Channel sharing in multi-hop WDM lightwave networks: realizationand performance of multicast traffic

A local lightwave network can be constructed by employing two-way fibers to connect nodes in a passive-star physical topology, and the available optical bandwidth may be effectively accessed by the nodal transmitters and receivers at electronic rates using wavelength division multiplexing (WDM). The number of channels, ω, in a WDM network is limited by technology and is usually less than the number of nodes, N, in the network. We provide a general method using channel sharing to construct practical multi-hop networks under this limitation. Channel sharing may be achieved through time division multiplexing. The method is applied to a generalized shuffle-exchange-based multi-hop architecture, called GEMNET. Multicasting-the ability to transmit information from a single source node to multiple destination nodes-is becoming an important requirement in high-performance networks. Multicasting, if improperly implemented, can be bandwidth-abusive. Channel sharing is one approach toward efficient management of multicast traffic. We develop a general modeling procedure for the analysis of multicast (point-to-multipoint) traffic in shared-channel, multihop WDM networks. The analysis is comprehensive in that it considers all components of delay that packets in the network experience-namely, synchronization, queuing, transmission, and propagation. The results show that, in the presence of multicast traffic, WDM networks with ω相似文献   

A system for seismocardiography-based identification of quiescent heart phases: implications for cardiac imaging

Seismocardiography (SCG), a representation of mechanical heart motion, may more accurately determine periods of cardiac quiescence within a cardiac cycle than the electrically derived electrocardiogram (EKG) and, thus, may have implications for gating in cardiac computed tomography. We designed and implemented a system to synchronously acquire echocardiography, EKG, and SCG data. The device was used to study the variability between EKG and SCG and characterize the relationship between the mechanical and electrical activity of the heart. For each cardiac cycle, the feature of the SCG indicating Aortic Valve Closure was identified and its time position with respect to the EKG was observed. This position was found to vary for different heart rates and between two human subjects. A color map showing the magnitude of the SCG acceleration and computed velocity was derived, allowing for direct visualization of quiescent phases of the cardiac cycle with respect to heart rate.     

Recruitment and phosphorylation of SH2-containing inositol phosphatase and Shc to the B-cell Fc gamma immunoreceptor tyrosine-based inhibition motif peptide motif

Recently, we and others have demonstrated that negative signaling in B cells selectively induces the tyrosine phosphorylation of a novel inositol polyphosphate phosphatase, p145SHIP. In this study, we present data indicating that p145SHIP binds directly a phosphorylated motif, immunoreceptor tyrosine-based inhibition motif (ITIM), present in the cytoplasmic domain of Fc gammaRIIB1. Using recombinant SH2 domains, we show that binding is mediated via the Src homology region 2 (SH2)-containing inositol phosphatase (SHIP) SH2 domain. SHIP also bound to a phosphopeptide derived from CD22, raising the possibility that SHIP contributes to negative signaling by this receptor as well as Fc gammaRIIB1. The association of SHIP with the ITIM phosphopeptide was activation independent, while coassociation with Shc was activation dependent. Furthermore, experiments with Fc gammaRIIB1-deficient B cells demonstrated a genetic requirement for expression of Fc gammaRIIB1 in the induction of SHIP phosphorylation and its interaction with Shc. Based on these results, we propose a model of negative signaling in which co-cross-linking of surface immunoglobulin and Fc gammaRIIB1 results in sequential tyrosine phosphorylation of the ITIM, recruitment and phosphorylation of p145SHIP, and subsequent binding of Shc.     

Integrating Security in the MAC Layer of WDM Optical Networks*

We introduce a new technique for providing security in a broadcast-and-select, wavelength-division-multiplexed (WDM) optical network. The approach provides privacy of communications by employing a novel challenge-response scheme and exploiting the tuning delay inherent in optical receivers. The proposed technique can be integrated easily into any existing WDM media-access-control (MAC) protocol that employs tunable receivers. The modified protocol would require every idle user, who is not scheduled to receive data, to tune in to a channel that does not contain sensitive data. A violation of the protocol can be detected with very high probability, and appropriate measures can be taken against the violator. The technique provides features that cannot be achieved with cryptography alone. Significant benefits of the proposed approach include the ability to detect security violations as they occur, and an efficient mechanism to provide privacy for multicast transmissions. We develop two simple solutions to deal with different levels of attack: (1) eavesdroppers working alone, and (2) eavesdroppers working in collaboration. We also introduce a dynamic channel allocation scheme that can further reduce the number of required overhead channels, with minimal loss in the capability to detect eavesdropping violations.     

Negative signaling in B cells causes reduced Ras activity by reducing Shc-Grb2 interactions

To elucidate the molecular basis for inhibition of B cell proliferation and differentiation by the Fc receptor for IgG (Fc(gamma)RII), we compared the signaling events in B cells stimulated by cross-linking surface Ig alone (positive signaling), or by co-cross-linking surface Ig and Fc(gamma)RII (negative signaling). Both modes of stimulation induced tyrosine kinase activation. Positive signaling induced activation of Ras, Raf-1 kinase, and mitogen-activated protein kinase; these events were significantly attenuated during negative signaling. Since Ras is activated by SOS and Vav, two known guanine nucleotide exchange factors, activation events associated with these molecules using the two different stimuli were examined. Results of these experiments indicated that tyrosine phosphorylation of Vav did not change upon co-cross-linking. In contrast, the association of Shc and Grb2 was abrogated under negative and induced under positive signaling conditions. Concomitantly, Shc was observed to associate with a tyrosine-phosphorylated 145-kDa protein, previously identified as Src homology 2-containing inositol phosphatase, only under conditions of negative signaling. Based on these results, we hypothesize that negative signaling via the Fc(gamma)RII in B cells is at least partly the result of a block in Ras activation, and that SOS, but not Vav, is the major guanine nucleotide exchange factor in B cells for Ras activation.