Performance analysis of a broadcast star local area network with collision avoidance. Part 1, Infinite station population model

Packet collisions and their resolution create a performance bottleneck in random access LANs. As a solution to this problem, a broadcast star network with collision avoidance has been proposed and studied in [3 - 17]. In a broadcast star network, collisions of simultaneously transmitted packets are avoided by means of hardware called a collision avoidance switch. While the channel is being used by one station, the collision avoidance switch blocks other stations from using it. This network implements random access protocols without the penalty of collisions among packets and combines the benefits of random access (low delay when traffic is light; simple, distributed, and therefore robust protocols) with excellent network utilization.In this paper, we analyze the performance of a broadcast star network, assuming synchronous operation of a network. In synchronous operation, the channel time is slotted, and stations transmit only at the beginning of a slot. The number of stations on a network is assumed to be infinite, and packets arrive at stations according to a Poisson process. An exact analysis is developed, and the distribution for the transmission delays is obtained. It is also shown through simulations that a broadcast star operating under synchronous mode yields better performance than that operating under asynchronous mode, where transmissions of packets are not confined to the beginning of slots, and stations start transmission any time

Tree LANS with collision avoidance : protocol, switch architecture, and performance

Packet collisions and their resolution create a performance bottleneck in random access LANs. A hardware solution to this problem is to use collision avoidance switches. These switches allow the implementation of random access protocols without the penalty of collisions among packets. We review and compare the designs of some tree LANs that use collision avoidance switches. They have the potential of combining the benefits of random access (low delay when traffic is light, simple and distributed, and therefore robust, protocols) with excellent network utilization and concurrency of transmission. The collision avoidance LANs we review are broadcast star, Hubnet-like tree, Tinker-Tree, and a treenet that allows concurrent broadcasts within non-intersecting subtrees. After this review, we present a slotted-time, infinite user analysis of the broadcast star network

55Mn NMR in Mn12 acetate: Hyperfine interaction and magnetic relaxation of cluster

The 55Mn NMR in oriented powder Mn12Ac has been investigated at 1.4-2.0 K in zero field and with external fields along the c-axis. Three kinds of 55Mn NMR composed of five-fold quadrupole-split lines for I=5/2 nuclei have been interpreted to arise from Mn4+ ion, and two crystallographically-inequivalent Mn3+ ions, respectively. It is found that the isotropic hyperfine field in the Mn4+ ion with 3d3 configuration indicates a large amount of reduction (26%) as compared with the theoretical evaluation. In the analysis for the hyperfine field of Mn3+ ions with 3d4 configuration, we have taken into account of the anisotropic dipolar contribution in addition to the Fermi-contact term in order to explain two kinds of 55Mn NMR frequencies in Mn3+ ions in inequivalent sites. By using the hyperfine coupling constants of twelve manganese ions in Mn12Ac, the total hyperfine interaction of the ferrimagnetic ground state of S=10 has been determined to amount to 0.3 cm-1 in magnitude at most, the magnitude of which corresponds to the nuclear hyperfine field he(0.32 kG seen by Mn12 cluster spin. The relaxation of the cluster magnetization was investigated by observing the recovery of the 55Mn spin-echo intensity in the fields of 0.20-1.90 T along the c-axis at 2.0 K. It was found that the magnetization of the cluster exhibits the (t-recovery in the short time regime. The relaxation time decreases with increasing external field following significant dips at every 0.45 T. This is interpreted to be due to the effects of thermally-assisted quantum tunneling between the spin states at magnetic level crossings.Comment: Text 25 pages, five figures and two table

Responses of Plasma Acetate Metabolism to Hop (Humulus lupulus L.) in Sheep

An isotope dilution method using [1-13C]sodium (Na) acetate was conducted to determine the effect of feeding hop (Humulus lupulus L.) residues on plasma acetate metabolism in six adult crossbred sheep. The sheep were fed 63 g/kg BW0.75/d of either mixed hay (MH-diet) of orchardgrass (Dactylis glomerata L.) and reed canarygrass (Phalaris arundinacea L.) at a 60:40 ratio or MH-diet and hop-residues (Hop-diet) at 85:15 ratio with a crossover design for each of 3 week period. The isotope dilution method using single injection of [1-13C]Na acetate was performed thrice; before feeding (BF), 2 h after feeding (2F) and 4 h after feeding (4F), on the 21st day of each dietary treatment. Plasma acetate concentration tended to increase (P= 0.06) and turnover rate was numerically higher (P= 0.16) for MH-diet than Hop-diet. Plasma glucose, NEFA, VFA and lactic acid concentrations were similar between dietary treatments. In both the diets, although plasma concentration of acetate did not change, turnover rate increased significantly (P= 0.02) 2F than BF. Hop-residues did not show any negative impacts on acetate metabolism as well as physiology of animals in the present experimental conditions, hence thereby it could be used as an alternative to MH-diet for rearing sheep