Luigi Ciminiera

A LOTOS extension for the performance analysis of distributed systems

Performance analysis and formal correctness verification of computer communication protocols and distributed systems have traditionally been considered as two separate fields. However, their integration can be achieved by using formal description techniques as paradigms for the development of performance models. This paper presents a novel extension of LOTOS, one of the two formal specification languages that were standardized by ISO. The extension is specifically conceived to integrate performance analysis and formal verification. The extended language syntax and semantics are formally defined, along with a mapping from extended specifications to performance models, The mapping preserves the specified observable behavior. Two simple examples, a stop-and-wait protocol and a time-sharing system, are used to concretely demonstrate the new approach and to validate it. >

Carry-save multiplication schemes without final addition

Carry-save multipliers require an adder at the last step to convert the carry-sum representation of the most significant half of the result into a non-redundant form. This paper presents n/spl times/n multiplication schemes where this conversion is performed with a circuit operating in parallel with the carry-save array. The most relevant feature of the proposed multipliers is that the full 2n-bit result is produced, unlike similar multiplication schemes presented in the literature.

Over-redundant digit sets and the design of digit-by-digit division units

Over-redundant digit sets are defined as those ranging from /spl minus/s to +s, with s/spl ges/B, B being the radix. This paper presents new techniques for the direct computation of division, that use an over-redundant digit set for representing the quotient, instead of simply redundant ones used previously. In particular, general criteria for synthesizing the digit selection rules and remainder updating are given for any radix and index of redundancy. A methodology combining the use of over-redundant digit sets with the prescaling of the divisor is also studied in order to achieve radix-B division units with trivial digit selection functions. It is also shown, for the specific case of radix-4 that using a prescaling slightly wider than in a radix-4 unit by M.D. Ercegovac and T. Lang (1990) possible to avoid the digit selection table. The paper also presents a modified algorithm for on-the-fly conversion of the result into the irredundant form. The proposed methodology can be considered as an alternative to existing division techniques. >

Higher radix square rooting

A general discussion on nonrestoring square root algorithms is presented, showing bounds and constraints delimiting the space of feasible algorithms, for all the choices of radix, digit set and representation of the partial remainder. Two classes of algorithms are then derived from the general discussion, and it is shown how it is possible to determine two parameters with a relevant impact on the implementation: the number of radicand bits to be inspected in order to obtain a starting value, and the number of partial remainder bits to be examined for digit selection. The algorithms for the specific case of radix 4 digit set (-2, -1, 0, +1, +2), and partial remainder represented in carry-save form are derived in order to show that the algorithms introduced can lead to better results than those obtained with algorithms previously presented. >

Some properties of timed token medium access protocols

Timed-token protocols are used to handle, on the same local area network, both real-time and non-real-time traffic. The authors analyze this type of protocol, giving worst-case values for the throughput of non-real-time traffic and the average token rotation time. Results are obtained for synchronous traffic generated according to a generic periodic pattern under heavy conditions for non-real-time traffic and express not only theoretical lower bounds but values deriving from the analysis of some real networks. A model which addresses the asynchronous overrun problem is presented. The influence of introducing multiple priority classes for non-real-time traffic on the total throughput of this type of message is shown. It is also shown that the differences between the values obtained under worst-case assumptions are close to those obtained under best-case assumptions; the method may therefore be used to provide important guidelines in properly tuning timed-token protocol parameters for each specific network installation. >

A Connecting Network with Fault Tolerance Capabilities

A new multistage interconnection network is presented in this paper. It is able to handle the communications between the connected devices correctly, even in the presence of fault(s) in the network. This goal is achieved by using redundant paths with a fast procedure able to dynamically reroute the message. It is also shown that the rerouting properties are still valid when broadcasting transmission is used.

Design of a radix 4 division unit with simple selection table

A radix 4 division architecture is presented which partially overlaps the updating of the remainder with the digit selection procedure. It is obtained by separating the radix 4 digit selection process into two concurrent substeps. The proposed unit requires a simple selection table and involves a small extra expense for the additional hardware compared to the usual radix 4 division units. Four possible implementations are derived from the general model, with different types of substeps. The high level evaluation shows that the proposed architectures offer an efficient alternative. >

Optimizing Deep Packet Inspection for High-Speed Traffic Analysis

Deep Packet Inspection (DPI) techniques are considered extremely expensive in terms of processing costs and therefore are usually deployed in edge networks, where the amount of data to be processed is limited. This paper demonstrates that, in case the application can tolerate some compromises in terms of accuracy (such as many measurement-based tasks) and in presence of normal traffic, the processing cost can be greatly reduced while even improving the classification precision, making DPI suitable also for high-speed networks.

A protocol for multirendezvous of LOTOS processes

It is noted that the implementation of the multiway rendezvous mechanism of the International Standards Organization (ISO) LOTOS specification language for protocols is very important in the development of tools for the execution of LOTOS. It involves problems such as global knowledge in a distributed environment and distributed agreement. The authors propose a novel algorithm which fully implements the multiway rendezvous of LOTOS within a distributed execution model based on a number of parallel processes. The processes are organized in a hierarchical topology and communicate with each other only by message transfers. The performance of the proposed algorithm is evaluated and is shown to be better than that achieved by other algorithms proposed in the literature. A formal specification of the algorithm in LOTOS is provided. >

Industrial IEEE 802.3 networks with short delivery time for urgent messages

Two solutions are investigated for introducing priority mechanisms in the CSMA/CD (carrier-sense multiple access with collision detection) protocol defined in the IEEE 802.3 standard to provide short delivery times for urgent messages even when the overall traffic on the channel is heavy. Thus the CSMA/CD protocol, attractive because of its low cost, also becomes suitable for a class of industrial control applications. The proposed schemes can easily be implemented by using already-available chips and boards because they are based on the dynamic management of a transmission parameter, the slot time; this feature is already present in the standard IEEE 802.3, where it is set statically when the network is configured. The first configuration considered implements a message priority scheme, where the priority level for the station can be updated at each new message transmission, varying the slot time according to the message characteristics. The second configuration introduces a fixed-station-priority mechanism which allows each node in the network to be assigned to a specific priority class. >