The different models that are used for building distributed computing systems can be classified as : **Minicomputer Model

Workstation Model

Workstation Server Model

Processor Pool Model and

Hybrid Model**

1.Minicomputer Model

The minicomputer model is a simple extension of the centralized time-sharing system. It consists of a many minicomputers interconnected by a communication network. Each minicomputer has multiple users logged on to it simultaneously. Many interactive terminals are connected to each minicomputer. Each user can have remote access to other minicomputers. The network allows a user to access remote resources that are available on some other machines in the network of distributed system. The minicomputer model was suitable when resource sharing with remote users is desired. Example : Early ARPA net.

2.Workstation Model

This model consists of many workstations interconnected by a communication network. In Some applications it is required to have several workstations located at different locations and communicate with each other with the help of communication network. Where each workstation is has its own disk & serves as a single-user computer.

This model was designed to utilize the power of lightly loaded or idle workstations. For some amount of time a significant proportion of the workstations are idle. If the idle workstations are not utilized properly it results in the waste of large amounts of CPU time.

Therefore, the idea of the workstation model is to interconnect all these workstations by a high-speed LAN so that idle workstations may be used to process jobs of users who are logged onto other workstations & do not have sufficient processing power at their own workstations to get their jobs processed efficiently. Example:Sprite system by Xerox PARC.

3.Workstation–Server Model

In the workstation model there is a network of personal workstations. Where each workstation has its own disk & a local file system (Diskful Workstation).

A workstation with its own local disk is usually called a diskful workstation & a workstation without a local disk is called a diskless workstation. Diskless workstations have become more popular in network environments than diskful workstations, making the workstation-server model more popular than the previous model for building distributed computing systems.

A distributed computing system based on the workstation-server model consists of a few minicomputers & multiple workstations interconnected by a communication network. In this model, a user logs onto a workstation called his or her home workstation. Special requests for services provided by special servers are sent to a server providing that type of service that performs the user's requested activity.

Therefore, in this model, the user's processes need not migrated to the server machines for getting the work done by those machines. Example:The V-System.

4.Processor–Pool Model:

The processor-pool model is based on the assumption that most of the time a user needs limited amount of computing power but sometimes he may need a very large amount of processing power for a short period. A single processor could not be enough to perform the task.

Therefore, here in processor-pool model the processors are pooled together. The power of multiple processors can be utilized as and when its required by the user.

The pool of processors consists of a large number of microcomputers & minicomputers attached to the network.Each processor in the pool has its own memory to load & run a system program or an application program of the distributed computing system.

In this model no home machine is present & the user does not log onto any machine. This model has better utilization of processing power & greater flexibility.

Example:Amoeba & the Cambridge Distributed Computing System.

5.Hybrid Model: The workstation-server model has a large number of computer users only performing simple interactive tasks &-executing small programs.

In a working environment that has groups of users who often perform jobs needing massive computation, the processor-pool model is more attractive & suitable.

The Advantages of workstation-server & processor-pool models can be obtained by implementing the hybrid model for a distributed system.

The processors in the pool can be allocated dynamically for computations that are too large or require several computers for execution.

The hybrid model gives guaranteed response to interactive jobs allowing them to be more processed in local workstations of the users.