text embedded in TV programs. In addition, WiVision is able to broadcast video streams on the wireless link while seamlessly working with commercially available media players. The paper presents the implementation details of the real-time acquisition and network transport components of a fully operational WiVision prototype, and the results of a performance evaluation study on that prototype.

Simplicity, cost effectiveness, scalability, and the economies of scale make Ethernet a popular choice for local area networks, as well as for storage area networks and increasingly metropolitan-area networks. These applications of Ethernet elevate it from a LAN technology to a ubiquitous networking technology, thus prompting a rethinking of some of its architectural features. One weakness of existing Ethernet architecture is its use of single spanning tree, which, while useful at avoiding routing loops, leads to low link utilization and long failure recovery time. To apply Ethernet to cluster networks and MANs, these problems need to be addressed. We propose a multi-spanning-tree Ethernet architecture, called Viking, that improves both aggregate throughput and fault tolerance by exploiting standard virtual LAN technology in a novel way. By supporting multiple spanning trees through VLAN, Viking makes the most of the inherent redundancies in most mesh-like networks and delivers a multi-fold throughput gain over single-spanning-tree Ethernet with the same physical network topology. It also provides much faster failure recovery, reducing the down-time to a sub-second range from that of multiple seconds in single-spanning-tree Ethernet architecture. Finally, based only on standard mechanisms, Viking is readily implementable on commodity Ethernet switches without any firmware modifications.

The increasing popularity of IEEE 802.11-based wireless local area networks (LANs) lends them credibility as a viable alternative to third-generation (3G) wireless technologies. Even though wireless LANs support much higher channel bandwidth than 3G networks, their network-layer handoff latency is still too high to be usable for interactive multimedia applications such as voice over IP or video streaming. Specifically, the peculiarities of commercially available IEEE 802.11b wireless LAN hardware prevent existing mobile Internet protocol (IP) implementations from achieving subsecond Mobile IP handoff latency when the wireless LANs are operating in the infrastructure mode, which is also the prevailing operating mode used in most deployed IEEE 802.11b LANs. In this paper, we propose a low-latency mobile IP handoff scheme that can reduce the handoff latency of infrastructure-mode wireless LANs to less than 100 ms, the fastest known handoff performance for such networks. The proposed scheme overcomes the inability of mobility software to sense the signal strengths of multiple-access points when operating in an infrastructure-mode wireless LAN. It expedites link-layer handoff detection and speeds up network-layer handoff by replaying cached foreign agent advertisements. The proposed scheme strictly adheres to the mobile IP standard specification, and does not require any modifications to existing mobile IP implementations. That is, the proposed mechanism is completely transparent to the existing mobile IP software installed on mobile nodes and wired nodes. As a demonstration of this technology, we show how this low-latency handoff scheme together with a wireless LAN bandwidth guarantee mechanism supports undisrupted playback of remote video streams on mobile stations that are traveling across wireless LAN segments.

A growing variety of edge network access devices appear on the marketplace that perform various functions which are meant to complement generic routers' capabilities, such as firewalling, intrusion detection, virus scanning, network address translation, traffic shaping and route optimization. Because these edge network access devices are deployed on the critical path between a user site and its Internet service provider, high availability is crucial to their design. This paper describes the design, construction and evaluation of a general implementation framework for supporting fault tolerance on edge network devices. This implementation framework, called Duplex, is designed to be independent of the functionality of the hosting edge network access device, such that only a minimal amount of programming is required to tailor this framework to a specific edge network access device implementation. Duplex can tolerate power failure, hardware failure, and software failure by supporting device mirroring and watchdog timer-based link bypassing. Empirical performance measurements of an instance of Duplex that is embedded in a commercial bandwidth management device show that the run-time overhead of its fault tolerance mechanisms is less than 1 msec 90% of the time, and the failure detection and recovery period is less than 1.3 sec when running at 100 Mbps.

Content replication and distribution is an effective technology to reduce the response time for Web accesses and has been proven quite popular among large Internet content providers. However, existing content distribution systems assume a store-and-forward delivery model and is mostly based on static content. This paper describes the design, implementation, and initial evaluation of a network resource management system for real-time Internet content distribution called Sago, which provides facilities to provision and allocate network resources so that multiple bandwidth-guaranteed and fault-tolerant multicast connections can be multiplexed on a single physical network. Sago includes a novel network resource mapping algorithm that takes into account both physical network topology and dynamic traffic demands, a network-wide fault tolerance mechanism that supports both node-level and link-level fault tolerance, and a hierarchical network link scheduler that provides performance protection among multicast connections sharing the same physical network link. Moreover, Sago does not require any IP multicasting support from underlying network routers because it performs application-level multicasting. The technologies underlying Sago are important building blocks for real-time content distribution networks, end-to-end quality of service guarantee over global corporate intranets, and application-specific adaptation of wide-area network services.