ESR Study of Nitroxide Radicals Generated from Triaz-2-en-1-ols

The triazenols 4-R¹? C₆H₄? N?N? N(OH)? R² ( 1 ), oxidized with t-BuO radicals, produced nitroxide radicals R¹? C₆H₄? N(O^?)? N?N(R²) ⁺O^? ( 5 ). The suggested radical structure was confirmed by ¹⁵N-labeling. The reaction of triazenols 1 with PbO₂ proceeded under N₂ elimination, in which case nitroxides R¹? C₆H₄? N(R₂)? O^?( 2 ) were observed as the final radical products. The intermediate R¹? C₆H radicals were identified by spin-trapping.     

Dispersion in the 10-30 GHz Frequency Range: Atmospheric Effects and Their Impact on Digital Satellite Communications

Earth-space communications links operating above 10 GHz may be adversely affected by the earth's atmosphere and hydrometeors, particularly rain. These propagation phenomena are usually evaluated only for fading and depolarization effects. However, dispersion is an important factor in wide-band digitally modulated satellite links. In this paper we examine the mechanisms responsible for dispersion over bandwidths as great as 500 MHz in the 10-30 GHz range. The amplitude and phase effects of dispersion are quantified for a wide range of propagation conditions. The resulting propagation model is then used to generate estimates of performance degradation in wide-band QPSK systems through the use of computer simulation.     

Performance of Multibeam Packet Satellite Systems with Conflict Free Scheduling

The concept of a multibeam satellite system With on-board processing and memory is studied. In this system multiple slotted ALOHA uplinks carry the traffic to the satellite. Packets are accepted at the satellite, when memory is available, and are routed to their destination zones using a TDM protocol. We present a model which can be used to evaluate a satellite system with conflict-free scheduling, i.e., a system in which several packets can be simultaneously chosen for downlink transmissions, given each earth zone is served by at most one satellite transponder in each slot. We compute the system throughput, packet delays, and buffer overflow probabilities for a general configuration. It is shown that for some configurations the one beam per zone restriction can have a significant effect on the system performance. The presented model can be also used to evaluate satellite systems when this restriction is removed and to evaluate a variety of other satellite systems.     

Partial Star Products: A Local Covering Approach for the Recognition of Approximate Cartesian Product Graphs

This paper is concerned with the recognition of approximate graph products with respect to the Cartesian product. Most graphs are prime, although they can have a rich product-like structure. The proposed algorithms are based on a local approach that covers a graph by small subgraphs, so-called partial star products, and then utilizes this information to derive the global factors and an embedding of the graph under investigation into Cartesian product graphs.     

Energy-conserving access protocols for identification networks

A myriad of applications are emerging, in which energy conservation is a critical system parameter for communications. Radio frequency identification device (RFID) networks, smart cards, and even mobile computing devices, in general, need to conserve energy. In RFID systems, nodes are small battery-operated inexpensive devices with radio receiving/transmitting and processing capabilities, integrated into the size of an ID card or smaller. These identification devices are designed for extremely low-cost large-scale applications, such that the replacement of batteries is not feasible. This imposes a critical energy constraint on the communications (access) protocols used in these systems, so that the total time a node needs to be active for transmitting or receiving information should be minimized. Among existing protocols, classical random access protocols are not energy conserving, while deterministic protocols lead to unacceptable delays. This paper deals with designing communications protocols with energy constraint, in which the number of time slots in which tags need to be in the active state is minimized, while the access delay meets the applications constraints. We propose three classes of protocols which combine the fairness of random access protocols with low energy requirements     

Intermediate-node initiated reservation (IIR): a new signaling scheme for wavelength-routed networks

A problem of many distributed lightpath provisioning schemes is wavelength contention, which occurs when a connection request attempts to reserve a wavelength channel that is no longer available. This situation results from the lack of updated global link-state information at every node. In networks with highly dynamic traffic loads, wavelength contention may seriously degrade the network performance. To overcome this problem, we propose a new framework for distributed signaling and introduce a class of schemes referred to as intermediate-node initiated reservation. In the new scheme, reservations may be initiated at any set of nodes along the route; in contrast, reservations can only be initiated by the destination node in the classic destination initiated reservation (DIR) scheme. As a result, the possibility of having outdated information due to propagation delay is significantly lowered. Specifically, we consider two schemes within this framework, for networks with no wavelength conversion and for networks with sparse wavelength conversion, respectively. Theoretical and simulation results show that, compared with the classic DIR scheme, the new schemes can significantly improve the network blocking performance. The accuracy of the analytical models is also confirmed by extensive numerical simulations.     

Distinguishing Cartesian powers of graphs

The distinguishing number D(G) of a graph is the least integer d such that there is a d‐labeling of the vertices of G that is not preserved by any nontrivial automorphism of G. We show that the distinguishing number of the square and higher powers of a connected graph G ≠ K₂, K₃ with respect to the Cartesian product is 2. This result strengthens results of Albertson [Electron J Combin, 12 ( 1 ), #N17] on powers of prime graphs, and results of Klav?ar and Zhu [Eu J Combin, to appear]. More generally, we also prove that d(G □ H) = 2 if G and H are relatively prime and |H| ≤ |G| < 2^|H| ? |H|. Under additional conditions similar results hold for powers of graphs with respect to the strong and the direct product. © 2006 Wiley Periodicals, Inc. J Graph Theory 53: 250–260, 2006     

Factoring cartesian-product graphs

In a fundamental paper, G. Sabidussi [“Graph Multiplication,” Mathematische Zeitschrift, Vol. 72 (1960), pp. 446–457] used a tower of equivalence relations on the edge set E(G) of a connected graph G to decompose G into a Cartesian product of prime graphs. Later, a method by R.L. Graham and P.M. Winkler [“On Isometric Embeddings of Graphs,” Transactions of the American Mathematics Society, Vol. 288 (1985), pp. 527–533] of embedding a connected graph isometrically into Cartesian products opened another approach to this problem. In both approaches an equivalence relation σ that determines the prime factorization is constructed. The methods differ by the starting relations used. We show that σ can be obtained as the convex hull of the starting relation used by Sabidussi. Our result also holds for the relation determining the prime decomposition of infinite connected graphs with respect to the weak Cartesian product. Moreover, we show that this relation is the transitive closure of the union of the starting relations of Sabidussi and Winkler [“Factoring a Graph in Polynomial Time,” European Journal of Combinatorics, Vol. 8 (1987), pp. 209–212], thereby generalizing the result of T. Feder [“Product Graph Representations,” Journal of Graph Theory, Vol 16 (1993), pp. 467–488] from finite to infinite graphs.     

Differential service in high-speed interconnection systems: analgorithm for prioritized access

The requirement to connect high-performance components of supercomputer systems has caused an increasing demand for connection-oriented, high-speed communication in a peer-to-peer environment. The importance of this type of communication is underscored by the advent of the high-performance parallel interface (HIPPI) standard, designed to support the commercial development of interconnection systems for this environment. For such interconnection systems, we present a connection management algorithm that supports priorities to provide preferential access for important classes of traffic, and that assures equitable access for all connections within the same priority class. At the same time, the algorithm utilizes the connection resources efficiently and assures low overhead at the nodes. We demonstrate the algorithm's applicability by outlining a centralized as well as a distributed implementation in an HIPPI-based interconnection system. Furthermore, we introduce an analytic priority model that can be used to evaluate an interconnection system's performance under the algorithm. Using this model, we analyze the efficacy of the nonpreemptive priority discipline and show the load/throughput behavior of a typical system