Ahmed K. Elhakeem

Performance analysis of ad hoc wireless LANs for real-time traffic

Compelling features of wireless local area networks (WLANs), put a variety of wireless service demands in place. In order to adjust system parameters to fulfill specific needs of different applications, a mathematical description of the system turns to be helpful. The inherent complexity of the wireless access, makes this description very challenging. We propose a new performance model for the IEEE 802.11 WLAN in ad hoc mode. The ad hoc mode has been chosen since we eventually aim at interconnected WLAN clusters where no base station exists. The model is based on the presentation of the system with a pair of one-dimensional state diagrams which can easily accommodate variations of many input parameters. The corresponding state variables are contention window size and buffer occupancy of each user in the system. The input parameters considered are: packet fragmentation factor, buffer size, and maximum allowable number of retransmissions. However, the approach taken is capable of ingesting many other probable parameters of interest. System performance criteria under study are: throughput, delay, and probability of fail to deliver. The last two are crucial for real-time applications.

Delay and throughput characteristics of TH, CDMA, TDMA, and hybrid networks for multipath faded data transmission channels

The new concepts of adaptive time hopping and variable frame code division (CDMA) multiple access are introduced. By a unified analysis, the probabilities of bit and packet errors in multipath fading environment for five time division (TDMA), code division, and time hopping (TH) related multiaccess networks are obtained; namely, TDMA, CDMA, CDMA/TDMA, Adaptive CDMA/TH, and variable frame CDMA/TDMA networks. The delay and useful throughputs of the five systems are also evaluated for data and voice traffic. All systems compared have the same channel power and bandwidth and support the same traffic. Though implementation issues are not covered, CDMA systems are put at a disadvantage (compared to cellular-type FDMA networks, for example) by ignoring such inherent advantages as voice silence utilizations and automatic frequency reuse. Nontheless, two CDMA systems outperform TDMA systems at low and medium input traffics. >

Traffic analysis of a local area network with a star topology

Two forms of fast circuit switching are modeled and studied through mathematical analysis and Monte Carlo simulation. The two examples described use the switching technique in an optical-fiber-based local area network with a star topology. The technique is compatible with time-division multiplexing techniques that are used for a range of traffic classes. Further, the technique trades transmission capacity for processing power, which is the critical limitation of the system. By means of a certain independence assumption, the first form of fast circuit switching is modeled as an M/G/1 queue. The results of the analysis show excellent agreement with simulation. The general result is that there is good system throughput, despite simplicity of processing. The second form, in which rather than first-come first-served the discipline is to search for a message whose destination queue is free, is studied by means of simulation alone. The results indicate an improved performance with a modest increase in processing power. >

Performance of ATM networks under hybrid ARQ/FEC error control scheme

In ATM networks, fixed-length cells are transmitted. A cell may be discarded during the transmission due to buffer overflow or detection of errors. Cell discarding seriously degrades transmission quality. This paper analyzes a hybrid automatic repeat request/forward error control (ARQ/FEC) cell-loss recovery scheme that is applied to virtual circuits (VCs) of ATM networks. FEC is performed based on a simple single-parity code, while a Go-Back-N ARQ is employed on top of that. Both throughput efficiency and reliability analysis of the hybrid scheme are presented. In the process we investigate the interactive effects of the network parameters (number of transit nodes, traffic intensity, ARQ packet length, ...) on the performance. The analysis provides a method for optimizing the FEC code size for a given network specification.

Congestion control in signalling free hybrid ATM/CDMA satellite network

We pursue a performance analysis for computing the various performance criteria in a hybrid time division/asynchronous transfer mode/code division multiple access network, i.e. TDMA/ATM/CDMA network. Users accessing this TDMA/ATM/CDMA uplink frame are assumed to belong to one of 4 service classes, namely video, voice, file and interactive data. Each user accesses only a portion of the subframe slots assigned to its class. A variable frame boundary strategy is used to adjust the subframe boundaries depending on the call load. To alleviate congestion in the assumed hubless signalling free satellite network, the satellite measures the uplink traffic of each class and issues pilot congestion control indicators to on-going calls of each class. These will be subsequently used by ground users to control their activities and police their calls using modified versions of leaky bucket and virtual leaky bucket congestion control techniques. The new techniques alleviate many of difficulties of specific slot assignment, onboard call management, superframe counting and management involved in the state of the art TDMA based systems, and yield a call establishment free yet dynamic and very well controlled access technique.

Performance evaluation of multihop ad hoc WLANs

Ongoing technological advances in portable devices, coupled with the need for continuous connectivity while mobile, have made ad hoc networks a compelling research and development topic, particularly in a challenging multimedia multihop scenario. The ability of IEEE 802.11s ad hoc mode of operation, as a dominating wireless local area network (WLAN) protocol, to serve multihop networks requires thorough investigation. In this article, through considering crucial real-life physical phenomena and avoiding as many confining assumptions as possible, system performance measures such as delay and packet failure rate are evaluated. As a result, the importance of adequate selection of the system parameters toward performance improvement is underscored. Moreover, the simulation results imply that by complementing through priority provisions, coordination, route reservation, clustering, and optimum channel coding considerations, the IEEE 802.11 medium access control (MAC) protocol can survive in a multihop scenario. The custom simulation environment developed features modularity, comprising traffic generator, mobility, wireless channel, and IEEE 802.11 protocol modules, and is capable of accommodating many more of the physical phenomena involved.

On Some Feature Selection Strategies for Spam Filter Design

Feature selection is an important research problem in different statistical learning problems including text categorization applications such as spam email classification. In designing spam filters, we often represent the email by vector space model (VSM), i.e., every email is considered as a vector of word terms. Since there are many different terms in the email, and not all classifiers can handle such a high dimension, only the most powerful discriminatory terms should be used. Another reason is that some of these features may not be influential and might carry redundant information which may confuse the classifier. Thus, feature selection, and hence dimensionality reduction, is a crucial step to get the best out of the constructed features. There are many feature selection strategies that can be applied to produce the resulting feature set. In this paper, we investigate the use of hill climbing, simulated annealing, and threshold accepting optimization techniques as feature selection algorithms. We also compare the performance of the above three techniques with the linear discriminate analysis. Our experiment results show that all these techniques can be used not only to reduce the dimensions of the e-mail, but also improve the performance of the classification filter. Among all the strategies, simulated annealing has the best performance which reaches a classification accuracy of 95.5%

Analysis of a movable boundary random/DAMA accessing technique for future integrated services satellites

In this paper, we pursue a performance analysis of a movable boundary accessing technique for a future integrated services multibeam satellite. We consider the integration of four user services video, voice, file data and interactive data using both random access and demand assigned access along with a moving boundary policy, to ensure that a maximum utilization of the frame capacity is achieved. It is shown that the potential user population is substantially increased with the use of a moving boundary policy and that minimal overhead is required to accommodate tens of thousands of users in the uplink spotbeam.<<ETX>>

Modified SUGAR/DS: a new CDMA scheme

The authors prove by analysis the possibility of gaining a minimum of 2 dB in signal-to-noise ratio by just, splitting the users to three or more groups and identifying each one by an orthogonal waveform (on top of his short Gold code) in a PSK/DS spread-spectrum network. User signals may arrive in a code-asynchronous fashion at the receiver, however, it is shown that by using the scheme the average code cross-correlation is minimal compared to the classic code division multiple access (CDMA) system. Both the chip-synchronous and asynchronous cases are investigated and the uniform and optimal cases of dividing the users into orthogonal groups are analyzed. The superior bit error and network data throughput results in the different fading and forward error correction environments make the system a strong candidate for competitive domestic applications. >

On Data Distortion for Privacy Preserving Data Mining

Because of the increasing ability to trace and collect large amount of personal information, privacy preserving in data mining applications has become an important concern. Data perturbation is one of the well known techniques for privacy preserving data mining. The objective of these data perturbation techniques is to distort the individual data values while preserving the underlying statistical distribution properties. Theses data perturbation techniques are usually assessed in terms of both their privacy parameters as well as its associated utility measure. While the privacy parameters present the ability of these techniques to hide the original data values, the data utility measures assess whether the dataset keeps the performance of data mining techniques after the data distortion. In this paper, we investigate the use of truncated non-negative matrix factorization (NMF) with sparseness constraints for data perturbation.