1.2 Goals:

    1.2.1 Advantages of Distributed Systems over Centralized Systems:

                 a) The real driving force behind the trend towards decentralization is economics. According to Grosch's law the computing power of a CPU is proportional to the square of its price. By paying twice as much, we could get four times the performance. This observation fit the mainframe technology of its time quite well.

          With microprocessor technology, Grosch's law no longer holds. For a few hundred dollars we can get a CPU chip that can execute more instructions per second than one of the largest 1980's mainframes. As a result, the most cost-effective solution is building a Distributed system with large number of cheap CPUs, which gives better price/performance ratio than a single large centralized system.

            A slight variation on the above theme is the observation that a collection of microprocessors cannot only give a better price/performance ratio than a single mainframe, but may yield an absolute performance that no mainframe can achieve at any price.  For example, with current technology it is possible to build a system from 10,000 modern CPU chips, each of which runs at 50MIPS, for a total performance of 500,000 MIPS. For a single processor to achieve this, it would have to execute an instruction in 0.002 nsec. No existing machine even comes close to this.

                b) A next reason for building a distributed system is that some applications are inherently distributed. For example  A super market chain might have many stores, each of which contains local transactions and, to keep track of inventory at each store on a local computer rather than centrally at corporate headquarters and top management may want to find out inventory time to time. One way to accomplish this goal is  to make the complete system look like a single computer to the application programs, but implementation decentrally, with one computer per store as we have described. This would then be a commercial distributed system.

                     Another inherently distributed system is what is often called computer supported cooperative work, in which group of people, located far from each other, are working together.

                  c)  Potential advantage of a distributed system over a centralized system is highly  reliability. By distributed the workload over many machines, a single chip failure will bring down at most one machine, leaving the rest intact.

                   d) In a distributed system incremental growth is potentially big plus. For example A company will buy a mainframe with the intention of doing all its work on it. if the company workloads grows, at a certain point the mainframe will no longer be adequate. The only solutions are either to replace the mainframe with a larger one or to add second mainframe. In contrast, with a distributed system, it may be possible simply to add more processors to the system, thus allowing it to expand gradually as the need arises.

                  Item                                                Description

    Economics                            Microprocessors offer a better price/performance than mainframes

    Speed                                   A distributed system may have more total computing power than a mainframe

    Inherent distribution               Some applications  involve spatially separated machines

    Reliability                              If one machine crashes, the system as a whole can still survive

    Incremental growth               Computing power can be added in small increments

1.2.2.    Advantages of Distributed system over independent PC's:

            a) In a distributed system data sharing is big advantage when compared with independent PC's. For example, Railway reservation clerks need access to the master data base of trains and existing reservation. Giving each clerk his own private copy of the entire data base would not work, since nobody would know which seats the other clerk had already sold. Shared data are absolutely essential to this and many other applications, so the machines mush be interconnected, this leads to the distributed system.

           b) In a distributed systems not only sharing the data, can also share expensive peripherals like color laser printers, phototypesetters and massive archival storage devices.

           c)  A distributed system connects a group of  isolated computers to achieve enhanced person-to-person communication. For example electronic mail, chatting, telephone, FAX and voice mail.

            d) Finally, a distributed system is potentially more flexible than giving each user an isolated personal computer. Although one model is to give each person a personal computer and connect them all with a LAN, this is not the only possibility. Another one is to have a mixture of personal and shared computers, perhaps of different sizes, and let jobs run on the most appropriate one, rather than always on the owner's machine. In this way, the workload can be spread over the computers more effectively, and the loss of a few machines may be compensated for by letting people run their jobs elsewhere.

            Item                                Description

    Data Sharing                    Allow may users access to a common data base

    Device sharing                 Allow many users to share expensive peripherals like color printers

    Communication               Make human-to-human communication easier, for example by electronic mail

    Flexibility                        Spread the workload over the available machines in the most cost effective way

1.2.3.  Disadvantages of Distributed Systems

           a) Software : With the current state-of-art, we do not have much experience in designing, implementing and using distributed software. It is difficult to find a operating system, programming languages and applications which  are compatible to distributed systems.

            b) A second problem is due to  the communication network. It can lose messages, which requires special software to be able to recover, and it can become overloaded. When network saturates, it must either be replaced or a second one must be added. In both cases, some portion of one or more buildings may have to be rewired at great expense, or network interface boards may have to be replaced.

            c) The third problem is security. In distributed system data in insecure when compared with independent PC's.

            Item                            Description

        Software                Little software exists at present for distributed system

        Networking            The network can saturate or cause other problems

        Security                 Easy access also applies to secret data