Buffer Threshold Research Articles

Throughput-delay characteristics of OCUM protocol for media access in multihop wireless networks with buffers

In this paper, we present the throughput and delay performance of a busy tone-based media access protocol in wireless networks with buffers at each node for the queueing of messages. Partially connected static topologies with a finite number of nodes that are globally synchronized to the slot duration are considered. Each node has unlimited buffer resources for transit messages and limited buffer capacity for newly arriving entry messages. The input buffer limiting policy rejects new message arrivals when the total number of queued messages (including entry and transit messages) exceeds a finite buffer threshold L at each node. Messages are serviced on a first-come-first-served basis. Simulation results for fully connected, ring, bus, multiconnected ring, square lattice and other arbitrary topologies are presented. The protocol offers constant throughput of the order of 0.8–0.9 at high arrival rates in a fully connected network. Throughput is found to improve at low arrival rates as the value of L is increased. However, the improvement becomes marginal for L > 4. For other multihop topologies like ring, bus, multiconnected ring and square lattice, the throughput performance improves beyond unity due to spatial reuse.

Yeast alcohol dehydrogenase bound to membranes: surface and microenvironment effects on activity and stability.

The enzyme, yeast alcohol dehydrogenase, was adsorbed to porous nitrocellulose and nylon membranes. The two membranes provide different surface chemistries as indicated by the results of the streaming potential, enzyme adsorption, and fluorescein isothiocyanate adsorption experiments. The stability of the enzyme, as determined by continually measuring the extent of coenzyme reduction as a function of time, appeared to be much less for the enzyme adsorbed to the positively charged membrane surface. Moreover, the enzyme adsorbed to the positively charged membrane was the least responsive to pulses of the reducing agent, dithiothreitol, and appeared to exhibit the highest transition temperature when subjected to differential scanning calorimetry analysis. These results indicate that the entropically spreading process observed for other adsorbed proteins may be occurring and the process is more rapid and extensive when enzyme is adsorbed to the nylon than the nitrocellulose membrane. In addition to the relative stability of the enzyme on two different surfaces being examined, the effect of the microenvironment on modulating the activity of the enzyme was investigated by using the reversibility of the enzyme-catalyzed reaction as a probe of the average local environment of the enzyme. It was found that a threshold buffer concentration existed that, once exceeded, the effect of proton production by the reaction could be suppressed.

A bang-bang principle for real-time transport protocols

A real-time protocol is usually concerned with the transportation of a real-time data stream over a packet switched network. Among the major issues distinguishing real-time protocols from ordinary transport protocols is the problem of trading delay for loss. That is, if some loss of packets may be acceptable, and usually inevitable, the objective of the protocol is to minimize the delay of packets, subject to constraints on the acceptable loss. This is unlike usual transport protocols, which are designed to guarantee no loss at the expense of increased delays. This paper presents a model for the delay-loss tradeoffs in a family of real-time transport protocols. It is demonstrated that, under very general assumptions, an optimal real-time protocol is a bang-bang protocol: there exists a threshold queue length such that as long as the packet queue length at the sender is less than the threshold the protocol should be an ordinary positive acknowledgement with retransmissions transport protocol. However, as soon as the threshold queue length is exceeded, a newly arriving packet causes the first (or any other) packet in the queue to be discarded. Closed form expressions for the threshold buffer size are obtained and analyzed in terms of the given parameters of the system.