Access control has been an important security issue in information systems. Multilevel hierarchical information access widely exists in present-day government, military, and business applications. Extending access control design to work in a hierarchical environment is natural and necessary but rarely addressed so far in the literature. In this paper, a dynamic group-oriented cryptographic scheme to access a multilevel data hierarchy is proposed. In the proposed scheme, a trusted central authority is in charge of the administrative activities among the organization hierarchy. At the beginning, each user class submits its associated information and a cryptographic key of its preference to the central authority. Next the central authority generates a public information for each class according to their location in the organization hierarchy. The cryptographic key held by each class can be used directly as an encryption key to encipher data. These keys need not be modified when adding/deleting a class to/from the system. Compare with other existing schemes, ours has the advantages of flexibility in choosing user preferred cryptographic keys, cryptographic keys not exceeding a fixed length, reduced storage space in publishing pubic information, and protection from conspiracy attack.

We propose a distributed operation system platform for optical cable networks. This distributed platform is an extension of the previously proposed platform for a flexible cable network operation. The concept of the unit platform has been proposed for the distributed operation system platform. By using this concept, we discuss the system upgrade including the connection to other operation systems. We use an object-oriented software technology for designing the distributed operation system platform. The prototype system has been constructed using C++ programing language and the evaluated results are shown.

This paper investigates the problems which inhibits the use of today's WDM networks. These are propagation delay, packet processing overhead, bit & frame synchronization, and tuning latency. So far, these problems, especially propagation delay, have been ignored in most performance analysis papers. They have always hindered network designers, but they are magnified by the order of magnitude increase in speed of optical communications systems as compared to previous media. This paper examines the impact of the propagation delay on the performance of WDM protocols with variations in the number of channels, packet length and system size, specifically in two reservation based protocols with control channels and two pre-allocation protocols without the control channels. Also the impact of three delay factors (packet processing overhead, bit & frame synchronization and tuning latency) are studied with different propagation delay parameters. In reservation protocols, each node has one agile transmitter and two receivers; one of them is fixed and the other one is agile. The fixed receiver continuously monitors the control channel, receives all control packets, and updates their own status tables in order to track the availability of the other nodes as a target and data channels to avoid the destination collisions and the data channel collisions, respectively. In pre-allocation protocols, each node has a tunable transmitter, a fixed or slow tunable receiver, and its own home channel to receive the packets. The performance of this protocol is evaluated through the discrete-event simulation in terms of the average packet delay and network throughput.

In this paper, we describe a distributed paged-object server to efficiently support the storage management for new generation database management systems. This storage server is based on distributed client/server architecture, and allows clients to directly map database files onto distributed shared virtual memory (DSVM). In this architecture, there is a server at each site and the server only supports the clients in the same site. This improves performance utilizing client machine resources and offloading the shared resourcer server machine and the network. Therefore, this architecture may avoid drawbacks such as bottleneck in a traditional centralized client/server architecture, and also may reduces network traffic and improve performance efficiency of remote file access using Net File System (NFS). Moreover, this architecture allows distributed shared objects to reside and execute anywhere and to be used by any clients on the network. A prototype system (called WAKASHI/D) is implemented under the Mach operating system. The distributed, shared, and transactional virtual memory that the system supports is either volatile or persistent and they can be accessed by user applications in a uniform way. This paper also presents a performance evaluation and analysis of WAKASHI/D to compare its centralized version WAKASHI/C and demonstrates that the distributed server has substantial performance benefits.

Efficient retrieval of nested objects is an important issue in advanced database systems. So far, a number of indexing methods for nested objects have been proposed. However, they do not consider retrieval of nested objects based on the set comparison operators such as and . Previouly, we proposed four set access facilities for nested objects and compared their performance in terms of retrieval cost, storage cost, and update cost. In this paper, we extend the study and present refined algorithms and cost formulas applicable to more generalized situations. Our cost models and analysis not only contribute to the study of set-valued retrieval but also to cost estimation of various indexing methods for nested objects in general.

A quasi-synchronous (QS) code division multiple access (CDMA) system is proposed for mobile communications. In the proposed method, which uses the time division duplex (TDD) mode of transmission, a mobile receiver can measure propagation delay changes. It then accordingly adjusts its transmission time so its signal can arrive at base station synchronously with other mobile units. A simple control unit is used at the mobile unit in order to reduce any error due to the propagation delay changes. The system operates as follows. At the start of a call, a mobile unit is quasi-synchronised through feedback control from the base station. The mobile unit then maintains synchronous status without any further base station feedback. The degree of the quasi-synchronous accuracy is determined by a clock in mobile units. This paper shows performance results based on using a clock rate of ten times faster than the spreading rate. Orthogonal codes are used for spreading the signals. The results demonstrate that the reverse link CDMA multiuser interference is to a great degree removed.

A uniquely decodable code (C1, C2, C3) is investigated for the three-user binary adder channel. The uniquely decodable code is constructed as follows: If C1 is an (n, k) linear code with a generator matrix, C2 is a coset of C1 and C3 is a set of all coset leaders, then the code (C1, C2, C3) is uniquely decodable and its total rate is equal to 1k/n, n2k. This code is easily decodable.

This paper deals with the method of integration of voice and data in wireless communication systems. By applying the DSV (Data Steal into Voice) technique to D-TDMA (Dynamic Time Division Multiple Access) systems, this paper presents an MAC (Media Access Control) method of integration of voice and data for the systems such as cellular radios and cordless phones. After a brief review of the D-TDMA scheme and the DSV technique, the protocol called D-TDMA/DSV is described. Then, a static analysis to derive the channel capacity and a dynamic analysis to evaluate the throughput and delay performance are presented. An extension of TFA (Transient Fluid Approximation) analysis is employed in the dynamic analysis. With the same system parameters, the capacity of D-TDMA/DSV is compared with that of the traditional D-TDMA. Under the limitation of the blocking probability required for cellular radios, some numerical examples of dynamic analysis are given to show the throughput and delay performance of the system.

This paper describes the multiple access performance of parallel combinatory spread spectrum (PC/SS) communication systems in nonfading and Rayleigh fading multipath channels. The PC/SS systems can provide the high-speed data transmission capability by transmitting multiple pseudo-noise sequences out of a pre-assigned sequence set. The performance is evaluated in terms of average bit error rate (BER) by numerical computation. In nonfading white gaussian channel, the PC/SS systems are superior to conventional direct sequence spread spectrum (DS/SS) systems under the identical spreading factor condition. In Rayleigh fading channel, the performance of the PC/SS system without diversity is poorer than that of the DS/SS system. By including the explicit and implicit diversity, the performance of the PC/SS system becomes better than that of conventional DS/SS systems. A longer spreading sequence is assignable to a PC/SS system having the spreading factor equal to that in the conventional DS/SS system. Hence, the error control coding is easily. It is found that the PC/SS systems including diversity and Reed-Solomon coding improves the multiple access performance.

A bottleneck identification methodology is proposed for the performance-oriented design of shared-bus multiprocessors, which are composed of several major subsystems (e.g. off-chip cache, bus, memory, I/O). A subsystem with the longest access time per instruction is the one that limits processor performance and creates a bottleneck to the system. The methodology also facilitates further refined analysis on the access time of the bottleneck subsystem to help identify the causes of the bottleneck. Example performance model of a particular shared-bus multiprocessor architecture with separate address bus and data bus is developed to illustrate the key idea of the bottleneck identification methodology. Accessing conflicts in subsystems and DMA transfers are also considered in the model.

Currently there is considerable world-wide speculation regarding the introduction of optical fiber cable into access networks, because optical fiber has a big potential for providing attractive multimedia services. Since optical fiber cable can provide a variety of grade of services, high-reliability of cable networks would be required compared with the conventional copper cable networks. To develop multimedia telecommunication networks as an infrastructure, it is urgent to clarify the highly reliable optical fiber cable network architecture. Since cable network architecture deeply depends on regional conditions such as demand, area size, duct layer networks (consisting of ducts, manholes, tunnels, feeder points etc.), it is necessary to develop a cable network designing tool with user-friendly interfaces for efficiently evaluating cable network architectures. This paper firstly proposes the heuristic algorithms enhanced by the disjoint-shortest-path and the depth-first-search methods that would be applicable for real access networks. Secondly, the design method of highly reliable optical fiber cable network based on the heuristic algorithms in terms of network cost and unavailability caused by cable breakdown is proposed. It can design the combination of star- and loop-shaped (where two diversified routes exist between a feeder point and central office) cable network. Furthermore, comparison with the conventional design method which simply applies star- or loop-shaped cable network is done in terms of economy and reliability on real access networks in the Tokyo metropolitan area. It is concluded that the proposed method can reduce the network cost further and realize a short unavailability value compared with the conventional method.

This paper describes a flexible point-to-multipoint access protocol for the fiber-optic passive double star (PDS) system. To provide various types of services, and permit flexibility in changing transport capacity, a time division multiple access (TDMA) scheme for the PDS system is considered. Dynamic time slot multiplexing based on TDMA is proposed to provide required time slots efficiently according to service changes. The effectiveness of dynamic time slot multiplexing is calculated and compared to fixed time slot multiplexing for telephony services. A TCM/TDMA frame structure and an access protocol enabling dynamic time slot multiplexing are proposed. ONU bandwidth is dynamically assigned by using a set of pointers. The ONU access protocol causes no interruption to operating ONUs on the same PDS system during the configuration or reconfiguration of an ONU. The access time is analyzed to estimate the performance of the access protocol. The probability density of access time is calculated for the number of ONUs connected. The calculation results indicate that a PDS system can accommodate up to around 60 ONUs within the maximum access time specified by ITU-T. The experimental results also agree fairly well with the theoretical values.

We investigate the multipacket message transmissions and variable length message transmissions in slotted ALOHA systems with capture effect. First, we propose an approach that the transmission power level is controlled probabilistically depending on message length for multipacket messages. We consider the multipacket messages model with capture. We derive explicit equations of the effective channel utilization of the model. It is demonstrated that if we increase the numbar of power levels, we can get more effective channel utilization of the system. Secondly, we propose how to assign the slot size and show that the effective utilization of the channel is improved for variable length messages using the approach proposed for multipacket messages. Channel design issue about length of the slot depending on the number of power levels used for transmission is discussed. Thirdly, we propose the multiple messages per slot model with capture. The analytical results show that the multiple messages per slot model can achieve the highest channel utilization among the models discussed in this paper.

We study behavior of multi-zone MCA (Multi-Channel Access) mobile communication systems with a finite number of channels in each zone. Three queueing schemes for channel requests named holding scheme, ready-nonready scheme, and optimum scheme are investigated. The delay performance of channel requests is studied through computer simulations.

Next generation personal communication systems will provide a range of different services to moving users. In parallel, packet switching is being proposed as a way to statistical multiplexing and hence to better resource exploitation. The co-existence of different services may prove difficult due to the different requirements on quality of service parameters like packet loss, delay, and so on. This requires a careful design of Call Admission Control policies, which are to be quite different from those used in fixed network, due to two phenomena which are typical of mobile systems, namely co-channel interference and handovers. In this paper we address these complex topics, and propose some basic rules for Call Admission Control policies suitable in this context.

This paper proposes a hybrid access control scheme for broadcast-based ATM-LANs. Broadcast-based ATM-LANs are shared media networks with star topology. In this network, packets are broadcast to all subscriber terminals by a CPYF (Cell COPY Function) node located in the ATM network and only relevant packets are extracted and sent to the destined user layer function by packet filtering functions in the terminals. The simplicity of the packet transfer mechanism makes the network very economical. In broadcast-based ATM-LANs, the hybrid access control scheme is effective in improving performance. In this scheme, short packets and long packets are transmitted respectively by means of a back-pressure type and a CSMA/CD type access control scheme. Throughput evaluation was performed by computer simulation and the results show that the proposed scheme achieves a high throughput characteristic.

The Distributed Queue Dual Bus protocol (DQDB) has been adopted as the metropolitan area network (MAN) standard by IEEE802.6 committee. Recently, the unfairness problem in the DQDB protocol, by which head stations benefit, has been pointed out. Although a fair bandwidth distribution among the stations is obtained by adding the so-called bandwidth balancing mechanism into the DQDB protocol (DQDB/BB), the DQDB/BB protocol leaves a portion of the available bandwidth unused, and it takes a considerable amount of time to converge to fair channel assignment. In this study, to overcome the drawbacks in DQDB and DQDB/BB, we introduce a new media access control protocol which is based on assigning each station a level according to some traffic information such as the queueing length, delay time etc. Only the station with the highest level is allowed to transmit. Through the operation of level assignment or the choice of level function, the transmission can be easily controlled in a distributed manner. This protocol is simple compared with DQDB/BB and can be implemented in the DQDB architecture. The simulation results show that the new protocol obtains not only fair throughput regardless of the distance between the stations, but also fair delay performance. In addition, the new protocol can easily provide preempty priority service through level assignment. The new protocol converges to fair distribution of the channel in the time required for only one or two round-trips. This is very fast compared with the DQDB/BB protocol.

A key element in the CDMA transmission is DS spreading. Spreading in a DS/SSMA system are provided in two categories-synchronization and data. For synchronization sequences, good auto-correlation and cross-correlation properties are required in order to guarantee fast acquistion with a minimum false alarm probability. On the other hand, the auto-correlation property may not be so important in data spreading since synchronization is obtained by synchronization spreading. In this paper we provide a set of synchronization sequences and a set of data sequences--each a set of binary N-tuples--that have the necessary correlation constraints.

Circuit access control is a traffic control technique of rejecting calls arriving at a group of specified circuits to make the group free at a target scheduled time so that the capacity may be dynamically reallocated to serve other traffic demand. This technique plays an important role for resource allocation control in state-of-the-art capacity reconfigurable networks as well as for switching calls on a reserved basis in the ISDNs. In this paper, we present a novel adaptive scheme for circuit access control in order to overcome the inefficiency of the conventional deterministic scheme. The presented scheme is based only on knowledge about service time and bandwidth characteristics of calls. The transitional behavior of the circuit group under the scheme is analyzed, and the gain in utilization achieved by the adaptive scheme is examined. We treat a model of the circuit group shared by multi-slot calls with different service times, and describe the results of the transient analysis and the approximation method for evaluating the gains.

Telecommunication services are expected to be upgraded from POTS to B-ISDN services in the future. This means that the conventional metallic access networks should be upgraded to optical fiber access networks because of providing high bit-rate services. Therefore, it is very important to clarify upgrade strategies in access networks. This paper proposes a dynamic evaluation method that can support decision-making on the upgrade strategy from the viewpoint of economy. This method can determine the most promising future access network and upgrade timing. Moreover, viability of various upgrade strategies can be evaluated by this method.