Authenticated routing for ad hoc networks

Initial work in ad hoc routing has considered only the problem of providing efficient mechanisms for finding paths in very dynamic networks, without considering security. Because of this, there are a number of attacks that can be used to manipulate the routing in an ad hoc network. In this paper, we describe these threats, specifically showing their effects on ad hoc on-demand distance vector and dynamic source routing. Our protocol, named authenticated routing for ad hoc networks (ARAN), uses public-key cryptographic mechanisms to defeat all identified attacks. We detail how ARAN can secure routing in environments where nodes are authorized to participate but untrusted to cooperate, as well as environments where participants do not need to be authorized to participate. Through both simulation and experimentation with our publicly available implementation, we characterize and evaluate ARAN and show that it is able to effectively and efficiently discover secure routes within an ad hoc network.     

Calcium elevation in astrocytes causes an NMDA receptor-dependent increase in the frequency of miniature synaptic currents in cultured hippocampal neurons

Astrocytes exhibit a form of excitability and communication on the basis of intracellular Ca2+ variations (Cornell-Bell et al., 1990; Charles et al., 1991) that can be initiated by neuronal activity (Dani et al., 1992; Porter and McCarthy, 1996). A Ca2+ elevation in astrocytes induces the release of glutamate (Parpura et al., 1994; Pasti et al., 1997; Araque et al., 1998;Bezzi et al., 1998), which evokes a slow inward current in neurons and modulates action potential-evoked synaptic transmission between cultured hippocampal cells (Araque et al., 1998), suggesting that astrocytes and neurons may function as a network with bidirectional communication. Here we show that a Ca2+ elevation in astrocytes increases the frequency of excitatory as well as inhibitory miniature postsynaptic currents (mPSCs), without modifying their amplitudes. Thapsigargin incubation, microinjection of the Ca2+ chelator BAPTA, and photolysis of the Ca2+ cage NP-EGTA demonstrate that a Ca2+ elevation in astrocytes is both necessary and sufficient to modulate spontaneous transmitter release. This Ca2+-dependent release of glutamate from astrocytes enhances mPSC frequency by acting on NMDA glutamate receptors, because it is antagonized by D-2-amino-5-phosphonopentanoic acid (AP5) or extracellular Mg2+. These NMDA receptors are located extrasynaptically, because blockage specifically of synaptic NMDA receptors by synaptic activation in the presence of the open channel blocker MK-801 did not impair the AP5-sensitive astrocyte-induced increase of mPSC frequency. Therefore, astrocytes modulate spontaneous excitatory and inhibitory synaptic transmission by increasing the probability of transmitter release via the activation of NMDA receptors.     

Glutamate-dependent astrocyte modulation of synaptic transmission between cultured hippocampal neurons

The idea that astrocytes merely provide structural and trophic support for neurons has been challenged by the demonstration that astrocytes can regulate neuronal calcium levels. However, the physiological consequences of astrocyte-neuron signalling are unknown. Using mixed cultures of rat hippocampal astrocytes and neurons we have determined functional consequences of elevating astrocyte calcium levels on co-cultured neurons. Electrical or mechanical stimulation of astrocytes to increase their calcium level caused a glutamate-dependent slow inward current (SIC) in associated neurons. Microinjection of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) into astrocytes to prevent the stimulus-dependent increase in astrocyte calcium level, blocks the appearance of the neuronal SIC. Pharmacological manipulations indicate that this astrocyte-dependent SIC is mediated by extracellular glutamate acting on N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors. Additionally, stimulation of astrocytes reduced the magnitude of action potential-evoked excitatory and inhibitory postsynaptic currents through the activation of metabotropic glutamate receptors. The demonstration that astrocytes modulate neuronal currents and synaptic transmission raises the possibility that astrocytes play a neuromodulatory role by controlling the extracellular level of glutamate.     

Constrained optimization of the performance indices of arbitrary array antennas

The solution to the optimization of performance indices of array antennas such as directive gain, efficiency index, and signal-to-noise ratio, do not provide information regarding the sidelobe region of the radiation pattern. It is shown that, with proper constraints on the sidelobes, a given performance index can be optimized to give a radiation pattern with desired sidelobe levels. As most of the performance indices of an array antenna can be expressed as a ratio of two Hermitian quadratic forms, an eigenvalue method is used for the constrained optimization. This method gives explicit expressions for the excitation vector and constrained values of the performance indices. An iterative technique is used to ensure that the specified field values occur at the sidelobe peak positions. The element excitations obtained by this technique for maximum gain and uniform sidelobe level are similar to these obtained by the Dolph-Chebyshev technique.     

Pre-Reply Probe and Route Request Tail: Approaches for Calculation of Intra-Flow Contention in Multihop Wireless Networks

Several applications have been envisioned for multihop wireless networks that require different qualities of service from the network. In order to support such applications, the network must control the admission of flows. To make an admission decision for a new flow, the expected bandwidth consumption of the flow must be correctly determined. Due to the shared nature of the wireless medium, nodes along a multihop path contend among themselves for access to the medium. This leads to intra-flow contention; contention between packets of the same flow forwarded by different hops along a multihop path, resulting in an increase in the actual bandwidth consumption of the flow to a multiple of its single hop bandwidth requirement. Determining the amount of intra-flow contention is non-trivial since interfering nodes may not be able to communicate directly if they are outside each other's transmission range. In this paper we examine methods to determine the extent of intra-flow contention along multihop paths in both reactive and proactive routing environments. The highlight of the solutions is that carrier-sensing data is used to deduce information about carrier-sensing neighbors, and no high power transmissions are necessary. Analytical and simulation results show that our methods estimate intra-flow contention with low error, while significantly reducing overhead, energy consumption and latency as compared to previous approaches. Kimaya Sanzgiri is a PhD candidate in the Department of Computer Science at the University of California, Santa Barbara. She is working with Prof. Elizabeth Belding-Royer in the Mobility Management and Networking (MOMENT) Laboratory. Kimaya received her B.E. (Hons.) in Computer Science from the Birla Institute of Technology and Science (BITS), Pilani, India in 1999. Her research interests are in the area of wireless networking, specifically mobility, quality of service support and security. See for more details. Ian D. Chakeres is an Ph.D. student in the Department of Electrical and Computer Engineering at the University of California, Santa Barbara. He is working with Prof. Elizabeth M. Belding-Royer in the Mobile Management and Networking (MOMENT) Laboratory. He completed his B.S. and M.S. in Electrical and Computer Engineering at Ohio State University in 1998 and 1999. He is also a co-chair of the IETF MANET working group. Ian's research interests include wireless communication and mobile networking, specifically routing protocols, MAC protocols, cross-layer coordination and quality of services in mobile wireless networks. See for further details. Elizabeth M. Belding-Royer is an Assistant Professor in the Department of Computer Science at the University of California, Santa Barbara. She completed her Ph.D. in Electrical and Computer Engineering at UC Santa Barbara in 2000. Elizabeth's research focuses on mobile networking, specifically routing protocols, multimedia, monitoring, and advanced service support. Elizabeth is the author of numerous papers related to ad hoc networking and has served on many program committees for networking conferences. Elizabeth is the TPC Co-Chair of ACM MobiCom 2005 and IEEE SECON 2005, and is currently on the editorial board for the Elsevier Science Ad hoc Networks Journal. Elizabeth is the recipient of an NSF CAREER award, and a 2002 Technology Review 100 award, awarded to the world's top young investigators. She is a member of the IEEE, IEEE Communications Society, ACM, and ACM SIGMOBILE. See for further details.     

Optimum aperture monopulse excitations

A unified formulation of the optimization of monopulse antenna performance indices for a specified sidelobe envelope function and/or specified nulls of the pattern is presented. The performance indices considered are beam efficiency, gain factor, and angular sensitivity factor of rectangular and circular apertures. The unconstrained optimization of beam efficiency result in an integral equation, the solutions of which are prolate spheroidal wave functions for rectangular aperture and hyperspheroidal wave functions for circular aperture. These functions reduce, respectively, to Legendre and Zernike polynomials in the case of gain factor and angular sensitivity factor. The double orthogonality properties of these eigenfunctions are used for constrained optimization. The results obtained by this technique for the near-in sidelobes constrained at a uniform level are shown to be in agreement with the earlier works. The method is applicable for other aperture surfaces such as elliptical, ellipsoidal, and spherical.     

A hybrid tile approach for Ka band subarray modules

A “thin” array construction using tile module architecture is presented in this paper. Several transmit-only tile modules operating at 30 GHz have been fabricated. The tile module involves a layered construction that contributes to reduced array thickness, weight, and cost. In such layered construction, the radiating aperture, active devices, and signal distribution functions are placed in different layers and involve vertical interconnections. The subarray module described involves a hybrid construction in which conventional wire bonding is utilized for interconnecting devices to the signal distribution layers. As in conventional modules, the devices are mounted on the carrier plates and fully tested before insertion in the module. The radiating elements which are integral to the module housing are connected to the devices through electromagnetic coupling. This hybrid construction is a first step towards a fully batch-processed tile module architecture. The construction approach and the performance results at the module and array level are presented