Showing papers by "Joe F. Chicharo published in 2000"

Unfairness and capture behaviour in 802.11 adhoc networks

Abstract: We address issues with the performance of IEEE 802.11, when used in the adhoc mode, in the presence of hidden terminals. We present results illustrating the strong dependence of channel capture behavior on the SNR observed on contending hidden connections. Experimental work has illustrated that in a hidden terminal scenario, the connection having the strongest SNR is able to capture the channel, despite the use of the RTS-CTS-DATA-ACK 4-way handshake designed to alleviate this problem. Our results indicate that the near-far SNR problem may have a significant effect on the performance of an adhoc 802.11 network.

Computing running DCTs and DSTs based on their second-order shift properties

Abstract: This paper presents a set of second-order recursive equations which are referred to as the second-order shift (SOS) properties of the discrete cosine transform (DCT) and the discrete sine transform (DST). The proposed SOS properties enable independent updating of the respective DCT and DST coefficients. This is in direct contrast with existing methodology for computing the running DCT and DST where there is an inherent interdependency between the DCT and DST coefficients. The SOS properties provide more efficient algorithms in terms of computational burden and memory requirements when implementing running DCTs and DSTs.

A new waveform interpolation coding scheme based on pitch synchronous wavelet transform decomposition

Abstract: This correspondence uses a pitch synchronous wavelet transform (PSWT) as an alternative characteristic waveform decomposition method for the waveform interpolation (WI) paradigm. The proposed method has the benefit of providing additional scalability in quantization than the existing WI decomposition to meet desired quality requirements. The PSWT is implemented as a quadrature mirror filter bank and decomposes the characteristic waveform surface into a series of reduced time resolution surfaces. Efficient quantization of these surfaces is achieved by exploiting their perceptual importance and inherent transmission rate requirements. The multiresolution representation has the additional benefit of more flexible parameter quantization, allowing a more accurate description of perceptually important scales, especially at higher coding rates. The proposed PSWT-WI coder is very well suited to high quality speech storage applications.

On the INFOMAX algorithm for blind signal separation

Abstract: This paper provides an analytical examination of the INFOMAX algorithm and establishes its effectiveness for blind signal separation using extensive simulation results. Results obtained show that the INFOMAX is not able to separate signal sources unless signal pre-processing is carried-out whereby the data to train the separating matrix is decorrelated. Further, results also show that if one uses the decorrelation pre-process alone it is able to effectively separate signal sources in many instances. Hence we conclude that the INFOMAX algorithm may not be a useful approach for signal separation.

Linear prediction incorporating simultaneous masking

Abstract: Whilst linear prediction is the cornerstone of most modern speech coders, few of these coders incorporate the perceptual characteristics of hearing into the calculation of the linear predictor coefficients (LPCs). This paper proposes a method of incorporating simultaneous masking into the calculation of the LPCs. This modification requires only a modest increase in computational complexity and results in the linear predictor removing more perceptually important information from the input speech signal. This results in a filter that better models the formants of the input speech spectrum. The net effect is that an improvement in quality is achieved for a given bit rate or alternately a bit rate reduction can be achieved while maintaining perceived quality. These results have been confirmed through subjective listening tests.

Differentiated service performance analysis

Abstract: Differentiated service (DiffServ) has been proposed as an alternative for Integrated service. It aims to provide the same service to a group of flows that have similar quality of service requirements. Assured forwarding (AF) and expedited forwarding (EF) are two proposals for DiffServ provision. We present a performance analysis of an N drop-precedences threshold dropping (TD) queue, which is one of the proposed mechanisms for AF. In this analysis, traffic flows are assumed Poisson with exponentially distributed service time. We present simulation results that verify the analysis. This paper is an extension of the work attempted by Bolot et al. (see Proceedings of IEEE INFOCOM, 1999) and Sahu (see Umass CMPCSI Technical Report 99-09, University of Massachusetts) since it considers the general case with multiple classes of flow. We also show that the Poisson based analysis can be shown to hold for aggregation of bursty Markov sources in some cases and not to hold in others.

Connection admission control in micro-cellular multi-service mobile networks

Abstract: This paper investigates the use of fixed bandwidth reservation for multi-service mobile networks. In particular it extends previous fixed multi-service network results to the mobile scenario and presents analytical and simulation results for new and handover call blocking probabilities. It also investigates the performance sensitivity to different traffic load ratios as well as the cell dwell time distribution.

Label switching using the IPv6 address hierarchy

Abstract: Current label switching protocols can use routing, address, and address hierarchy information to group flows for cut-throughs that bypass IP forwarding This paper examines a label switching solution that uses the IP version 6 (IPv6) address structure to classify and cut-through flows based on address hierarchy The performance of this approach is examined using actual backbone traffic traces with associated hierarchical address information obtained from Internet address registries, routing arbiter databases and route servers This hierarchical address information is used to map a hierarchical address structure over the packet level trace We investigate the relationship between aggregation bit-mask size versus label switching performance We show that aggregation greater than IPv6 destination site address does not significantly improve performance Our trace driven simulation studies show that it is possible to reduce the network layer packet forwarding requirements to below 015% of total packets at boundary routers within the core network by using IPv6 address hierarchy linked label switching

An examination of IP/ATM cut-through forwarding in dynamically routed networks

Abstract: Multiprotocol over ATM (MPOA), IP switching and multiprotocol label switching (MPLS) have distinctly different mechanisms for cut-through packet forwarding. MPOA and IP switching use flow-based cut-through (FBC) forwarding while MPLS uses routing table linked cut-through forwarding (TLC). This paper examines the sensitivity of each these cut-through forwarding mechanisms to changes in underlying routing tables. We examine a scenario where a congestion-sensitive dynamic routing protocol, such as OSPF optimised multipath, leads to frequently changing routing tables. We show that FBC forwarding reacts significantly worse than flow length distributions predict, taking up to 1200 seconds to react to route changes and forward at least 50% of packets on the new route. Flow characteristics are examined to determine ways to improve FBC sensitivity. We show that implementing a maximum flow length of 200 seconds improves the response to route changes significantly with a minimal decrease in the number of switched packets (1.5%).

Performance analysis of QoS mechanisms in IP networks

Abstract: Integrated services IP networks are expected to provide a variety of services with differentiated QoS. This requires the implementation of mechanisms that can discriminate service classes in terms of QoS. The IETF has recently proposed a differentiated services (Diffserv) framework for provision of QoS. In this paper we analyse the performance of two Diffserv mechanisms: threshold dropping and priority scheduling in terms of packet loss and mean packet delay. A comparison of the two mechanisms is carried out with the requirement that both mechanisms provide the same level of packet loss for the preferred flow. This comparison extends the results reported in the literature for these two mechanisms. In particular, in this paper we determine the impact of buffer threshold and buffer size on packet loss and mean packet delay in these mechanisms.

An improved channel model for ADSL and VDSL systems

Abstract: This paper examines existing channel models used with xDSL systems and identifies a key shortcoming - namely, the implicit assumption that all impulse noise originates at the transmitter. Based on extensive data collected from the local loop, a new model is proposed which addresses this problem by combining a digital filter model of the transmission line with a distributed noise source. This better reflects the nature of a real telephone line, and thus provides a more solid basis for simulation and optimisation of xDSL systems.

IP forwarding alternatives in cell switched optical networks

Abstract: Optical switching will enable core Internet packet switching to scale with future transmission rate increases. Currently proposed optical ATM switches do not allow packet reassembly, which is necessary for packet level forwarding. This results in the requirement to create end to end ATM virtual connections for flows even if they contain only one packet. In electronically switched networks MPOA and MPLS allow both cell and packet level forwarding to overcome this problem. This paper examines the feasibility of implementing such protocols over an optically switched network. Two different architectures are examined: use of an adjunct electrical router; and native optical packet reassembly. An examination of the optical reassembly buffer requirements show that the use of MPLS will require significantly more buffering than MPOA.