With the increasing popularity of full-duplex, switched Ethernet-a point-to-point technology as opposed to classical CSMA/CD-efforts to standardize a vehicle for flow control at or below the MAC (media access control) sublayer have been initiated. In this paper, the end-to-end transport layer flow control and congestion avoidance mechanisms of TCP, as well as a new hop-by-hop MAC layer flow control being standardized by the IEEE, are considered. Simulation studies are performed to evaluate network performance with and without the explicit congestion indication provided by the MAC layer approach.

A credit-based controller (CBC), which has a provision for cell tagging, is proposed for usage parameter control (UPC). Similar in concept to the leaky bucket, the CBC has a credit counter and a data buffer. Credits are accumulated at a rate initiated from the network (equivalent to the sustainable cell rate), and depleted at a rate pertaining to the network traffic load. A source has the prerogative whether or not to send its cells as tagged cells when the credit level is nonpositive. The queueing behavior of the CBC fed by a Marcov modulated source, is analyzed using a stochastic fluid flow model. It is shown that the performance of the CBC can be altered by tuning the weighting parameter within the CBC. Also, adjustments can be made to balance the ratio of tagged and untagged cells sent into the network.

The performance of a multiversion conservative timestamp algorithm is evaluated by analyzing the response time of operations in a distributed database system. The random communication delay in the transmission channels of a long-haul network supporting the distributed database system causes transmitted operations to be received out of order. According to rules that are dependent on the number of versions, the scheduler reorders the operations to preserve the consistency of the database. Operations encounter extra delay in the process of reordering. The system is modeled by a queuing network. Response-time distributions are obtained analytically. Numerical results are presented to show the effect of the number of versions on the performance.>