Performance modelling of the Cambridge Fast Ring protocol

The Cambridge Fast Ring is high-speed slotted ring. The features that make it suitable for use at very large transmission rates are the synchronous transmission, the simplicity of the medium-access-control protocol, and the possibility of immediate retransmission of erroneous packets. A novel analytical model of the Cambridge Fast Ring with normal slots is presented. The model is shown to be accurate and usable over wide range of parameters. A performance analysis based on this model is presented

A Slotted Ring Test Bed for the Study of ATM Network Congestion Management

This thesis addresses issues raised by the proposed Broadband Integrated Services Digital Network which will provide a flexible combination of integrated services traffic through its cell-based Asynchronbus Transport Mode (ATM). The introduction of a cell-based, connection-oriented, transport mode brings with it new technical challenges for network management. The routing of cells, their service at switching centres, and problems of cell congestion not encountered in the existing network, are some of the key issues. The thesis describes the development of a hardware slotted ring testbed for the investigation of congestion management in an ATM network. The testbed is designed to incorporate a modified form of the ORWELL protocol to control media access. The media access protocol is analysed to give a model for maximum throughput and reset interval under various traffic distributions. The results from the models are compared with measurements carried out on the testbed, where cell arrival statistics are also varied. It is shown that the maximum throughput of the testbed is dependent on both traffic distribution and cell arrival statistics. The testbed is used for investigations in a heterogeneous traffic environment where two classes of traffic with different cell arrival statistics and quality of service requirements are defined. The effect of prioritisation, media access protocol, traffic intensity, and traffic source statistics were investigated by determining an Admissible Load Region (ALR) for a network station. Conclusions drawn from this work suggest that there are many problems associated with the reliable definition of an ALR because of the number of variable parameters which could shift the ALR boundary. A suggested direction for further work is to explore bandwidth reservation and the concept of equivalent capacity of a connection, and how this can be linked to source control parameters

Transmission of variable bit rate video over an Orwell ring

Asynchronous Transfer Mode (ATM) is fast emerging as the preferred information transfer technique for future Broadband Integrated Services Digital Networks (BISON), offering the advantages of both the simplicity of time division circuit switched techniques and the flexibility of packet switched techniques. ATM networks with their inherent rate flexibility offer new opportunities for the efficient transmission of real time Variable Bit Rate (VBR) services over such networks. Since most services are VBR in nature when efficiently coded, this could in turn lead to a more efficient utilisation of network resources through statistical multiplexing. Video communication is typical of such a service and could benefit significantly if supported with VBR video over ATM networks. [Continues.

Parallel simulation techniques for telecommunication network modelling

In this thesis, we consider the application of parallel simulation to the performance modelling of telecommunication networks. A largely automated approach was first explored using a parallelizing compiler to speed up the simulation of simple models of circuit-switched networks. This yielded reasonable results for relatively little effort compared with other approaches. However, more complex simulation models of packet- and cell-based telecommunication networks, requiring the use of discrete event techniques, need an alternative approach. A critical review of parallel discrete event simulation indicated that a distributed model components approach using conservative or optimistic synchronization would be worth exploring. Experiments were therefore conducted using simulation models of queuing networks and Asynchronous Transfer Mode (ATM) networks to explore the potential speed-up possible using this approach. Specifically, it is shown that these techniques can be used successfully to speed-up the execution of useful telecommunication network simulations. A detailed investigation has demonstrated that conservative synchronization performs very well for applications with good look ahead properties and sufficient message traffic density and, given such properties, will significantly outperform optimistic synchronization. Optimistic synchronization, however, gives reasonable speed-up for models with a wider range of such properties and can be optimized for speed-up and memory usage at run time. Thus, it is confirmed as being more generally applicable particularly as model development is somewhat easier than for conservative synchronization. This has to be balanced against the more difficult task of developing and debugging an optimistic synchronization kernel and the application models

Simulation of packet and cell-based communication networks

This thesis investigates, using simulation techniques, the practical aspects of implementing a novel mobility protocol on the emerging Broadband Integrated Services Digital Network standard. The increasing expansion of telecommunications networks has meant that the demand for simulation has increased rapidly in recent years; but conventional simulators are slow and developments in the communications field are outstripping the ability of sequential uni-processor simulators. Newer techniques using distributed simulation on a multi-processor network are investigated in an attempt to make a cell-level simulation of a non-trivial B.-I.S.D.N. network feasible. The current state of development of the Asynchronous Transfer Mode standard, which will be used to implement a B.-I.S.D.N., is reviewed and simulation studies of the Orwell Slotted Ring protocol were made in an attempt to devise a simpler model for use in the main simulator. The mobility protocol, which uses a footprinting technique to simplify hand- offs by distributing information about a connexion to surrounding base stations, was implemented on the simulator and found to be functional after a few 'special case' scenarios had been catered for

'Slowly becoming sales promotion men?': Negotiating the career of the sales representative in Britain, 1920s–1970s

The commercial traveler, or traveling salesman, was an agent of commercialism and modernization as well as a stock character in British popular culture. To C. Wright Mills, salesmen faced particularly challenging demands to conform to managerial direction. This article examines how British salesmen negotiated their occupational identity during the twentieth century. Developments in marketing, corporate growth, and periods of war and recession all challenged salesmen’s status and autonomy. These influences prompted a lengthy and recurring debate about how best to present, defend, and justify their work and identity. New marketing techniques and management systems evolved steadily, rather than producing sudden or uniform changes in the ways in which salesmen worked. Their culture of enterprise and individualism persisted, in part as it was shared by employers and managers. The impact of new marketing methods proved greatest in confectionery, tobacco, and other consumer goods trades as sales of branded, packaged goods expanded. Even then, salesmen contributed to shaping their work and occupational identity, proving unable to establish professional credentials and dividing over whether adopting trade union methods could improve their position

Imagination: A sine qua non of science

What role does the imagination play in scientifi c progress? After examining several studies in cognitive science, I argue that one thing the imagination does is help to increase scientifi c understanding, which is itself indispensable for scientifi c progress. Then, I sketch a transcendental justification of the role of imagination in this process

Queueing models for token and slotted ring networks

Currently the end-to-end delay characteristics of very high speed local area networks are not well understood. The transmission speed of computer networks is increasing, and local area networks especially are finding increasing use in real time systems. Ring networks operation is generally well understood for both token rings and slotted rings. There is, however, a severe lack of queueing models for high layer operation. There are several factors which contribute to the processing delay of a packet, as opposed to the transmission delay, e.g., packet priority, its length, the user load, the processor load, the use of priority preemption, the use of preemption at packet reception, the number of processors, the number of protocol processing layers, the speed of each processor, and queue length limitations. Currently existing medium access queueing models are extended by adding modeling techniques which will handle exhaustive limited service both with and without priority traffic, and modeling capabilities are extended into the upper layers of the OSI model. Some of the model are parameterized solution methods, since it is shown that certain models do not exist as parameterized solutions, but rather as solution methods