Richard O. LaMaire

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.

Two-dimensional round-robin schedulers for packet switches with multiple input queues

Presents a new scheduler, the two-dimensional round-robin (2DRR) scheduler, that provides high throughput and fair access in a packet switch that uses multiple input queues. We consider an architecture in which each input port maintains a separate queue for each output. In an N/spl times/N switch, our scheduler determines which of the queues in the total of N/sup 2/ input queues are served during each time slot. We demonstrate the fairness properties of the 2DRR scheduler and compare its performance with that of the input and output queueing configurations, showing that our scheme achieves the same saturation throughput as output queueing. The 2DRR scheduler can be implemented using simple logic components, thereby allowing a very high-speed implementation. >

On the randomization of transmitter power levels to increase throughput in multiple access radio systems

To enhance the throughput of a slotted random access protocol in a radio communication system, we describe the use of a scheme in which multiple power levels are used at the transmitters. We first consider a situation in which n transmitters are simultaneously trying to send a packet to a central receiving station using a time‐slotted access protocol, like slotted ALOHA. Each of these transmitters randomly chooses one of m discrete power levels during each attempt to send a packet. One of the simultaneously sent packets can often be successfully received due to the power capture effect. We consider two types of capture models: (1) one in which the transmitter with the largest received power captures the channel, and (2) one in which the transmitter captures the channel only if its signal‐to‐interference ratio is above some threshold when received at the central station. In this paper, we determine the optimal transmit probabilities for the power levels as well as the optimal values of the power levels themselves, when their range is constrained and for cases both with and without Rayleigh fading. After determining the precise optimal power levels and probabilities for maximizing the capture probabilities (i.e., for a given n), we propose a less complex, but nearly optimal, approximate approach based on using logarithmically equi‐spaced levels. After demonstrating the closeness of our suboptimal results to the optimal results, we apply our approach to the problem of optimizing the throughput of the slotted ALOHA protocol for a case in which the input traffic is generated according to a Poisson process. Several numerical examples are presented along with a demonstration of how the optimal choice of power levels and probabilities can enhance throughput relative to previous ad hoc methodologies.

Distributed topology construction of Bluetooth wireless personal area networks

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.

A frequency-domain estimator for use in adaptive control systems

The paper presents a frequency-domain estimator which can identify both a nominal model of a plant as well as a frequency-domain bounding function on the modeling error associated with this nominal model. This estimator, which we call a robust estimator, can be used in conjunction with a robust control-law redesign algorithm to form a robust adaptive controller.

Effect of correlation in diversity systems with Rayleigh fading, shadowing, and power capture

With the growth of wireless personal communications networks and wireless local area networks (WLANs), the need for increased reliability of the radio link has become evident. The use of diversity techniques, such as dual receiving antennas, helps mitigate the effect of multipath fading in both the in-building and land mobile radio environments. A significant issue in the design of such systems is the degree to which correlation between the two or more diversity signals can be tolerated. In this paper, we consider the use of diversity techniques in radio systems that are subject to correlation. Rayleigh fading, lognormal shadowing, and the radio capture effect. In the presence of two simultaneously transmitting stations, the throughput, conditioned on the local-mean power, is determined exactly for the case of a dual diversity receiving station. The insight gained from the two-station analysis is used to develop an accurate approximation for cases with more than two stations. The degree to which correlation can be tolerated without significant performance loss relative to the case of independent diversity signals is quantified, as are the effects of different system parameters (i.e., the capture ratio, power roll-off coefficient, and the amount of shadowing). Furthermore, the relationship between the envelope and power correlation coefficients is presented. An application of the capture results to the slotted ALOHA protocol is also included.

An M/G/1 vacation model of an FDDI station

The author defines and analyzes an M/G/1 vacation model that can be used to describe a single station in the fiber distributed data interface (FDDI). The M/G/1 model uses a service discipline called the exhaustive limited with limit variation discipline. According to this discipline, the server provides service until either the system is emptied or a randomly chosen limit of l frames has been served. The server then goes on a vacation before returning to service the queue again. The model can be used to gain insight into how the varying (timer-controlled) limit on the number of frames that can be transmitted during token visit at a station affects the mean waiting time in the timed-token protocol of FDDI. The analytical results of the M/G/1 vacation model are applied to an FDDI simulation example. >

Analysis of a wireless MAC protocol with client-server traffic and capture

We analyze an efficient medium access control (MAC) protocol for use in a single cell of a wireless local area network (LAN). A fixed frame structure with two periods is used; one period for making reservations using a slotted Aloha protocol and a second period for data transmission. The MAC protocol operates in a centralized manner in which a single station, the base station, accepts reservations (transmission or data requests) that are made by the remote stations, and then schedules the times in which they will transmit or receive data. We consider two different types of client-server traffic models, an open-loop and a closed-loop model. In the open-loop model, a remote station can generate reservation requests for the base station whether or not it has received a response from the base station. In the closed-loop case, the remote station waits for a response before generating a new request. The performance of the MAC protocol was analyzed exactly for both the open and closed-loop traffic models and for cases in which transmission errors and a mean-value type of radio capture model are included. In addition, different policies were considered for the transmission strategy that is used by the remote stations when they attempt to make reservations. We derive exact results for the mean throughput and waiting times as well as for the queue length distributions. We also derive an approximate Markov chain to treat a case in which a fixed-position capture model is used. Several types of behavior are illustrated through the use of numerical examples. >

Design issues in wireless LANs

Wireless Local Area Networks (LANs), while similar in some respects to wired LANs, have many differences that impact their design. The key differences are the unreliable natnre of wireless communication and the mobility of stations enabled by their wireless capability. Various design alternatives for wireless LANs are discussed and the advantages and disadvantages of each choice are argued. The discussion is restricted to the design alternatives that are impacted by the wireless media and the mobile natnre of stations for the physical layer, Medium Access Control (MAC) protocol, topology choices for the connectivity of stations, and the networking layer. The choices that have been made in current systems are briefly described imd the potential advantages of alternative designs are discussed.

Analysis of a wireless MAC protocol with client-server traffic

An efficient medium access control (MAC) protocol for use in a single cell of a wireless local area network (LAN) is analyzed. A fixed frame structure with two periods is used, one period for making reservations using a slotted Aloha protocol and a second period for data transmission. The MAC protocol operates in a centralized manner in which a single station, the base station, accepts reservations (transmission or data requests) that are made by the remote stations, and then schedules the times in which they will transmit or receive data. Two different types of client-server traffic models, open loop and closed loop, are considered. Exact results are derived for the mean throughput and waiting times as well as for the queue length distributions. Several types of behavior are illustrated by numerical examples.<<ETX>>