Pravin Bhagwat

Highly dynamic Destination-Sequenced Distance-Vector routing (DSDV) for mobile computers

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 RIP [5], to make 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.

MSOCKS: an architecture for transport layer mobility

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.

Enhancing throughput over wireless LANs using channel state dependent packet scheduling

Unlike wired networks, packets transmitted on wireless channels are often subject to burst errors which cause back to back packet losses. Most wireless LAN link layer protocols recover from packet losses by retransmitting lost segments. When the wireless channel is in a burst error state, most retransmission attempts fail thereby causing poor utilization of the wireless channel. Furthermore, in the event of multiple sessions sharing a wireless link, FIFO packet scheduling can cause the HOL blocking effect, resulting in unfair sharing of the bandwidth. This observation leads to a new class of packet dispatching methods which explicitly take the wireless channel characteristics into consideration in making packet dispatching decisions. We compare a variety of channel state dependent packet (CSDP) scheduling methods with a view towards enhancing the performance of the transport layer sessions. Our results indicate that by employing a CSDP scheduler at the wireless LAN device driver level, significant improvement in the channel utilization can be achieved in typical wireless LAN configurations.

Dynamic adaptation in an image transcoding proxy for mobile Web browsing

Transcoding proxies are used as intermediaries between generic World Wide Web servers and a variety of client devices in order to adapt to the greatly varying bandwidths of different client communication links and to handle the heterogeneity of possibly small-screened client devices. Such transcoding proxies can adaptively adjust the amount by which a data stream is reduced, using an aggressive lossy compression method (e.g., an image becomes less clear, text is summarized). We present an analytical framework for determining whether to transcode and how much to transcode an image for the two cases of store-and-forward transcoding as well as streamed transcoding. These methods require prediction of transcoding delay, prediction of transcoded image size (in bytes), and estimation of network bandwidth. We discuss methods of adaptation based on fixed quality as well as fixed delay (automated/dynamic transcoding). We conclude with a description of the practical adaptation policies that have been implemented in our adaptive image transcoding proxy.

Network layer mobility: an architecture and survey

We explore various network layer concepts that play a crucial role in the design of mobile networking systems. We show that mobility is essentially an address translation problem and is best resolved at the network layer. We describe services that must be supported at the network layer to carry out the task of address translation. Using these service primitives as building blocks, we describe a network-layer architecture which enables smooth integration of mobile end systems within the existing Internet. A summary of some of the key mobile IP proposals is presented, and it is shown that each proposal can be viewed as a special case of the architecture outlined in this article.

Declustering using fractals

A method for achieving declustering for Cartesian product files on M units is proposed. The focus is on range queries, as opposed to partial match queries that older declustering methods have examined. The method uses a distance-preserving mapping, the Hilbert curve, to impose a linear ordering on the multidimensional points (buckets); then, it traverses the buckets according to this ordering, assigning buckets to disks in a round-robin fashion. Because of the good distance-preserving properties of the Hilbert curve, the end result is that each disk contains buckets that are far away in the linear ordering, and, most probably, far away in the k-d address space. This is exactly the goal of declustering. Experiments show that these intuitive arguments lead to good performance: the proposed method performs at least as well as or better than older declustering schemes.<<ETX>>

Using channel state dependent packet scheduling to improve TCP throughput over wireless LANs

In recent years, a variety of mobile computers equipped with wireless communication devices have become popular. These computers use applications and protocols, originally developed for wired desktop hosts, to communicate over wireless channels. Unlike wired networks, packets transmitted on wireless channels are often subject to burst errors which cause back to back packet losses. In this paper we study the effect of burst packet errors and error recovery mechanisms employed in wireless MAC protocols on the performance of transport protocols such as TCP. Most wireless LAN link layer protocols recover from packet losses by retransmitting lost segments. When the wireless channel is in a burst error state, most retransmission attempts fail, thereby causing poor utilization of the wireless channel. Furthermore, in the event of multiple sessions sharing a wireless link, FIFO packet scheduling can cause the HOL blocking effect, resulting in unfair sharing of the bandwidth. This observation leads to a new class of packet dispatching methods which explicitly take wireless channel characteristics into consideration in making packet dispatching decisions. We compare a variety of channel state dependent packet (CSDP) scheduling methods with a view towards enhancing the performance of transport layer sessions. Our results indicate that by employing a CSDP scheduler at the wireless LAN device driver level, significant improvement in channel utilization can be achieved in typical wireless LAN configurations.

A routing vector method (RVM) for routing in Bluetooth scatternets

Bluetooth is a promising new technology for short range wireless connectivity between mobile devices. Initially, Bluetooth will be used as a replacement for point to-(multi)point cables. However, in due course of time, solutions for forming multihop ad hoc networks over Bluetooth (referred to as scatternets) will be needed. In this paper, we explore the design space of routing protocols over Bluetooth scatternets. We argue that ad hoc routing protocols for Bluetooth must make a different set off design compromises compared to those being developed by the MANET working group. These differences result primarily from the specific characteristics of the Bluetooth physical and link layer. Our proposed routing method is based on the concept of route vector, which is an efficient method for encoding source route paths in Bluetooth scatternets. We describe the protocols for route discovery and packet forwarding. Our design illustrates three main design compromises, namely minimization of soft-state, protocol simplicity, and bandwidth conservation, all of are crucial for efficient operation over small size Bluetooth scatternets.

DSDV routing over a multihop wireless network of mobile computers

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.

Effect of topology on performance of reliable multicast communication

The authors examine the performance implications of providing reliability in conjunction with multicast transport over a high speed wide area network. They use a block based acknowledgement and selective retransmission protocol to evaluate the impact of the loss rate and the multicast tree topology on the achievable throughput. Their results show that even when the buffer overflow probability at switches and receivers is low, the cumulative loss probability seen by a source may be quite high. They also demonstrate that the average throughput increases significantly if the transport protocol delivers packets to the application layer out-of-sequence. They investigate the scaling properties of the error control mechanism and show that the multicast tree topology that results in minimum transfer time is not necessarily the same as the one constructed using minimal bandwidth or shortest path algorithms.<<ETX>>