Mostafa A. Bassiouni

Data Compression in Scientific and Statistical Databases

Scientific and statistical database systems heavily depend on data compression techniques to make possible the management and storage of their large databases. The efficiency of data compression methods has a signficant impact on the overall performance of these systems. The purpose of this paper is to show the importance of data compression to scientific/statistical databases, to discuss the pros and cons of data compression, and to survey data compression techniques relevant to scientific/statistical databases. The emphasis is on the basic idea, motivation, and tradeoffs of each approach. Both software and hardware methods are covered. The paper is concluded by a discussion of several points of research that seem worthy of further investigation.

Improving fairness in optical-burst-switching networks

Approaches for dealing with the beat-down unfairness problem have been mainly limited to electronic packet-switching networks. We investigate two methods for alleviating this problem in optical-burst-switching (OBS) networks. When OBS is used with just-in-time or just-enough-time scheduling, the control packet transmitted ahead of the data burst may fail to secure a free channel, causing the data burst to be dropped at some node along its path. The burst dropping probability increases as the number of hops in the lightpath of the burst increases, resulting in unfairness to bursts with long paths. Our first scheme to deal with this problem is based on a simple equation that adjusts the size of the search space for a free wavelength based on the number of hops traveled by the burst. Our second scheme uses proactive discarding to reduce the probability of dropping bursts with large hop count at the expense of an increase in the dropping probability of bursts with small hop count. The scheme adapts the random-early-discard (RED) strategy to the OBS environment and prioritizes the levels of discarding on the basis of the length of the lightpath. The proactive burst discarding is done in the network access station (NAS) of the source node that generated the burst. Thus discarded bursts do not waste any bandwidth resources in the core of the optical network. Both schemes have simple logic, do not use preemption, do not require complex modification to the lightpath setup scheme, and alleviate the beat-down unfairness without negatively affecting the overall throughput of the system. Detailed performance evaluation of the two schemes is presented and analyzed.

Efficient decoding of compressed data

In this article, we discuss the problem of enhancing the speed of Huffman decoding. One viable solution to this problem is the multibit scheme which uses the concept of k‐bit trees to decode up to k bits at a time. A linear‐time optimal solution for the mapping of 2‐bit trees into memory is presented. The optimal solution is derived by formulating the memory mapping problem as a binary string mapping problem and observing that at most four different 4‐bit patterns can occur within any 2‐bit Huffman tree. In addition to reducing the processing time of decoding, the optimal scheme is storage efficient, does not require changes to the encoding process, and is suitable for hardware implementations.

Data compression in real-time distributed systems

A scheme that can be used to compress image and textlike data efficiently and to meet the real-time constraints of distributed simulation networks is presented. An initial transformation step is applied to image data in order to induce desirable properties that allow the efficient application of popular lossless text compression schemes to image data. The VLSI design of a new run-length encoding chip that can provide on-the-fly compression of images within real-time networks is described.<<ETX>>

A multi-group technique for data compression

An efficient compression technique that is particularly attractive for the storage of large commercial files and the transfer of such files within a distributed communication network is outlined. The technique, constructed as a two-level hierarchy of Huffman-type binary trees, is a reversible semantic-independent variable-length character encoding method that makes use of the group locality of character reference behavior and the variable frequency of occurrence of various characters within the different subgroupings of the character set. The compression efficiency and cost of the Multi-Group scheme has been found to be superior to the well-known Huffman encoding algorithm.

Handling Time in Query Languages

In this paper, an approach to handle time in relational query languages is outlined. The approach is based on extending Boolean and comparison operators by allowing their operands to be sets of intervals [BASS87]. The proposed temporal logic is shown to satisfy the properties of the normal Boolean logic. New syntax for retrieval statements is defined in order to separate the process of selecting entities (tuples) and the process of selecting required values of temporal attributes from the chosen entities. The extensions presented in this paper offer a good degree of flexibility in expressing different temporal requirements.

A logic for handling time in temporal databases

An approach to the handling of time in query languages of historical database management systems is outlined. The approach is based on extending Boolean and comparison operators by allowing their operands to be sets of time intervals. The proposed temporal logic is shown to satisfy the properties of the normal Boolean logic. A relational-calculus query language using the extended logic is proposed. A new syntax for retrieval statements is defined in order to separate the process of selecting entities (tuples) and the process of selecting required values of temporal attributes from the chosen entities. The extension presented in this paper offer a good degree of flexibility in expressing different temporal requirements.<<ETX>>

Transport control protocols for wireless connections

W/sup e/ study the effect of bit error rates and handoff disconnections on the performance of different TCP implementations (Tahoe, Reno, New-Reno, and SACK). Test scenarios with larger link-up periods, when the mobile is connected, but higher disconnection probability yielded better performance than scenarios with smaller link-up periods and smaller disconnection probability. Our performance tests show that the performance of TCP is more sensitive to the length of the link-up period than the error rate of the wireless link, or the mobile disconnection probability. While SACK TCP is well known to perform better than Reno TCP in wired networks, the tests show that this is not the case in mobile networks. The paper presents performance results for the four TCP implementations in a mobile wireless environment and provides comparisons based on important performance metrics (throughput, goodput, transfer time) and multimedia QoS measures (average packet delay).

Performance evaluation of position-based channel reservation for handoff of cellular calls

In this paper, we evaluate the performance implications of incorporating guard channels and reservation queuing into a position-based predictive channel reservation scheme. The paper also investigates the impact of the value of the distance threshold, a parameter used to control the timing of sending channel reservations and examines the benefits of reservation pooling, a mechanism introduced to reduce the negative impact of cell congestion on the QoS of ongoing calls. The simulation results give good insight into several design approaches that significantly improve the predictive scheme.

Generalized logical operators for temporal languages

Previous proposals of temporal languages have used the standard TRUE/FALSE model of Boolean and comparison operators. Since this standard binary model introduces ambiguity as to which time value is desired, expressing nontrivial temporal conditions in these languages can be quite complicated. The authors model overcomes this problem by generalizing the TRUE/FALSE model into one that uses sets of time intervals. The set-valued temporal logic satisfies the properties of the normal Boolean logic. The proposed logic can be very useful to different applications of temporal languages. Database query languages are used to demonstrate the effectiveness of the set-valued logic. The basic features of a relational-calculus query language based on the set-valued logic are presented. Applicability of the logic to relational-algebra query languages is discussed.<<ETX>>