Giorgio Gambosi

On-line algorithms for the channel assignment problem in cellular networks

We consider the on-line channel assignment problem in the case of cellular networks and we formalize this problem as an on-line load balancing problem for temporary tasks with restricted assignment. For the latter problem, we provide a general solution (denoted as the cluster algorithm ) and we characterize its competitive ratio in terms of the combinatorial properties of the graph representing the network. We then compare the cluster algorithm with the greedy one when applied to the channel assignment problem: it turns out that the competitive ratio of the cluster algorithm is strictly better than the competitive ratio of the greedy algorithm. The cluster method is general enough to be applied to other on-line load balancing problems and, for some topologies, it can be proved to be optimal.

Automatic construction of an opinion-term vocabulary for ad hoc retrieval

We present a method to automatically generate a term-opinion lexicon. We also weight these lexicon terms and use them at real time to boost the ranking with opinionated-content documents. We define very simple models both for opinion-term extraction and document ranking. Both the lexicon model and retrieval model are assessed. To evaluate the quality of the lexicon we compare performance with a well-established manually generated opinion-term dictionary. We evaluate the effectiveness of the term-opinion lexicon using the opinion task evaluation data of the TREC 2007 blog track.

Interval routing schemes

In this paper the problem of routing messages along shortest paths in a distributed network without using complete routing tables is considered. In particular, the complexity of deriving minimum (in terms of number of intervals) interval routing schemes is analyzed under different requirements. For all the cases considered NP-hardness proofs are given, while some approximability results are provided. Moreover, relations among the different cases considered are studied.

On relevance, time and query expansion

We present the results of our exploratory analysis on the relationship that exists between relevance and time. We observe how the amount of documents published in a given interval of time is related to the probability of relevance, and, using the time series analysis, we show the existence of a correlation between time and relevance. As an initial application of this analysis, we study query expansion exploiting the detection of publication time peaks over the Blog06 collection. We finally propose an effective approach for the query expansion in the blog search domain. Our approach is based on the documents publication trend being so completely independent of any external resource.

Time-Stamp Approach to Store-and-Forward Deadlock Prevention

This paper deals with the problem of store-and-forward deadlock prevention in store-and-forward networks. The presented solution uses time stamping of all messages in the network, and a nonpreemptable message exchange mechanism. By combining these ideas, a new distributed flow control procedure is derived which guarantees that all messages are delivered to their own destinations, thus avoiding both deadlock and livelock without any message loss. It is shown that some properties of this procedure depend on the policy of the allocation of exchange buffers to nodes. On the one hand, an optimal allocation strategy is presented which results in a maximally optimal deadlock prevention procedure. The procedure is network sizeand topology-independent and allows unrestricted packet routing. On the other hand, the allocation of one exchange buffer per node is discussed, which, even if not optimal, makes the derived deadlock prevention procedure completely independent of network reconfigurations. The last feature is extremely important from the practical point of view and, therefore, such a solution is strongly recommended. When compared to store-and-forward deadlock prevention procedures described so far, which lack some or all of these desirable properties, the procedure presented here behaves favorably. However, it imposes other drawbacks, i.e., the possibility of extra hops as a result of exchange operations. It is argued that this drawback appears rarely in practice, and some strategies which aim at a reduction of it are proposed.

The incremental maintenance of a depth-first-search tree in directed acyclic graphs

We propose an incremental algorithm to maintain a DFS-forest in a directed acyclic graph under a sequence of arc insertions in O(nm) worst case total time, where n is the number of nodes and m is the number of arcs after the insertions. This compares favorably with the time required to recompute DFS from scratch by using Tarjans Θ(n + m) algorithm any time a sequence of Ω(n) arc insertions must be handled. In particular, over a sequence of Θ(m) arc insertions our algorithm requires O(n) amortized time per operation, and its worst case time is O(n + m). Our algorithm relies on an original characterization of a DFS-forest in terms of a relaxed planar embedding of the graph. Besides the basic representation of the graphs in term of adjacency lists, O(n) additional space is required. Although the problem of the dynamic maintenance of a DFS-tree was pointed out about one decade ago, this paper provides the first solution to this problem for nontrivial classes of graphs.

The effect of cut-through switching on the performance of buffered banyan networks

Abstract This paper briefly reports on the results of some testing activities performed at IASI on the efficiency of multimedia conferencing on both local and wide-area packet switched networks. The aim of the testing was to study under which hypothesis it was possible to obtain acceptable - in terms of rate of information exchanged - multimedia conferencing sessions. A specific goal was to verify the possibility of obtaining an acceptable quality by using widely available, cheap resources. The tests performed have been carried out on an Ethernet LAN, connected to Internet by a 64Kbps link, using TCP/IP with multicast extension as transport protocol, in order to avoid IP datagrams multiplication when a multimedia conference involves more than two participants.Fast packet switches based on banyan interconnection networks and their variations are being proposed for Broadband Integrated Services Digital Network (B-1SDN). Previous analyses on buffered banyan networks mainly assume the store-and-forward operation of the switching systems. In this paper we study the effect of cut-through switching on buffered banyan networks. Two types of buffered banyan networks are considered: those with the switching elements employing input buffers and those employing output buffers. We extend the analytical method of Theimer et al. to multibuffered banyan networks, and to buffered banyan networks with output buffering in the switching elements. We present thc analyses of buffered banyan networks operating under cut-through switching. The results of these analyses confirm the simulation results, originally made by Bubenik and Turner. that cut-through switching in buffered banyan networks; improves both throughout and delay performances.

On devising Boolean routing schemes

In this paper, the problem of routing messages along shortest paths in a network of processors without using complete routing tables is considered.

A CONVEX HULL ALGORITHM FOR POINTS WITH APPROXIMATELY KNOWN POSITIONS

We consider the problem of deriving good approximations of the convex hull of a set of points in the plane in the realistic case that only arbitrary finite approximations of the real valued coordinates can be known. In particular, the algorithm we introduce derives sequences of improved certified approximations converging to the exact solution, at the same time allowing the insertion of new points to the problem instance. The complexity analysis of the algorithm is performed by referring to a suitable computation model, based on a RAM with logarithmic costs, and the derived space and time bounds are shown to be competitive with respect to current off-line algorithms.

Multidimensional interval routing schemes

Interval Routing Scheme (k-IRS) is a compact routing scheme on general networks. It has been studied extensively and recently been implemented on the latest generation INMOS Transputer Router chip. In this paper we introduce an extension of the Interval Routing Scheme k-IRS to the multi-dimensional case (k, d)-MIRS, where k is the number of intervals and d is the number of dimensions. Whereas k-IRS only represents compactly a single shortest path between any two nodes, with this new extension we are able to represent all shortest paths compactly. This is useful for fault-tolerance and traffic distribution in a network. We study efficient representations of all shortest paths between any pair of nodes for general network topologies and for specific interconnection networks such as rings, grids, tori and hypercubes. For these interconnection networks we show that for about the same space complexity as k-IRS we can represent all shortest paths in (k, d)-MIRS (as compared to only a single shortest path in k-IRS). Moreover, tradeoffs are derived between the dimension d and the number of intervals k in multi-dimensional interval routing schemes on hypercubes, grids and tori.