Luigi Fratta

Bio: Luigi Fratta is an academic researcher from Polytechnic University of Milan. The author has contributed to research in topics: Network packet & Time division multiple access. The author has an hindex of 32, co-authored 115 publications receiving 5051 citations. Previous affiliations of Luigi Fratta include University of California, Los Angeles & Stanford University.

Performance evaluation and enhancement of the CSMA/CA MAC protocol for 802.11 wireless LANs

Abstract: The IEEE 802.11 protocol for wireless local area networks adopts a CSMA/CA protocol with exponential backoff as medium access control technique. As the throughput performance of such a scheme becomes critical when the number of mobile stations increases, in this paper we propose an adaptive contention window mechanism, which dynamically selects the optimal backoff window according to the estimate of the number of contending stations. We show that this technique leads to stable behavior, and it outperforms the standard protocol when the network load and the number of mobile stations are high. We also investigate the CSMA/CA with the optional RTS/CTS technique, and we show that our adaptive technique reaches better performance only when the packet size is short. Finally, the performance of a system environment with hidden terminals show that the RTS/CTS mechanism, which can also be used in conjunction with the adaptive contention window mechanism, provides significant improvements.

The flow deviation method: An approach to store-and-forward communication network design

Abstract: mo problems reZevant to the design of a store-and-forward communication network (the message routing problem and the channeZ capacity assignment problem) are formulated and are recognized to be essentiaZ Zy non- Zinear, uncoMnA;tcained muZticomodity (m. c. 1 fZm problems. A "FZm Deviation" (FD) method for the soZution of these non-linear, unconstrained m.c. fZm probZems is described which is quite simiZar to the gradient method for functions of continuous variables; here the concept of gradient is repZaced by the concept of "shortest route" flow. As in the gradient method, the application of successive flow deviations leads to ZocaZ minima. of the FD method to the design of the ARPA Computer Network are discussed. FinaZ ly, two interesting applications

ADHOC MAC: new MAC architecture for ad hoc networks providing efficient and reliable point-to-point and broadcast services

Abstract: Ad-hoc networking, though an attractive solution for many applications, still has many unsolved issues, such as the hiddenterminal problem, flexible and prompt access, QoS provisioning, and efficient broadcast service. In this paper we present a MAC architecture able to solve the above issues in environments with no power consumption limitations, such as networks for inter-vehicle communications. This new architecture is based on a completely distributed access technique, RR-ALOHA, capable of dynamically establishing, for each active terminal in the network, a reliable single-hop broadcast channel on a slotted/framed structure. Though the proposed architecture uses a slotted channel it can be adapted to operate on the physical layer of different standards, including the UMTS Terrestrial Radio Access TDD, and IEEE 802.11. The paper presents the mechanisms that compose the new MAC: the basic RR-ALOHA protocol, an efficient broadcast service and the reservation of point-to-point channels that exploit parallel transmissions. Some basic performance figures are discussed to prove the effectiveness of the protocol.

On improving relaxation methods by modified gradient techniques

Abstract: Relaxation methods have been recently shown to be very effective, for some large scale linear problems. The aim of this paper is to show that these procedures can be considerably improved by following a modified gradient step direction.

ATM: bandwidth assignment and bandwidth enforcement policies

Abstract: Two techniques are presented: the class-related rule and the virtual leaky bucket mechanism. It is shown that the former can effectively solve the bandwidth-assignment problem for ATM (asynchronous transfer mode) when different classes of services are multiplexed on the same network trunk. The virtual leaky bucket technique is used as a bandwidth-enforcement policy. It has the advantage of combining the policing function on each source with congestion control on the cell loss rate in the network. The implementation complexity is not cumbersome, and both techniques can be realized with existing technology at the B-ISDN (broadband integrated services digital network) transmission speed. >

Performance analysis of the IEEE 802.11 distributed coordination function

Abstract: The IEEE has standardized the 802.11 protocol for wireless local area networks. The primary medium access control (MAC) technique of 802.11 is called the distributed coordination function (DCF). The DCF is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary slotted exponential backoff. This paper provides a simple, but nevertheless extremely accurate, analytical model to compute the 802.11 DCF throughput, in the assumption of finite number of terminals and ideal channel conditions. The proposed analysis applies to both the packet transmission schemes employed by DCF, namely, the basic access and the RTS/CTS access mechanisms. In addition, it also applies to a combination of the two schemes, in which packets longer than a given threshold are transmitted according to the RTS/CTS mechanism. By means of the proposed model, we provide an extensive throughput performance evaluation of both access mechanisms of the 802.11 protocol.

A performance comparison of multi-hop wireless ad hoc network routing protocols

Abstract: An ad hoc networkis a collwtion of wirelessmobilenodes dynamically forminga temporarynetworkwithouttheuseof anyexistingnetworkirrfrastructureor centralizedadministration.Dueto the limitedtransmissionrange of ~vlrelessnenvorkinterfaces,multiplenetwork“hops”maybe neededfor onenodeto exchangedata ivithanotheracrox thenetwork.Inrecentyears, a ttiery of nelvroutingprotocols~geted specificallyat this environment havebeen developed.but little pcrfomrartwinformationon mch protocol and no ralistic performancecomparisonbehvwrrthem ISavailable. ~Is paper presentsthe results of a derailedpacket-levelsimulationcomparing fourmulti-hopwirelessad hoc networkroutingprotocolsthatcovera range of designchoices: DSDV,TORA, DSR and AODV. \Vehave extended the /~r-2networksimulatorto accuratelymodelthe MACandphysical-layer behaviorof the IEEE 802.1I wirelessLANstandard,includinga realistic wtrelesstransmissionchannelmodel, and present the resultsof simulations of net(vorksof 50 mobilenodes.

Learning Bayesian Networks: The Combination of Knowledge and Statistical Data

Abstract: We describe a Bayesian approach for learning Bayesian networks from a combination of prior knowledge and statistical data. First and foremost, we develop a methodology for assessing informative priors needed for learning. Our approach is derived from a set of assumptions made previously as well as the assumption of likelihood equivalence, which says that data should not help to discriminate network structures that represent the same assertions of conditional independence. We show that likelihood equivalence when combined with previously made assumptions implies that the user's priors for network parameters can be encoded in a single Bayesian network for the next case to be seen—a prior network—and a single measure of confidence for that network. Second, using these priors, we show how to compute the relative posterior probabilities of network structures given data. Third, we describe search methods for identifying network structures with high posterior probabilities. We describe polynomial algorithms for finding the highest-scoring network structures in the special case where every node has at most k e 1 parent. For the general case (k > 1), which is NP-hard, we review heuristic search algorithms including local search, iterative local search, and simulated annealing. Finally, we describe a methodology for evaluating Bayesian-network learning algorithms, and apply this approach to a comparison of various approaches.

On the capacity of a cellular CDMA system

Abstract: It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

Fibonacci heaps and their uses in improved network optimization algorithms

Abstract: In this paper we develop a new data structure for implementing heaps (priority queues). Our structure, Fibonacci heaps (abbreviated F-heaps), extends the binomial queues proposed by Vuillemin and studied further by Brown. F-heaps support arbitrary deletion from an n-item heap in O(log n) amortized time and all other standard heap operations in O(1) amortized time. Using F-heaps we are able to obtain improved running times for several network optimization algorithms. In particular, we obtain the following worst-case bounds, where n is the number of vertices and m the number of edges in the problem graph: O(n log n + m) for the single-source shortest path problem with nonnegative edge lengths, improved from O(mlog(m/n+2)n);O(n2log n + nm) for the all-pairs shortest path problem, improved from O(nm log(m/n+2)n);O(n2log n + nm) for the assignment problem (weighted bipartite matching), improved from O(nmlog(m/n+2)n);O(mβ(m, n)) for the minimum spanning tree problem, improved from O(mlog log(m/n+2)n); where β(m, n) = min {i | log(i)n ≤ m/n}. Note that β(m, n) ≤ log*n if m ≥ n.Of these results, the improved bound for minimum spanning trees is the most striking, although all the results give asymptotic improvements for graphs of appropriate densities.