Prioritized Buffer Management in Photonic Packet Switches for Synchronously Arriving Fixed-length Packets

We investigate a photonic packet switch architecture that enables a high node throughput and provides priority services. We describe PBSO (partial buffer sharing with overwriting) method that allows control of an optical fiber-delay-line buffer by prioritized buffer management under conditions of synchronous arrival of fixed length optical packets at a packet switch. The PBSO method is based on a single queue and its complexity is O(p), where p is the number of priority classes. We first present photonic buffer architectures which can support the PBSO method. We also develop an analytical method for PBSO where p =2. Through analysis and simulation results, we show that PBSO improves the packet loss probability in each priority class more than the existing PBS (partial buffer sharing) does, and that it can be actually applied to prioritized buffer management of an optical buffer. PBSO is especially effective when the arrival rate of higher priority class packets is much lower than that of lower priority class packets. In that case, PBSO dramatically improves the performance of higher priority class packets while the degradation in the performance of lower priority class packets is small. In other words, in PBSO, a larger number of higher priority class packets can be accepted at a given packet loss probability than in PBS or non-priority methods

Density functional study of selected mono-zinc and gem-dizinc radical carbenoid cyclopropanation reactions: observation of an efficient radical zinc carbenoid cyclopropanation reaction and the influence of the leaving group on ring closure

We report a theoretical study of the cyclopropanation reactions of EtZnCHI, (EtZn)2CH EtZnCHZnI, and EtZnCIZnI radicals with ethylene. The mono-zinc and gem-dizinc radical carbenoids can undergo cyclopropanation reactions with ethylene via a two-step reaction mechanism similar to that previously reported for the CH2I and IZnCH2 radicals. The barrier for the second reaction step (ring closure) was found to be highly dependent on the leaving group of the cyclopropanation reaction. In some cases, the (di)zinc carbenoid radical undergoes cyclopropanation via a low barrier of about 5–7 kcal/mol on the second reaction step and this is lower than the CH2I radical reaction which has a barrier of about 13.5 kcal/mol for the second reaction step. Our results suggest that in some cases, zinc radical carbenoid species have cyclopropanation reaction barriers that can be competitive with their related molecular Simmons-Smith carbenoid species reactions and produce somewhat different cyclopropanated products and leaving groups

Effect of strain reversals on the processing of high-purity aluminum by high-pressure torsion

High-purity aluminum was processed by high-pressure torsion (HPT) under conventional monotonic (m-HPT) and cyclic (c-HPT) conditions where strain reversals are introduced in c-HPT during processing. Measurements show higher values of the Vickers microhardness in the center regions of all disks but these values are higher when processing by c-HPT by comparison with m-HPT for the same total number of turns. Slightly smaller grain sizes are observed in the c-HPT samples. It is shown that all of the microhardness values correlate with the estimated values of the equivalent strain and the results are consistent with earlier data reported under c-HPT conditions when it is recognized that the variation of hardness with equivalent strain is dependent upon the level of recovery within the material. <br/

Alternative parameter choices for multi-step quasi-Newton methods

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The strength–grain size relationship in ultrafine-grained metals

Metals processed by severe plastic deformation (SPD) techniques, such as equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), generally have submicrometer grain sizes. Consequently, they exhibit high strength as expected on the basis of the Hall–Petch (H–P) relationship. Examples of this behavior are discussed using experimental data for Ti, Al, and Ni. These materials typically have grain sizes greater than ~50 nm where softening is not expected. An increase in strength is usually accompanied by a decrease in ductility. However, both high strength and high ductility may be achieved simultaneously by imposing high strain to obtain ultrafine-grain sizes and high fractions of high-angle grain boundaries. This facilitates grain boundary sliding, and an example is presented for a cast Al-7 pct Si alloy processed by HPT. In some materials, SPD may result in a weakening even with a very fine grain size, and this is due to microstructural changes during processing. Examples are presented for an Al-7034 alloy processed by ECAP and a Zn-22 pct Al alloy processed by HPT. In some SPD-processed materials, it is possible that grain boundary segregation and other features are present leading to higher strengths than predicted by the H–P relationshi