Magda El Zarki

Error control schemes for networks: an overview

In this paper, we investigate the issue of error control in wireless communication networks. We review the alternative error control schemes available for providing reliable end-to-end communication in wireless environments. Through case studies, the performance and tradeoffs of these schemes are shown. Based on the application environments and QoS requirements, the design issues of error control are discussed to achieve the best solution.

Dynamic multi-path routing and how it compares with other dynamic routing algorithms for high speed wide area network

In this paper we describe briefly a dynamic multi-path routing scheme that has been considered for connection oriented homogeneous high speed networks. The fundamental objective of the scheme is to bridge the gap between routing and congestion control as the network becomes congested. Because propagation delay far out shadows queueing and transmission delay in high speed networks, the proposed routing scheme works as a shortest path (minimum hop) first algorithm under light traffic conditions. However as the shortest path becomes congested, the source node uses multiple paths when and if available in order to distribute the load and reduce packet loss. The scheme is a cross between Alternate Path routing and Trunk Reservation. We compare the performance of the proposed scheme with the Shortest Path Only algorithm, the Alternate Path routing algorithm, the Random Routing algorithm, and the Trunk Reservation scheme. The throughput and packet loss performance are compared via simulations. These have been carried out concentrating on a 5 node network with varying traffic patterns, the intention being to gain insight into the strengths and weaknesses of the various schemes.

Adaptive source rate control for real-time wireless video transmission

Hybrid ARQ schemes can yield much better throughput and reliability than static FEC schemes for the transmission of data over time-varying wireless channels. However these schemes result in extra delay. They adapt to the varying channel conditions by retransmitting erroneous packets, this causes variable effective data rates for current PCS networks because the channel bandwidth is constant. Hybrid ARQ schemes are currently being proposed as the error control schemes for real-time video transmission. An important issue is how to ensure low delay while taking advantage of the high throughput and reliability that these schemes provide for. In this paper we propose an adaptive source rate control (ASRC) scheme which can work together with the hybrid ARQ error control schemes to achieve efficient transmission of real-time video with low delay and high reliability. The ASRC scheme adjusts the source rate based on the channel conditions, the transport buffer occupancy and the delay constraints. It achieves good video quality by dynamically changing both the number of the forced update (intracoded) macroblocks and the quantization scale used in a frame. The number of the forced update macroblocks used in a frame is first adjusted according to the allocated source rate. This reduces the fluctuation of the quantization scale with the change in the channel conditions during encoding so that the uniformity of the video quality is improved. The simulation results show that the proposed ASRC scheme performs very well for both slow fading and fast fading channels.

Channel access and interference issues in multi-code DS-CDMA wireless packet (ATM) networks

Multi-Code Direct-Sequence Code-Division-Multiple-Access (MC-CDMA) has been proposed as a flexible multiple access scheme for wireless packet networks that support a large variety of mobiles with different and even time-varying rates. Using MC-CDMA, traffic streams with significantly different transmission rates can be easily integrated into a unified architecture, with all the transmissions occupying the same bandwidth and having the same spread spectrum processing gain. In this paper, we address medium-access and interference issues in MC-CDMA wireless packet networks. For medium access, we propose and study Multi-Code CDMA (MC-CDMA) with Distributed-Queueing Request Update Multiple Access (DQRUMA) to form a unified bandwidth-on-demand fair-sharing platform for multi-rate wireless services. DQRUMA is an efficient demand-assignment multiple access protocol for wireless access and scheduling. Pseudo-Noise (PN) codes (primary codes) and optimal power levels are allocated to the mobiles on a slot-by-slot basis, and a Maximum Capacity Power Allocation (MCPA) criterion exploits the sub-code concatenation property of the MC-CDMA transmission. Simulation results show that the system provides close to ideal-access performance for multi-rate mobiles, both with homogeneous traffic characteristics and with a mix of heterogeneous traffic characteristics. Finally, we analyze the effects of MC-CDMA intercell interference on the reverse link (i.e., mobile to cell site) and investigate interference reduction by using the Maximum Capacity Power Allocation (MCPA) criterion. Our results show significant reduction in reverse-link MC-CDMA intercell interference is possible using the MCPA criterion.

Analysis of error concealment schemes for MPEG-2 video transmission over ATM-based networks

When cell loss occurs, error concealment can play a critical role in recovering the viewing quality of impaired video. In this paper, we first present a Slice Interleaving (SI) algorithm which has been demonstrated to be able to effectively prevent vertical adjacent slice loss, thereby enhancing the performance of error concealment techniques that rely heavily on interpolation. Experimentation on approximately 100 video clips using a time-variant Markov Chain (MC) cell loss model that emulates a fluctuating bursty cell loss environment, allowed us to conduct a comprehensive and comparative study of error concealment mechanisms in different coding domains. We then designed a hybrid concealment algorithm, such that, for an arbitrary video sequence, several concealment mechanisms can be adaptively merged together to achieve the best performance. Simulation results show that our hybrid algorithm can effectively detect frames or macroblocks with scene change and/or excessive irregular motion, and adaptively switch to an appropriate concealment module to achieve the optimal video quality.

Routing in virtual path based ATM networks

The issue of routing requests for virtual circuits in a virtual path (VP) based asynchronous transfer mode (ATM) network is addressed. It is supposed that the VPs are partitioned in a manner such that all the virtual circuits (VCs) making use of any given VP have the same traffic characteristics and quality of service demands. A virtual circuit request can be accepted in the network only if the specified loss and delay bounds can be guaranteed for the cells belonging to it. After suitable simplifying assumptions about the operation of the network are made, simple admission rules for virtual circuit requests are proposed. Routing policies are then proposed that try to minimize the fraction of virtual circuit requests denied. Simulations are carried out to evaluate the proposed routing policies.<<ETX>>

A Dynamic Multi-Path Routing Algorithm for ATM Networks

In this paper we describe briefly a dynamic multi-path routing scheme that has been considered for connection oriented homogeneous high-speed wide area Asynchronous Transfer Mode (ATM) networks. The fundamental objective of the scheme is to bridge the gap between routing and congestion control as the network becomes congested. Because propagation delay far out shadows queueing and transmission delay in high-speed networks, the proposed routing scheme works as a shortest path (minimum hop) first algorithm under light traffic conditions. However as the shortest path becomes congested, the source node uses multiple paths when and if available in order to distribute the load and reduce packet loss. The scheme is a cross between Alternate Path Routing and Trunk Reservation.We compare the performance of the proposed scheme with the Shortest Path Only Algorithm, the Alternate Path Routing Algorithm, the Random Routing Algorithm, and the Trunk Reservation Scheme. The throughput and packet loss performance are compared via simulations. These have been carried out concentrating on a five-node network, each with varying traffic patterns, the intention being to gain insight into the strengths and weaknesses of the various schemes.

Transmitting scalable MPEG-2 video over ATM-based networks

For high-end real-time video services, a prioritized transmission scheme (PT) should match the video information distribution of the high priority and low priority bitstreams generated by a layered source coder. This paper studies two PT schemes, which are respectively coupled- concatenated transmission (CCT) and coupled-interlaced transmission (CIT). Simulation results reveal that: given the same amount of network resource, the CIT scheme outperforms both the CCT scheme and the non-layered scheme in terms of the delivered video quality and the network resource utilization.

MPEGTool: an X window based MPEG encoder and statistics tool

In this paper, we describe MPEGTool, an X window-based tool that can be used to generate a motion picture expert group (MPEG) encoded bit stream for video sequences and to study the statistical properties of the encoded data. The tool was designed to study the characteristics of variable bit rate video sources for transmission over asynchronous transfer mode (ATM) based broadband integrated services digital network (B-ISDN). The tool, which has a window-based graphic user interface, allows a user to specify several of the MPEG parameters such as the intraframe-to-interframe ratio and the quantizer scale. The tool also includes a statistical package that allows the user to plot graphs of various statistics including bit distributions, ATM cell distributions, time se ries, autocorrelation functions and cell interarrival times.

MPEG-2 encoded VoD series over fixed wireless channels using ARQ schemes

In this paper, we investigated the possibility and effectiveness of applying ARQ schemes to the transmission of MPEG-2 encoded video streams over fixed wireless links in B- FWANs in video-on-demand services. Simulations were conducted to evaluate the overall system performance, which is then compared to a scheme using FEC with cell interleaving. Important metrics relevant to the broadband video services are analyzed, including the quality of the reconstructed video sequences, the excess delay and delay variation introduced by the ARQ scheme, the throughput, and the required average channel bandwidth. A hidden Markov model was also introduced to model the variation of the wireless channel.