Srikant Sharma

Viking: a multi-spanning-tree Ethernet architecture for metropolitan area and cluster networks

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.

MiNT: a miniaturized network testbed for mobile wireless research

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.

Low-latency mobile IP handoff for infrastructure-mode wireless LANs

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.

MiNT-m: an autonomous mobile wireless experimentation platform

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.

Omnicon:a mobile ip-based vertical handoff system for wireless LAN and GPRS links

Wi-Fi based hotspots offer mobile users broadband wireless Internet connectivity in public work spaces and corporate/ university campuses. Despite aggressive deployment of these hotspots in recent years, high-speed wireless Internet access remains restricted to a small number of geographical areas because of 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 GPRS or 3G whenever wireless LAN connectivity is not available. The resulting handoff mechanism allows a network connection on a mobile node to operate over multiple wireless access networks in a way that is completely 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 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.

Design considerations for a multihop wireless network testbed

Limited fidelity of simulators has prompted researchers to build wireless network testbeds for realistic testing. Unlike simulators, which have broad applicability, most of these testbeds are tailored to specific projects and cannot be used by a wider research community. Recognizing the growing importance of testbeds, this article is one of the first attempts to identify a comprehensive set of requirements for a general-purpose multihop wireless network testbed and the challenges therein. The issues range from initial testbed deployment and routine management to individual experimental configuration and data collection. We survey state-of-the-art wireless testbeds and highlight their salient features. The article is intended to provide an initial reference for researchers, application developers, and administrators dealing with various aspects of wireless network testbeds.

Implementation experiences of bandwidth guarantee on a wireless LAN

Rether was originally developed to support guaranteed Quality of Service (QoS) for shared Ethernet LANs. With the growing popularity of wireless LANs, we modified the Rether protocol to provide QoS guarantee on wireless networks. In this paper, we present the design and implementation of the Wireless Rether protocol for 802.11 networks. We also describe our experiences with wireless LAN hardware. Wireless Rether supports QoS for TCP and UDP traffic in both upstream and downstream directions. The protocol can seamlessly inter-operate with any priority-based QoS mechanisms (such as Diffserv on the wired networks that connect the wireless access network to the rest of the Internet. QoS requirements of real-time applications are specified as a simple configurable policy table. Legacy networking applications can benefit from QoS guarantees provided by Wireless Rether without requiring any modifications.

Quality of service guarantee on 802.11 networks

Rether was originally developed to support guaranteed Quality of Service (QoS) for shared Ethernet LANs. With the growing popularity of wireless LANs, we modified the Rether protocol to provide QoS guarantee on wireless networks. In this paper we present the design and implementation of the wireless Rether protocol for 802.11 networks. Wireless Rether supports QoS for TCP, UDP and ICMP traffic in both upstream and downstream directions. The protocol can seamlessly inter-operate with any priority-based QoS mechanisms (such as Diffserv) on the wired networks that connect the wireless access network to the rest of the Internet. QoS requirements of real-time applications are specified as a simple configurable policy table. Legacy networking applications can benefit from QoS guarantees provided by Wireless Rether without requiring any modifications.

WiVision: a wireless video system for real-time distribution and on-demand playback

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.

A case for network-centric buffer cache organization

The emergence of clustered and networked storage architecture gives rise to a new type of server which acts as a data conduit over the network for remotely stored data. These servers, which we call pass-through servers, are mainly responsible for passing the data through them without interpreting it in any way. In this paper, we put forward a scheme of network-centric buffer cache management. This scheme can facilitate the data transmission through pass-through servers by avoiding redundant data copying, and by caching the data in a network-ready form, while having no modifications to the existing buffer cache organization. The performance measurement on an NFS server, using iSCSI storage, running on Linux, with this scheme shows throughput improvement of more than 50% compared to an NFS server on common Linux, while consuming about 40% less CPU resource.