Global Knowledge:

It is not possible to implement centralized control in distributed system due to the problem imposed by the architecture itself of the system, so decentralized control is used that is all control functions are distributed among various sites of the system.

But in distributed system due to the absence of global shared memory, it is not possible to have upto date knowledge regarding processes and resources and in the absence of updated global state of distributed system, it is also difficult to implement decentralized control.

Another important function of Operating system is scheduling which is mainly done on the basis of arrival time of the processes. But in case of distributed systems due to the absence of global physical clock, scheduling is also difficult to implement.

Naming:

A name refers to an object such as computer, printer, file, a service etc. One of the service is naming service which is implemented using look up procedure. In table look up procedure, tables or directories are used, which contains the logical name. Such directories can be implemented in two ways.

Approach1

A directory is replicated at each site in order to avoid single point of failure & in order to increase the availability of naming service.

Disadvantage: More space is required.

Consistancy problem that is any modification in a directory at one site should also be reflected in same copy at other site.

Approach 2

Directory is partitioned into various blocks and then these blocks are distributed among various sites. The problem is that when a name is referred then how it will be possible to find the location of the corresponding block.

Solution: Another directory is created which contains the information regarding location of these blocks and then this directory is replicated at various sites.

Scalability:

It refers to the concept that when system grows then it should not result in the unavailability of the system or performance degradation. Example: Broadcast based protocols works well for small system but not for large systems. Suppose a distributed system is locating the file by broadcasting the queries then in such system each computer has to handle the message overhead & as the system grows there will be more such queries resulting in more message overhead & degrading the performance of the system.

Compatability:

Compatability refers to inter-operability among resources that is a resource can be used from any computer or it can be used in combination with other resource. There are three levels of compatibility.

Binary Level Compatability: A system is binary level compatable, if all machines are executing same instruction set that is all of them have same architecture. Most of the operating system functionalities are dependent on underline machine architecture & since architecture is same for all machines so operating system codes will be easy to implement.

Execution Level Compatability: A system is having execution level compatability if the same code can be compiled & executed on any machine of the system.

Protocol Level Compatability: A system is having protocol level compatability, if all machines are sharing same set of rules. It is only due to protocol level compatibility, machines running on different operating system is possible.

Process Synchronization:

Processes running on different machines must access the shared resource in a mutually exclusive manner. In such system, a process can request or release resources at any timeand in any order which may lead to deadlock. Such deadlocks must be detected as early as possible otherwise system performance will degrade.

Resource Management:

In such system, a user must access the remote resources with as much ease as it can access the local resources. Resources can be made available in three ways:

Data Migration: In this approach data is transferred from its source to the location of computation and if any modification is made in the data then it is also reflected at the source side.

Computation Migration: In this approach, computation itself is migrated to the location where required data is present.

Process Synchronization: In this approach, the entire process is migrated to the location where data is present. In distributed system, usually the processes are executed on  the sites where they are generated in order to support load sharing.

Security

Two main aspect of security are: Authorization, it basically involves assigning access rights to the users regarding resources. Authentication, it involves the checking of access rights of users while accessing a particular resource.

Structuring

The structuring of distributed operating system can be performed in three ways:

Monolithic Kernel: Distributed operating system is designed in the form of a monolithic kernel where the kernel contains all the services provided by operating system. A copy of huge kernel is running on all the machines of the system. The limitation of this approach is that most of the machines will not require most of the services but the kernel is still providing it.

Collective Kernel Approach: Operating system is designed as a collection of independent processes where each process represents some service such as distributed scheduling, distributed file system etc. The kernel consists of a nucleus of operating system called micro kernel which is installed on all the machines and provides basic functionalities. The micro kernel also provides interaction between services running on different machines example Galaxy, V-Kernel.

Object Oriented Kernel: All services of operating system are implemented in the form of objects. Each object encapsulate a data structure and also a set of operations for those data structure example Amoeba, CLOUDS.

FTP is an abbreviation for File Transfer Protocol. FTP is a part of TCP/IP suite. Its existence is as old as the Internet itself. The first version of FTP dates back to early 1970's and is still commonly used on the Internet. In the early days of Internet, several programs such as e-mail existed for moving files from one computer to another. But these programs did most of their work behind the scenes without any user interaction. As companies and research institutions began to store & share their data on computers a method of data warehousing came into existence. FTP was created as a way for an individual to contact those computers rind retrieves those files interactively. FTP allowed people on the network to browse through directories of files, select what they wanted & retrieve those ales to their own computers. Besides downloading a file, FTP can also be used to upload ales from your computer to mother computer on the Internet. Like most of the other Internet applications. FTP use, are Client-Server approach. In order to transfer files using FTP, your need to have a FTP program or FTP client to be installed on your computer. The client may be a command line FTP (a utility program provided as a part of the operating system), a modem web browser (success Internet Explorer, Netscape Navigator) or FTP software’s such as Cute FTP, WS_FTP which you can easily download run the Internet.

FTP server allows you to download & upload files such can be of wide range of files types & formats such as ASCII, EBCDIC & binary. Some files on FTP servers are available for everyone, while some have authorized access. In Windows system, inarch a FTP server that is integrated won MS Web Server Internet Information Services (IIS) & on Unix or Linux system, the FTP server is installed as a separate program and not integrated with web server.

Working of FTP

FTP works on Client server approach.

• Whenever a FTP client wants to download files from a remote FTP server it contacts the sercer which prompts the user to enter a username and a password.
• After you have specified the username and password, a successful connection known as commend link is established between the client and the server. Now, FTP server program is ready to server the client’s request for the File Transfer.
• FTP Server displays a large number of directories or folders. After selecting a particular folder from where you want to download a file, a second connection known as data connection is opened up which can transfer file either in ASCII mode or binary mode. After the file is downloaded, the data connection is closed automatically but other command link remains open.

Similarly, you can download more files. When your work is finished, you log off and command link connection closes. You are no longer connected to the FTP Server.

Fig: Conceptual Representation of working of FTP

Advantages:

FTP provides numerous advantages which is the reason why people prefer to download and upload files using FTP. Some of these include:

• Any FTP client running on a Microsoft Windows, Unix, Linux and other operating system can use the F I P Server.
• FTP can be used to upload or download files of almost any size from or to a central server anywhere in the world, unlike email that has limited downloading facility.
• It is very simple, fast & reliable to transfer files from FIP servers either with the help of commands or windows based softwares such as CuteFTP, WS_FTP etc.
• It shields user from variations in a file storage system among FTP servers. It downloads the files from the Unix based FTP Server on the Window based client.
• It provides you the facility to transfer files either in binary mode or in ASCII mode hut some softwares like MS FrontPage do not have such choice.
• You can download multiple files with a single VIP command.

Disadvantages:

• FTP has little security protection when performing file transfer. The logon information that the user enters before connecting is sent unencrypted across the network. This logon information can be used by unauthorized user to access other systems.

To make file Transfer more secure, several enhancements have been made on the FTP including SFTP, SSH protected FTP and BBFTP.

• If the FTP client is accessing FTP Server using command line software then it needs to be aware about the working of the Unix operating system as most of the FTP Servers are Unix based.
• Although there are tens of thousands of anonymous FTP Servers, but there is no central directory in which you can look up for a particular file.
• It is not always user friendly. To retrieve a file, the user needs to know which FTP Server it is on and the directory where it is located along with the specific filenames of the file.

Softwares Available For FTP

Depending on what you are planning to do with your FTP software you should pick best one. There are many software available. The three software packages are:

• Cute FTP
• FTP Explorer
• Elite FTP

Each of these has its own good and bad points.

• Cute FTP used to be the best shareware FTP program around. It is easy to use and has many functions. Unfortunately, because it has become so popular, the latest version only allows you to transfer one file at a time unless you register.
• FTP Explorer is not such a good program as Cute FTP but it is freeware so there are no annoying nag-screens.
• Elite FTP is not such a good program for uploading standard files but better if you are working with CGI as you can send commands to the server by typing them in. This will work for CGI as it does not work with some servers properly and for some reason can’t rename files.

*An operating system is a program that acts as an interface between interface between the user and the computer. Operating system has earned the reputation for being the most critical software in a computer system. Operating system primarily provides a convenient interface to its users and at the same time manages the computer’s resources processor, memory, and I/O devices. In a nutshell operating system can be defined as a resource manager. A general organization of an operating system is as follows-accounting, input/output, command interpreter, data management, program development tools, time-sharing, security, communication. Without an operating system the most powerful computer in the world would be useless. No matter how powerful and elegant your programs are, they can’t function without the assistance of an operating system. *The operating system is characterized in three types-single user operating system, multi user operating system and multiprocessor operating system. 1>Single user operating system is popularly known as personal computer operating system. Their job is to provide a good interface to a single user. The two popular operating systems under this category are DOS and Windows. DOS is an example for a single user operating system that is single user with no multitasking. Multitasking means running more than one program concurrently. Windows is an example for a single user operating system with multitasking capability. There are various versions of operating system like, window 95, window 98 and window XP These operating systems are widely used for small applications like word processing, Internet access, managing small databases etc. 2>Multi user system is that operating which can serve for more than one user at a time, are known as multi user operating systems. Each user can run his own program. The operating system allots a quantum of time for each user for processing his tasks. The most popular multi user operating system is UNIX Operating system. 3>Multi processor operating system is those in which some computer systems involve more than one CPU. Depending on precisely how these CPU’s are connected and what is shared, these computers are called parallel computers or multiprocessor computers. They need a special type of operating system to manage the additional resources.

10 OPEN SOURCE WINDOWS APPLICATIONS

Mostly we all know that open source applications are used for linux os. But Windows too have some open source applications.

They are listed below.

1. VLC

2. Gnumeric

3. Abiword

4. Audacity

5. Inkspace

6. X-Chat 2

7. FreeMind

8. TurboCASH

9. Amaya Web Browser

10.ClamWin

Now let us briefly about these.

1. VLC

VLC is one of the most flexible multimedia players available. It supports a vast number ofaudio and video formats, including H.264, Ogg, DivX, MKV, TS, MPEG-2, mp3, MPEG-4, and aac, and it supports streaming and TV capture cards. VLC isn't limited to viewing multimedia files, either. It can convert and transcend formats, too.

2. Gnumeric

Gnumeric is the spreadsheet portion of the GNOME office suite(as well as a stand-alone tool). Gnumeric has been around for quite some time and is an outstanding entry in the spreadsheet word.

3. Abiword

Abiword is also part of the GNOME office suite and it can serve all your word processing needs. With Abiword, you can create and collaborate.

4. Audacity

It is software for recording live audio, converting analog to digital, edit sounds, remove noise and add effects.

5. Inkspace

It is simillar to illustrator, corel draw and Xara X. Its nice part is availability of many tutorials.

6. X-Chat 2

It is one of the best IRC clients available. It is a valuable resource for consultants and IT admins

7. Freemind

It is like 'to-do' lists software but it has many features. It is a mind mapping software.

8. TurboCASH

It is accounting software used by more than 100,000 companies

9. Amaya Web Browser

It is web browser and also a web editor. It is hosted by W3C.

10.ClamWin

It is an anti-virus software. But it has only manual scan only.