Performance evaluation of a reservation random access scheme for packetized wireless systems with call control and hand-off loading

We consider a packet switched wireless network where each cell's communication channel is shared among packet voice sources. In this paper, we present a method for the design and analysis of wireless cells using a reservation random access (RRA) scheme for packet access control. This scheme is integrated with a call admission control procedure. We model the state process of a single cell as a vector Markov chain. We compute the steady state distribution of the Markov chain. This result is used to calculate the packet dropping probability and the call blocking probability. By setting limits on maximum permissible levels for the call blocking probability and the packet dropping probability, we obtain the Erlang capacity of a single cell, with and without hand-off traffic. For an illustrative RRA scheme, the Erlang capacity of a single cell is shown to be about twice that attained by a comparable fixed assigned TDMA scheme. We show that a cellular network using this RRA scheme and which applies can be no blocking of hand-off calls, exhibits similar call capacity levels.This work is supported by a University of California MICRO and Pacific-Bell Grant No. 94-107.     

Elastic–plastic spherical contact under combined normal and tangential loading in full stick

The behavior of an elastic–plastic contact between a deformable sphere and a rigid flat under combined normal and tangential loading with full stick contact condition is investigated theoretically. Sliding inception is treated as a plastic yield failure mechanism, which allows static friction modeling under highly adhesive conditions. Several contact parameters such as: junction tangential stiffness, static friction force and static friction coefficient are extensively investigated. The phenomenon of junction growth and the evolution of the plastic zone in the contact region are briefly described. It is found that at low normal dimensionless loads the static friction coefficient decreases sharply with increasing normal load, in breach with the classical laws of friction. As the normal load further increases the static friction coefficient approaches a constant value that is about 0.3 for many material properties combinations.     

The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact

Laser surface texturing (LST) is an emerging effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were conducted with a pin-on-disk apparatus at sliding speeds in the range of 0.015–0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two oils with different viscosities (54.8 and 124.7 cSt at 40 °C) were used as lubricants. The test results showed that laser texturing expanded the contact parameters in terms of load and speed for hydrodynamic lubrication, as indicated by friction transitions on the Stribeck curve. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.     

Lymphangiomatosis with splenic involvement

Severe splenomegaly and anemia developed in a 5-year-old girl with diffuse lymphangiomatosis of the upper part of the body. Radioisotope scanning and celiac angiography demonstrated lymphangiomatosis of the spleen, a rare but diagnosable condition. Intractable infection in areas of ulcerated skin led to her death from overwhelming sepsis.     

Vaporization in Face Seals Operating with Liquid Mixtures

The problem of vaporization in face seals operating on mixtures where the minority liquid is discontinuous and has the lower boiling point is analyzed. A model based on several simplifications is presented and used to solve a numerical example for water-in-oil mixtures. It is shown that even the smallest amount (in the order of 1 %) of water in the oil may considerably alter the seal performance and result in seal failure.     

Generalized satellite link model and its application to the transmission plan

A generalized model for the satellite link that includes non-linear effects is presented. The transponder is handled as a black box with nominal gain g_s. At a specific operating point each signal in the transponder has its own gain, g_sci, that may be different from g_s. The parameter g_sci depends on the operating point of the transponder, the power of the signal and the distribution of the residual transponder power among the other carriers. The intermodulation between the carriers and between them and the up-link noise is included in the model as well. The generalized model is represented by the conventional traditional equation, modified with correction factors which account for the nonlinear effects. Application of the model to typical transmission plans is given and comparison of the results to field experiments and transponder analysis program is included. Emphasis is placed on the use of the transponder with mixed TV and SCPC operation.     

Efficient non‐linear reanalysis of skeletal structures using combined approximations

Non‐linear reanalysis of large‐scale structures usually involves much computational effort, because the set of non‐linear equations must be solved repeatedly during the solution process. Various approximations that are often used for linear reanalysis are not sufficiently accurate for non‐linear problems. In this study, solution procedures based on the combined approximations approach are developed and compared in terms of efficiency and accuracy. Various path‐independent non‐linear analysis and reanalysis problems are considered, including material non‐linearity, geometric non‐linearity and buckling analysis. Numerical examples demonstrate the effectiveness of the procedures presented. It is shown that in various cases accurate results can be achieved efficiently. Copyright © 2007 John Wiley & Sons, Ltd.     

Dynamics of shells of revolution under axisymmetric load involving shear deformation

A general solution procedure, based on the linear theory, is presented for arbitrary shells of revolution subjected to arbitrary axisymmetric dynamic loads. The equations of motion admit shear deformation and rotational inertia. The numerical solution is obtained by Houbolt's method and by finite differences.     

Robust throughput and routing for mobile ad hoc wireless networks

Flows transported across mobile ad hoc wireless networks suffer from route breakups caused by nodal mobility. In a network that aims to support critical interactive real-time data transactions, to provide for the uninterrupted execution of a transaction, or for the rapid transport of a high value file, it is essential to identify robust routes across which such transactions are transported. Noting that route failures can induce long re-routing delays that may be highly interruptive for many applications and message/stream transactions, it is beneficial to configure the routing scheme to send a flow across a route whose lifetime is longer, with sufficiently high probability, than the estimated duration of the activity that it is selected to carry. We evaluate the ability of a mobile ad hoc wireless network to distribute flows across robust routes by introducing the robust throughput measure as a performance metric. The utility gained by the delivery of flow messages is based on the level of interruption experienced by the underlying transaction. As a special case, for certain applications only transactions that are completed without being prematurely interrupted may convey data to their intended users that is of acceptable utility. We describe the mathematical calculation of a network’s robust throughput measure, as well as its robust throughput capacity. We introduce the robust flow admission and routing algorithm (RFAR) to provide for the timely and robust transport of flow transactions across mobile ad hoc wireless network systems.     

Modification of pool boiling regimes by sand deposition

Sand spots, attached to a copper ball surface by means of polyvinyl acetate adhesive and distributed over the surface with areal density that ranges between one spot per 1.18 cm² (for low‐density spots) and one spot per 0.51 cm² (for high‐density spots), serve as a temporary heat transfer enhancer during the quenching in liquid nitrogen. Highest heat flux densities, achieved during quenching, lie in the range 10.8 to 20.2 W/cm², depending on the sand layer structure. Application of the temporary enhancer increases an amount of heat, evacuated by highly effective nucleate and transition boiling, by factor of 4.5 as compared with the bare sample. The process of sand layer preparation, data acquisition peculiarities, relationship between heat exchange efficiency and the spots areal density, along with sand grit size are discussed in this paper.