Kuriacose Joseph

Packet CDMA versus dynamic TDMA for multiple access in an integrated voice/data PCN

A comparative evaluation of dynamic time-division multiple access (TDMA) and spread-spectrum packet code-division multiple access (CDMA) approaches to multiple access in an integrated voice/data personal communications network (PCN) environment are presented. After briefly outlining a cellular packet-switching architecture for voice/data PCN systems, dynamic TDMA and packet CDMA protocols appropriate for such traffic scenarios are described. Simulation-based network models which have been developed for performance evaluation of these competing access techniques are then outlined. These models are exercised with example integrated voice/data traffic models to obtain comparative system performance measures such as channel utilization, voice blocking probability, and data delay. Operating points based on typical performance constraints such as voice blocking probability 0.01 (for TDMA), voice packet loss rate 10/sup -3/ (for CDMA), and data delay 250 ms are obtained, and results are presented. >

Statistically based buffer control policies for constant rate transmission of compressed digital video

An investigation of statistically based approaches to the design of buffer control algorithms for interfacing a compressed digital video source to a constant rate channel is presented. As a first step to a quantitative methodology for the design and evaluation of adaptive buffer control algorithms, a detailed statistical characterization of the various encoding modes of an example broadcast quality intra/interframe differential-pulse-code-modulation (DPCM) algorithm is obtained from extensive simulation. The statistics presented include simple measures such as the overall encoded rate distributions for each encoding mode and more detailed intermode and intramode statistics. Two specific adaptive mode control algorithms are proposed. Using simulation over a large sea of representative images, the performance of the proposed statistically based algorithms is compared to that of a conventional buffer-level-based control heuristic which does not require source characterization. >

Performance evaluation of slotted ALOHA with generalized retransmission backoff

An analytical investigation is presented of generalized retransmission backoff policies for slotted-ALOHA random-access channels. Backoff techniques, of which the well-known exponential backoff is a special case, are based on adaptation of average retransmission delay as a function of the number of collisions experienced by each message accessing the contention channel. An analytical model applicable to slotted-ALOHA channels using general backoff functions has been developed and used to assess the performance advantages offered by either exponential backoff or alternative policies motivated by heuristic considerations. Numerical results for a sample satellite channel scenario are presented, demonstrating that the use of appropriate backoff policies can result in significant improvements in stable throughput-delay characteristics relative to those of nonadaptive systems. >

Throughput of unslotted direct-sequence spread-spectrum multiple-access channels with block FEC coding

An analysis of unslotted random-access direct-sequence spread-spectrum multiple-access (DS/SSMA) channels with block forward error correction (FEC) coding is presented. Extending a methodology that was introduced in an earlier paper on unslotted packet code-division multiple access (CDMA) without coding, a procedure for calculating the error probability of an L-bit packet in the variable message length, FEC-coded, DS/SSMA environment is described. This procedure is then used in conjunction with appropriate flow equilibrium traffic models to compute channel throughput. Using BCH block coding as an example, the analytical model is exercised to obtain throughput versus channel traffic curves over a range of code rates, leading to an assessment of maximum achievable throughput and the associated optimum FEC code rate. The results show that the use of block FEC coding provides a significant improvement in the bandwidth-normalized channel throughput (utilization), approaching values competitive with those for comparable narrowband ALOHA channels. >

Source traffic smoothing and ATM network interfaces for VBR MPEG video encoders

This paper describes approaches for shaping and smoothing traffic from VBR MPEG video encoders and interfacing to ATM networks, by introducing delay and rate buffering between the video encoding and decoding processes, and by flow controlling the data rate out of the encoder buffer into the network. Three algorithms for flow control are defined and compared in terms of their resulting traffic profile. The performance of these algorithms in the presence of a leaky bucket ATM network access controller is studied. The relationship between the leaky bucket algorithm parameters and VBR MPEG encoder parameters is discussed. It is shown that significant gains in performance can be obtained by appropriate selection of traffic smoothing algorithms at the output of a VBR MPEG encoder.

Performance of cellular packet CDMA in an integrated voice/data network

This paper presents a simulation-based study of cellular packet CDMA systems operating in an integrated voice/data traffic scenario. Spread-spectrum CDMA provides a suitable framework for resource-shared packet transport capable of combining isochronous (voice, ISDN) and bursty data services. In this work, a general network model for cellular packet CDMA with mixed voice/data traffic is described and used to evaluate the capacity/performance impact of several key system parameters. First, the effect of spreading factor (N) and forward error correction (FEC) rate are studied, confirming earlier work indicating a weak dependence onN and a well-defined optimum code rate in the range of 0.5–0.7 (with BCH coding). Next, the effect of propagation loss coefficient (γ) on network capacity is investigated over a range of possible assumptions forγ, including both constant and distance-dependent models. The results show that system capacity depends strongly onγ, varying by as much as a factor of 2 over the range of parameters considered. For a given distance-dependentγ assumption, performance results are also obtained for different cell sizes in order to understand the overall spatial reuse efficiency achievable in different cellular and microcellular scenarios. This is followed by an investigation of traffic source model effects: first the capacity improvement from voice activity detection VAD) is presented, showing the expected ∼2∶1 gains. Results for varying proportions of voice and data traffic intensities indicate that the operating efficiency does not change significantly as the proportion of bursty data relative to voice is varied.

Simulation models for performance evaluation of satellite multiple access protocols

Simulation models are described for the performance evaluation of satellite multiple-access protocols used in VSAT (very small aperture terminal) data networks. Since the design of VSAT-based wide-area networks for interactive applications is critically dependent on the delay-throughput characteristics of the channel access protocol used, detailed simulation models have been developed to augment analytical performance-evaluation methods. Simulation models for several candidate VSAT access protocols, including random-access techniques such as Aloha, selective reject Aloha and slotted Aloha and demand-assignment multiple-access based techniques, are discussed in terms of traffic-source models, access algorithms, and performance-evaluation criteria. Numerical results for a realistic interactive data VSAT network environment are presented. >

Packet transport of VBR interframe DCT compressed digital video on a CSMA/CD LAN

The feasibility of providing teleconference/picturephone video services over 10-Mb/s IEEE 802.3 CSMA/CD (carrier sense multiple access with collision detection) local area networks is examined. The system considered is based on variable-bit-rate (VBR) video codecs which produce an interframe discrete cosine transform/differential pulse code modulation (DCT/DPCM) codec output format compatible with the evolving CCITT H.261 standard for px64 kb/s motion video. Transport-level protocol functions (such as delay compensation, flow control, and error control) required to support real-time video at an acceptable quality level are identified and discussed. A simulation model used for assessing the performance of the LAN with several video and data users is described. The model provides quantitative measures of network performance (such as packet loss or delay vs. throughput). Preliminary performance evaluation results demonstrate that, with appropriate transport-level delay compensation, potentially acceptable video and data quality levels can be provided on a well-loaded ( approximately 55%) CSMA/CD LAN. With appropriate transport buffering, acceptably low video packet loss rates can be achieved, although the corresponding subjective quality has yet to be established.<<ETX>>

CDMA versus dynamic TDMA for access control in an integrated voice/data PCN

The authors present a comparative evaluation of dynamic TDMA and spread spectrum CDMA approaches to medium access control (MAC) in an integrated voice/data PCN environment. After a brief discussion of general technological issues related to the selection of a MAC approach for PCN, dynamic TDMA and packet CDMA protocols appropriate for the support of integrated voice/data traffic are described. Performance evaluation methodologies for determining the system capacities of these competing MAC techniques are described. System-level performance measures such as channel utilization, voice blocking probability and data delay are obtained from a detailed simulation model for the D-TDMA case, and compared with a preliminary set of corresponding results for asynchronous packet CDMA. Overall, the results show that while CDMA does achieve a higher normalized capacity than TDMA, the gains are very sensitive to the propagation loss constant, and must be balanced against higher message transfer delay for certain data services.<<ETX>>

Performance evaluation of cellular packet CDMA networks with transmit power constraints

A throughput analysis is presented of cellular spread spectrum CDMA packet networks with transmit power control limitations. Scenarios in which regulatory and/or equipment considerations limit the maximum transmitted power are considered with the objective of assessing the impact on system capacity. An analytical model is developed for an architecture in which uniformly distributed remote stations use asynchronous direct-sequence code division multiple access (DS-CDMA) to access base stations laid out in a square grid pattern. The model is used to assess the degradation in system performance caused by transmit power constraints, both in terms of a global capacity measure (average throughput per unit area) and a service uniformity measure (local packet success probability function).<<ETX>>