Mark W. Garrett

Joint source/channel coding of statistically multiplexed real-time services on packet networks

The interaction of congestion control with the partitioning of source information into components of varying importance for variable-bit-rate packet voice and packet video is investigated. High-priority transport for the more important signal components results in substantially increased objective service quality. Using a Markov chain voice source model with simple PCM speech encoding and a priority queue, simulation results show a signal-to-noise ratio improvement of 45 dB with two priorities over an unprioritized system. Performance is sensitive to the fraction of traffic placed in each priority, and the optimal partition depends on network loss conditions. When this partition is optimized dynamically, quality degrades gracefully over a wide range of load values. Results with DCT encoded speech and video samples show similar behavior. Variations are investigated such as further partition of low-priority information into multiple priorities. A simulation with delay added to represent other network nodes shows general insensitivity to delay of network feedback information. A comparison is made between dropping packets on buffer overflow and timeout based on service requirements. >

Distance Realization Problems with Applications to Internet Tomography

In recent years, a variety of graph optimization problems have arisen in which the graphs involved are much too large for the usual algorithms to be effective. In these cases, even though we are not able to examine the entire graph (which may be changing dynamically), we would still like to deduce various properties of it, such as the size of a connected component, the set of neighbors of a subset of vertices, etc. In this paper, we study a class of problems, called distance realization problems, which arise in the study of Internet data traffic models. Suppose we are given a set S of terminal nodes, taken from some (unknown) weighted graph. A basic problem is to reconstruct a weighted graph G including S, with possibly additional vertices, that realizes the given distance matrix for S. We will first show that this problem is not only difficult bu the solution is often unstable in the sense that even if all distances between nodes in S decrease, the solution can increase by a factor proportional to the size of S in the worst case. We then proceed to consider a weaker version of the realization problem that only requires the distances in G to upper bound the given distances. We will show that this weak realization problem is NP-complete and that its optimum solutions can be approximated to within a factor of 2. We also consider several variants of these problems and a number of heuristics are presented. These problems are of interest for monitoring large-scale networks and for supplementing network management techniques.

Rate constraints for video transmission over ATM networks based on joint source/network criteria

In this paper, we study the rate constraints required for variable rate video transmission over atm networks. Our objective is to achieve both good source quality and efficient network utilization. We show that these objectives may not be achieved simultaneously unless both network and source coding considerations are taken into account. In particular, we show that, given a rate constraint, a greedy source coding strategy will reduce the potential for statistical multiplexing gain in the network. We propose two alternative methods to achieve our goals. The first method requires non-greedy source coding techniques, implementable through rate control, such that video encoders will only use the bit rate needed to achieve a certain, nearly constant quality level. As a consequence, low activity scenes will use a fraction of the maximum allowable bit rate. The second method calls for increasing the number of rate constraints imposed on each connection so that the amount of bandwidth used in the worst case (it by greedy coders) is limited. Experimental results for a medium length (5 min) video sequence are given.RésuméDans cet article les contraintes devant être imposées sur le débit de transmissions vidéo sur réseaux atm sont étudiées. Ľ objectif est ďobtenir une bonne qualité pour la source, ainsi qu’une utilisation efficace du réseau. Ces deux objectifs ne peuvent pas être atteints si les critères de source et de réseau ne sont pas pris en compte simultanément. En particulier, un codage de source glouton réduit le potentiel pour le gain de multiplexage statistique. Deux methodes pour atteindre ce but sont proposées. Une première methodefait appel à un codage de source non glouton, tel que ľ objectif du codeur vidéo est de maintenir une qualite constante. Dans ce cas, les scènes contenant peu ďactivité utiliseront un nombre de bits réduit. La deuxième méthode consiste à augmenter le nombre de contraintes imposées sur le débit vidéo de façon à limiter le nombre de bits utilisés dans le pire cas (c’ est-à-dire dans le cas du codeur glouton). Des résultats expérimentaux pour une séquence vidéo de durée moyenne (5 min) sont présentés.

Lightwave Gigabit Packet Communication Networks

The capacity of fiber optic systems is generally limited by electronics rather than the fiber itself. Recent advances in electronics technologies, notably Gallium Arsenide, has increased the potential bandwidth for LANs to several Gigabits per second. This high speed together with new protocols and architectures will allow true integration in a packet switched network, of video and high-speed data (e.g. LAN interconnection) with lower rate data and voice traffic. We discuss research efforts on Gigabit/second packet network prototypes and briefly mention the emerging technologies that will impact such experiments.