Communications
You have already seen in the module on Local Area Networks how computers can be linked together to form networks, enabling them to communicate, share information and resources. While information requirements within a limited area (like an office building or university campus) can be handled by Local Area Networks (LANs), communication requirements beyond that need Metropolitan Area Networks (MANs) or Wide Area Networks (WANs).
In this session, you will see how the services available on Wide Area Networks help in expanding the usage of a PC. The rapid increase of such networks has been responsible for transforming the PC from an information-processing machine into a communications center.
Wide Area Networks may be commercial networks which anyone can subscribe or they may be run privately by companies for their internal use only. There are international networks with thousands of users as well as smaller ones run by local computer clubs. Computer equipment manufacturers have networks to give their customers the latest information on their products. Customers can also report problems or place orders through them.
Networks like CompuServe offer their users (subscribers) many useful facilities like:
- Communication with other users through E-Mail (Electronic Mail) or FAX (Facsimile).
- Transfer of files from one computer to another. You may want to send a file to a particular user, or you may want to share a game or utility that you have developed with other users. In such a case, you can upload (send) it to the network, from where other users can download it for their use.
- Access to centralized databases. These are like libraries on computers containing business, technical or academic information. They have complete text of publications or abstracts that you can browse through, as in a library. Some well known centralized databases are:
- Dow Jones & Co's Dow Jones News/Retrieval Service.
- Lockheed's Dialog Information Service. It has 518 separate databases covering science, engineering, business and economics.
- Mead Data Central's NEXIS/LEXIS. It offers full text from 15 newspapers like New York Times and 31 magazines like Byte, Newsweek and Business Week, etc.
- West Publishing Co's Westlaw. It allows lawyers to browse through court cases and federal regulations.
- Special Interest Groups (SIGs) or forums where you can ask questions, get responses and exchange information about your specific areas of interest, that could be medicine, cricket or computers.
- On-line services offered by various banks, airlines and shopping centers allow you to transact with them through the network.
- Support forums maintained by various hardware and software companies provide technical expertise and solutions to problems to their customers.
- Computing Time, i.e. allowing you to link to mainframes and utilize their processing capacity.
Wide Area Networks that offer such services are called Bulletin Board Services (BBSs).
Network Operating System
A network operating system does everything a stand alone operating system does, and more in a much more complex environment. In addition to the usual lower level computing tasks, a network operating system is responsible for all the following tasks:
- Directing data traffic through the network
- Allowing and preventing access to data based on security requirements
- Preventing access to data files while they are being processed
- Managing the flow of information between a variety of workstations
- Managing requests for printer services
- Managing communication and messages between network users
- Managing connections between the network and remote sites
- Make services as transparent as possible to the user
Components of the Network Operating System
There are two main components of the network operating system. They are:
- Client software - installed on clients.
- Server software - installed on servers.
Client Software
In a network environment, when a user initiates a request to use a resource that exists on a server in another part of the network, the request has to be forwarded or redirected to the server with the requested resource. The component of the client software that does this task is the redirector.
Redirector
The redirector may also be referred to as a shell or a requester. It is a small part of the network operating system that:
- Intercepts requests in the computer.
- Determines if the requests can be serviced by the local system itself or it needs to be forwarded to the server.
Designators
A designator is an alphabet that is assigned to each network resource. The redirector keeps track of which drive designators are associated with which network resources.
For example, to access a particular shared directory on a remote computer, we can assign a letter of the alphabet, say H, to it. We can then refer to the shared directory on the remote computer as H and the redirector will locate it.
Server Software
Server software makes it possible for users working on other machines to share the server's data and peripherals including shared directories, printers, plotters and disks.
Resource Sharing
The server software not only allows sharing of resources, but also determines the degree of sharing. The degree of sharing includes:
- Allowing different users different levels of access to the resources. For example, a file server could give read or write or read and write permissions to different users.
- Coordinating access to the resources to make sure that two users do not use the same resource at the same time.
Managing Users
Network operating systems make it possible for a network administrator to determine who will be able to use the network. The server software can be used to:
- Create user privileges, which indicates who gets to use the various resources on the network.
- Validate user names and passwords at the time of logging on.
- Grant or revoke user privileges on the network.
- Remove users from the list of users having access to the server.
Network Services
Network services are network operating application programs that run on the network. The network operating system installation program ensures that the user has a minimum of network services installed by default.
Some examples of services installed by the NT operating system, by default are:
- Alerter service - notifies selected users and computers, of administrative alerts that occur on the computer. As an example, an alerter service could be set to issue alerts whenever the used space of a hard disk exceeds a particular percentage of the total hard disk space.
- Event log service - records system, security and program events in the event log.
- Messenger service - sends and receives messages sent by administrators or by the alerter service.
- Other services such as the telephony service, which is used by programs to make data, fax, voice calls. These services have to be manually started by the user.
Shared Network Applications
Applications such as word processors and databases can be shared on the network like any other resource.
The advantages of shared network applications are:
- It makes application programs less expensive because buying a site license for 200 users on an application is usually cheaper than buying 200 individual copies of the application.
- It ensures that everyone will be using the same version of the product.
- The information in the database that is stored on the server, can be accessed by the clients on remote machines. This model of the client/server network is the most efficient way for:
- Database access and management.
- Network management.
- Centralized file storage.
Access Methods
The set of rules defining how a computer puts data onto the network cable and retrieves data from the cable is called an access method. Access methods prevent simultaneous access to the cable. By ensuring that only one computer can put the data on the network cable, access methods keep the transmission of network data an orderly process.
- Carrier-Sense Multiple Access
- Token Passing
- Demand Priority
Carrier-Sense Multiple Access with Collision Detection
In the Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) method, each computer on the network senses any signal passing on the cable. The computer can only transmit if the cable is free. If two or more computers send data at exactly the same time, it will result in data collision. When a collision occurs, the two computers involved stop transmitting and then attempt to re-transmit after a certain period of time. The collision detection method is only effective up to 2,500 meters.
With more traffic, collisions tend to increase, which slows down the network. So, CSMA/CD can be a slow access method.
Carrier-Sense Multiple Access with Collision Avoidance
In the Carrier-Sense Multiple Accesses with Collision Avoidance (CSMA/CA) method, each computer signals its intent to transmit before it actually transmits data. This way, the computer can avoid collisions. CSMA/CA is a slower access method and is less popular than CSMA/CD.
Token Passing
In Token Passing, a special packet called a token circulates around the ring network in one direction. When any computer on the network wants to send data, it takes control of a free token. The computer can then transmit data. Until the token has been released by the computer sending data, no other computer can transmit. As only one computer can use the token at a time, the possibility of a collision is ruled out.
Demand Priority
Demand priority is an access method designed for the 100 Mbps Ethernet standard called 100 VG-AnyLAN. The 100VG-Anyl_AN network comprises repeaters and end nodes. An end node could be a computer, bridge, router or switch. The function of the repeater is to search for requests from all nodes on the network. In a demand priority network, computers can receive and transmit at the same time. This is because four pairs of wires are used in the cabling scheme defined for this method. In demand priority, communication is only between the sending computer, the hub or the repeater, and the destination computer. This method is more efficient than CSMA/CD, which sends transmission to the entire network.
Define Access Method
Access method is a set of rules defining how computer puts data onto the network cable and retrieves data from the cable.
Components of a Network
The following are the components and features that all networks have in common:
- Servers - Computers that provide shared resources to network users.
- Clients - Computers that access shared network resources provided by a server.
- Media - The way in which the computers are connected.
- Resources - Files, printers or other items to be used by network users.
Even with these shared components, networks can be divided into two broad categories:
Peer-to-Peer Networks
In a peer-to-peer network, there are no dedicated servers. All the computers are equal and, therefore, are termed as peers. Normally, each computer functions as both a client and a server.
A peer-to-peer network has the advantage of simplicity in design and maintenance. It is usually less expensive to set up as compared to server-based networks. Peer-to-peer networks are also called workgroups. The term workgroup implies a small group of users.
Peer-to-peer networks are suitable for environments where:
- There are limited users (usually 10 or less).
- The users are located in the same area.
- Security is not an important issue.
- The organization and the network have limited growth.
- Users need to freely access data and programs that reside on other computers across the network
In a peer-to-peer network, there is no one assigned to be an administrator responsible for the entire network. Users administer their own computers. All users can share any of their resources.
Note: Network administration involves tasks like managing users and security, making resource available, installing and upgrading software.
Why Networks Exist
During the 80's, the concept of desktop computers working independently was very popular. This was, and still is called the stand-alone environment.
Slowly, businessmen all over the world began to realize that information was useful only when it was communicated between human beings. The process of distributing and processing information among, individuals, each with an independent desktop computer being slow and prone to error, led to the concept of connecting computers together to form computer networks.
Computer Networks
A computer network is a communication system where a group of computers and other devices like printers are connected by cables and other hardware. The data is shared between the computers. A network, besides facilitating data communication, allows resources to be shared among all the systems connected to the network (refer Figure 1.3). Thus, users on the second floor of a building can use the printer on the eleventh floor if their computer and the printer are connected to the network. This concept of connected computers sharing resources is called networking.
Computers that are connected in a network can share:
- Data
- Messages
- Printers
- Hard Disks
- CD-ROMs
- Modems
- Other hardware resources