Pravin Bhagwat’s research while affiliated with University of Maryland, College Park and other places

An ad-hoc network is the cooperative engagement of a collection of Mobile Hosts without the required intervention of any centralized Access Point. In this paper we present an innovative design for the operation of such ad-hoc networks. The basic idea of the design is to operate each Mobile Host as a specialized router, which periodically advertises its view of the interconnection topology with other Mobile Hosts within the network. This amounts to a new sort of routing protocol. We have investigated modifications to the basic Bellman-Ford routing mechanisms, as specified by the Routing Information Protocol, making it suitable for a dynamic and self-starting network mechanism as is required by users wishing to utilize ad-hoc networks. Our modifications address some of the previous objections to the use of Bellman-Ford, related to the poor looping properties of such algorithms in the face of broken links and the resulting time dependent nature of the interconnection topology describing the links between the Mobile Hosts. Finally, we describe the ways in which the basic network-layer routing can be modified to provide MAC-layer support for ad-hoc networks.

Wireless LAN (WLAN) market consists of IEEE 802.11 MAC standard conformant devices (e.g., access points (APs), client adapters) from multiple vendors. Certain third party certifications such as those specified by the Wi-Fi alliance have been widely used by vendors to ensure basic conformance to the 802.11 standard, thus leading to the expectation that the available devices exhibit identical MAC level behavior. In this paper, however, we present what we believe to be the first ever set of experimental results that highlight the fact that WLAN devices from different vendors in the market can have heterogeneous MAC level behavior. Specifically, we demonstrate with examples and data that in certain cases, devices may not be conformant with the 802.11 standard while in other cases, they may differ in significant details that are not a part of mandatory specifications of the standard. We argue that heterogeneous MAC implementations can adversely impact WLAN operations leading to unfair bandwidth allocation, potential break-down of related MAC functionality and difficulties in provisioning the capacity of a WLAN. However, on the positive side, MAC level heterogeneity can be useful in applications such as vendor/model level device fingerprinting.

Bluetooth, a wireless technology based on a frequency-hopping physical layer, enables portable devices to form short-range wireless ad hoc networks. Bluetooth hosts are not able to communicate unless they have previously discovered each other through synchronization of their timing and frequency-hopping patterns. Thus, even if all nodes are within proximity of each other, only those nodes which are synchronized with the transmitter can hear the transmission. To support any-to-any communication, nodes must be synchronized so that the pairs of nodes, which can communicate with each other, form a connected graph. Using Bluetooth as an example, we first provide deeper insights into the issue of link establishment in frequency-hopping wireless systems. We then introduce an asynchronous distributed protocol that begins with nodes having no knowledge of their surroundings and terminates with the formation of a connected network topology satisfying all constraints posed by Bluetooth. An attractive protocol feature is its ease in implementation using the communication primitives offered by the Bluetooth Specification.

The past decade has seen communication revolution in the form of cellular telephony as well as the Internet, but much of it has been restricted to the developed world and metropolitan pockets in the developing world. While the use of cellular technologies can cut down on the time to deploy access networks, the cost economics make this non-viable in growing telecom economies. In the Digital Gangetic Plains (DGP) project, we have been exploring the use of 802.11 as a long-distance access technology. 802.11, popularly known as WiFi, is currently cost-priced since it is an open/inter-operable standard, and hence is attractive for low cost and rapid deployment in rural areas. We have built an extensive testbed in a rural setting consisting of directional 802.11 links, spanning up to about 80km at its longest. The testbed is set in the Kanpur-Lucknow corridor of the Gangetic Plains. To our knowledge such a long-distance, multi-hop testbed based on 802.11 is unique thus far. While 802.11 is attractive in terms of cost economics, it was inherently designed for indoor use. Our novel use of the technology for outdoor, long-distance access links presents several challenges. Our experience with the testbed has brought several research as well as operational issues to the fore. Our contributions in this project thus far include: (1) demonstration of technical fea- sibility of using 802.11 for rural Internet connectivity, (2) an understanding of several operational and cost issues, (3) various technical contributions, (4) demonstration of and experience with various services on the testbed network, as well as (5) identification of various technical and operational issues going forward. In this report, we document our experiences with this technology, our contributions in this project, as well as the novel technical and operational challenges that lie ahead in using 802.11 to bridge the digital divide.

The past decade has seen communication revolution in the form of cellular telephony as well as the Internet, but much of it has been restricted to the developed world and metro pockets in the developing world. While the use of cellular technologies can cut down on the time to deploy access networks, the cost economics make this non-viable in growing telecom economies. In the Digital Gangetic Plains (DGP) project, we are exploring the use of 802.11 as a long-distance access technology. 802.11 is currently cost-priced due to competitive mass production and hence is attractive for low cost and rapid deployment in rural areas.

Mobile nodes of the future will be equipped with multiple network interfaces to take advantage of overlay networks, yet no current mobility systems provide full support for the simultaneous use of multiple interfaces. The need for such support arises when multiple connectivity options are available with different cost, coverage, latency and bandwidth characteristics, and applications want their data to flow over the interface that best matches the characteristics of the data. In this paper we introduce and analyze an architecture called Transport Layer Mobility that allows mobile nodes to not only change their point of attachment to the Internet within a corporate domain, but also to control which network interfaces are used for the different kinds of data leaving from and arriving at the mobile node. We implement our transport layer mobility scheme using a split–connection proxy architecture and a new technique called TCP Splice that gives split–connection proxy systems the same end-to-end semantics as normal TCP connections. We introduce the architecture, present its system aspects, investigate its performance and present its reliability properties. The analytical aspects of the protocol, in particular its pseudo-code, its properties and its validation are given in a related Technical Report.

Mobile nodes of the future will be equiped with multiple network interfaces to take advantage of overlay networks, yet no current mobility systems provide full support for the simultaneous use of multiple interfaces. The need for such support arises when multiple connectivity options are available with different cost, coverage, latency and bandwidth characteristics, and applications want their data to flow over the interface that best matches the characteristics of the data. We present an architecture called transport layer mobility that allows mobile nodes to not only change their point of attachment to the Internet, but also to control which network interfaces are used for the different kinds of data leaving from and arriving at the mobile node. We implement our transport layer mobility scheme using a split connection proxy architecture and a new technique called TCP splice that gives split connection proxy systems the same end to-end semantics as normal TCP connections

Bluetooth, an innovation in short-range radio technology, has gone through the first stage of standardization, and commercial products based on ν1.0 specifications will be appearing. While much work has gone into developing the radio technology and hardware for this system, little effort has been focused on additional infrastructure that is necessary for applications in this environment. We examine the issue of supporting ubiquitous computing applications (Weiser, 1991) in a Bluetooth network. The Bluetooth standard defines a multi-hop routing structure, called a scatternet, to address the limitations caused by short-range and small fanout of the underlying link technology. We identify several characteristics, the combination of which makes scatternets different from previously considered networks. Importantly, Bluetooth links are connection-oriented with low-power link modes. We show that the unique aspects of the technology require a redesign of the protocol structure for link formation, IP routing and service discovery. When existing approaches to these protocols are applied to scatternets, the multiple protocol layers would operate without knowledge of each other, resulting in inefficient use of power in many cases. We suggest an alternative approach where there is a single protocol layer providing a level of indirection within the scope of a scatternet. That is, we argue for extensive cross-layer optimizations

Wireless ad hoc networks have been a growing area of research. While there has been considerable research on the topic of routing in such networks, the topic of topology creation has not received due attention. This is because almost all ad hoc networks to date have been built on top of a single channel, broadcast based wireless media, such as 802.11 or IR LANs. For such networks the distance relationship between the nodes implicitly (and uniquely) determines the topology of the ad hoc network. Bluetooth is a promising new wireless technology, which enables portable devices to form short-range wireless ad hoc networks and is based on a frequency hopping physical layer. This fact implies that hosts are not able to communicate unless they have previously discovered each other by synchronizing their frequency hopping patterns. Thus, even if all nodes are within direct communication range of each other, only those nodes which are synchronized with the transmitter can hear the transmission. To support any-to-any communication, nodes must be synchronized so that the pairs of nodes (which can communicate with each other) together form a connected graph. Using Bluetooth as an example, this paper first provides deeper insights into the issue to link establishment in frequency hopping wireless systems. It then introduces the Bluetooth topology construction protocol (BTCP), an asynchronous distributed protocol for constructing scatternets which starts with nodes that have no knowledge of their surroundings and terminates with the formation of a connected network satisfying all connectivity constraints posed by the Bluetooth technology. To the best of our knowledge, the work presented in this paper is the first attempt at building Bluetooth scatternets using distributed logic and is quite “practical” in the sense that it can be implemented using the communication primitives offered by the Bluetooth 1.0 specifications

Bluetooth, a recent innovation in short-range radio technology, has gone through the first stage of standardization, and commercial products based on v1.0 specifications will be appearing soon. While much work has gone into developing the radio technology and hardware for this system, little effort has been focused on additional infrastructure that is necessary for applications in this environment. In this paper, we examine the issue of supporting ubiquitous computing applications [21, 14] in a Bluetooth network. The Bluetooth standard defines a multi-hop routing structure, called a scatternet, to address the limitations caused by short-range and small fanout of the underlying link technology. We identify several characteristics, the combination of which makes scatternets different from previously considered networks. Importantly, Bluetooth links are connection-oriented with low-power link modes. We show that the unique aspects of the technology require a redesign of the protocol stru...