Elan Amir

Improving TCP/IP performance over wireless networks

TCP is a reliable transport protocol tuned to perform well intraditional networks made up of links with low bit-error rates.Networks with higher bit-error rates, such as those with wirelesslinks and mobile hosts, violate many of the assumptions made byTCP, causing degraded end-to-end performance. In tbis paper, wedescribe the design and implementation of a simple protocol, calledthe snoop protocol, that improves TCP performance in wirelessnetworks. The protocol modifies network-layer software mainly at abase station and preserves end-to-end TCP semantics. The main ideaof the protocol is to cache packets at the base station and performlocal retransmissions across the wireless link. We have implementedthe snoop protocol on a wireless testbed consisting of IBM ThinkPadlaptops and i486 base stations communicating over an AT&TWavelan. Our experiments show that it is significantly more robustat dealing with unreliable wireless links as compared to normalTCP; we have achieved throughput speedups of up to 20 times overregular TCP in our experiments with the protocol.

An application level video gateway

The current model for multicast transmission of video over the Internet assumes that a fixed average bandwidth is uniformly present throughout the network. Consequently, sources limit their transmission rates to accommodate the lowest bandwidth links, even though high-bandwidth connectivity might be available to many of the participants. We propose an architecture where a video transmission can be decomposed into multiple sessions with different bandwidth requirements using an application-level gateway. Our video gateway transparently connects pairs of sessions into a single logical conference by manipulating the data and control information of the video streams. In particular, the gateway performs bandwidth adaptation through transcoding and rate-control. We describe an efficient algorithm for transcoding Motion-JPEG to H.261 that runs in real-time on standard workstations. By making the Real-time Transport Protocol (RTP) an integral component of our architecture, the video gateway interoperates with the current Internet video tools in a transparent fashion. We have built a prototype of the video gateway and used it to redistribute multi-megabit JPEG video seminars from the Bay Area Gigabit Network as 128 kb/s H.261 MBone sessions.

A network architecture for heterogeneous mobile computing

This article summarizes the results of the BARWAN project, which focused on enabling truly useful mobile networking across an extremely wide variety of real-world networks and mobile devices. We present the overall architecture, summarize key results, and discuss four broad lessons learned along the way. The architecture enables seamless roaming in a single logical overlay network composed of many heterogeneous (mostly wireless) physical networks, and provides significantly better TCP performance for these networks. It also provides complex scalable and highly available services to enable powerful capabilities across a very wide range of mobile devices, and mechanisms for automated discovery and configuration of localized services. Four broad themes arose from the project: (1) the power of dynamic adaptation as a generic solution to heterogeneity, (2) the importance of cross-layer information, such as the exploitation of TCP semantics in the link layer, (3) the use of agents in the infrastructure to enable new abilities and to hide new problems from legacy servers and protocol stacks, and (4) the importance of soft state for such agents for simplicity, ease of fault recovery, and scalability.

Receiver-driven bandwidth adaptation for light-weight sessions

Current Internet multicast conferencing tools treat all sources with equal importance in that they either statically allocate a fixed bandwidth to each source in a session, or they automatically adapt each source’s transmission rate independently of all other sources. But not all sources are of equal interest to all receivers. We believe that to effectively support human to human communication, this disparity in receiver interest should be reflected in the rate-adaptation process. To this end, we propose a protocol called “SCUBA” that enables media sources to intelligently account for receiver interest in their rate-adjustment algorithms. SCUBA is orthogonal to and complements existing rate-adaptation schemes and can interoperate with either senderor receiverdirected control systems. To scale the SCUBA protocol with multicast session size, we decouple the receiver-feedback process from the session size through sampling. This approach introduces a “tunable” tradeoff between convergence time and sampling accuracy that for large sessions is solely dependent on the control traffic bandwidth. In addition to its applicability in video conferencing, our control scheme can be combined with media transcoders to intelligently manage a bottleneck link at a well-known and fixed location in the network. We implemented SCUBA within our video conferencing tool vie and our media gateway rtpgw and feedback from their preliminary deployment indicates that the efficacy of the overall multimedia communication system has been greatly enhanced.

Efficient TCP over networks with wireless links

TCP is a reliable transport protocol tuned to perform well in traditional networks made up of wired links with stationary hosts. Networks with wireless links and mobile hosts violate many of the assumptions made by TCP, causing degraded performance. We describe a simple protocol that improves TCP performance by modifying network-layer software only at a basestation without violating end-to-end TCP semantics. The main idea is to cache packets at the basestation and perform focal retransmissions. Simulations of this protocol show that is it significantly more robust in the presence of multiple packet losses in a single transmission window as compared to TCP. This enables our protocol to tolerate at least 10 times as high an error rate without any performance degradation.

A layered DCT coder for Internet video

Several compression schemes for Internet video utilize block-based conditional replenishment (CR) where block updates are coded independently of the past. In the current Internet video tools, blocks are compressed with a single-layer representation. We propose a new approach that compresses image blocks using a layered representation. Our layered-DCT (LDCT) compression algorithm, derived from progressive JPEG, has been combined with CR and optimized for efficient software implementation to provide an improved solution for Internet packet video. Although LDCT is constrained to a layered representation, its compression performance is as good or better than the single layer Intra-H.261 and baseline JPEG coding schemes.

A low-power, lightweight unit to provide ubiquitous information access application and network support for InfoPad

Some of the most important trends in computer systems are the emerging use of multimedia Internet services, the popularity of portable computing, and the development of wireless data communications. The primary goal of the InfoPad project is to combine these trends to create a system that provides ubiquitous information access. The system is built around a low-power, lightweight wireless multimedia terminal that operates in indoor environments and supports a high density of users. The InfoPad system uses a number of innovative techniques to provide the high-bandwidth connectivity, portability, and user interface needed for this environment. The article describes the design, implementation, and evaluation of the software network and application services that support the InfoPad terminal. Special applications, type servers, and recognizers are developed for the InfoPad system. This software is designed to take advantage of the multimedia capabilities of the portable terminal and the additional computational resources available on the servers. The InfoNet system provides low-latency, high bandwidth connectivity between the computation and the portable terminal. It also provides the routing and handoff support that allows users to roam freely. The performance measurements of the system show that this design is a viable alternative, especially in the indoor environment.