Networking Notes

Introduction

A collection of two or more computers interconnected by the telephone lines, co-axial cable, satellite links, radio and microwave transmission and some other communication techniques.  A computer network is a group of computers that are connected together and that communicate with one another for a common purpose.

Although the computer industry is young compared to anther industries (e.g., automobiles air transportation), computer have made spectacular progress in a short time.  During the first two decades of their extrinsic, computer system highly centralized, usually a single large room.  A medium size company or university might have had one or two computers, while large instructions had at most a few dozen. The idea that with in 20 years equally powerful computers smaller than postage stamps would be mass produced by the millions was pure science fiction.

The merging of computers & communications has had a profound influence on the way computer systems are organized. The concept of the computer center as a room with a large computer to which users bring their work for processing is now totally obsolete. The old model of single computer serving all of the organizations computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computer networks.

Two computers are said to be interconnected if they are able to exchange information’s. The connection need not be via a copper wire; fiber optics, microwaves, and communication satellites can also be used. By requiring the computer to be autonomous, we which to execute from our definitions systems in which there is a clear master/slave relation. If one computer can forcibly starts , stop, or control another one, the computers are not autonomous. A system with one control unit and many slaves is not a network; nor is a large computer with remote printers and terminals.

There is considerable confusion in the literature between a computer network and a distributed system. The key distinction is that in a distributed system, the extence of multiple autonomous computers is transparent to the user. He can type a command to run a program. And it runs. It is up to the operating system to select the best processor, find and transport all the input files to the processor, and put the result in the appropriate place. In other words, the user of a distributed system is not aware that there are multiple processors; it looks like a virtual uni-processer. A location of jobs to processors and files to disks, moment of files between where they are stored and where they are needed, and all other system functions must be automatic.

With a network, user must explicitly log on to one machine, explicitly submit jobs remotely, explicitly move files around and generally handle all the network management personally. With the distributed system, nothing has to be done explicitly it is all automatically done by the system without the users knowledge.

In effect, a distributed system is a software system built on top of a network. The software gives it a high degree of cohesiveness and transparency. Those distinction between a network and a distributed system lies with the software rather than with the hardware. Nevertheless, there is considerable our lap between the two subjects. E.g., both distributed system and computers networks need to move file around. The difference lies in whom in vokes the movement, the system or the user.

Local Area Networks (LAN):

Local area network, generally called LANs, is privately-owned networks with in a single building or campus of up to a few KM in size. They are widely used to connect personal computers and workstation in company offices and factories to share resources (e.g., printers) and exchange information. LANs are distinguished from other kinds of networks by three characteristics:

1.    Their size,

2.    Their transmission technology,

3.    Their topology.

LANs are restricted in size, which means that the worst-case transmission time is bounded and known in advance. Knowing this bound makes it possible to use certain kinds of designs that would not otherwise be possible. It also simplifies network management.

Metropolitan Area Networks (MAN):

A metropolitan area network, or MAN (plural: MANs, not MEN) is basically a bigger version of a LAN and normally uses similar technology. It might cover a group of nearby corporate offices or a city and might be either private or public. A MAN can support both data and voice, and might even be related to the local cable television network. A MAN just has one or two cables and does not contain switching elements, which shunt packets over one of several potential output lines.

Wide Area Networks (WAN):

A wide area network, or WAN, spans a large geographical area, often a country or continent. It contains of machines intended for running user (i.e., application) programs. We will follow traditional usage and call these machines hosts. The term end system is sometimes also used in the literature. The hosts are connected by a communication subnet, or just subnet for short. The job of the subnet is to carry messages from host to host, just as the telephone system carries words from speaker to listener. By separating the pure communication aspects of the network (the subnet) from the application aspects (the hosts), the complete network design is greatly simplified.

In most wide area networks, the subnet consists of two distinct components: transmission lines & switching elements. Transmission lines (also called circuits, channels, or trunks) move bits between machines.

The switching elements are specialized computers used to connect two or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line to forward them on.

In most WANs, the network contains numerous cables or telephone lines, each one connecting a pair of routers. If two routers that do not share a cable nevertheless wish to communicate, they must do this indirectly, via other routers.  When a packet is send from one router to another via one or more intermediate routers, the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and the forwarded.  A subnet using this principle is called a Point to Point, store and forward or packet switched subnet.  Nearly all wide area networks (except those using satellite) have store and forward subnet.  When the packet are small and all the same size, they often called cells.

Wireless network:

Mobile computers, such as notebook computers and personal digital assistants (PDA) are the fastest growing segment of the computer industries.  Many of the owners of these computers have desktop machines on LAN’s and WAN’s back at the office and want to be connected to their home base even when away from home or en route.  Since having a wired connection is impossible in cars and airplanes, there is a lot of interest in wireless network.

Wireless networks come in many forms. Some universities are already installing antennas all over campus to allow students to sit under the trees and consult the libraries card catalogs.  Here the computers communicate directly with the wireless LAN in a digital form.  Another possibility is using a cellular (i.e. portable) telephone with a traditional analog modem.  Direct digital cellular service, called CDPD (Cellular Digital Packet Data) is becoming available in many cities.

Network Hardware

It is now time to turn our attention from the application and social aspects of networking to the technical issues involved in network design. There is no generally excepted taxonomy into which all computer network fit, but two dimensions stand out as important: transmission technologies and scale. We will now examine each of these in turns.

Broadly speaking, there are two types of transmission technologies:

1. Broadcast networks.
2. Point-to-point networks.

Broadcast networks: It have a single communication channel that is shared by all the machines on the network. Short messages, called packets in certain contexts, sent by any machine are received by all the others. An address field within the packet specifies for whom it’s intended. Upon receiving a packet, a machine checks the address field. If the packet is intended for itself, it process the packet, if the packet is intended for the other machine, it is just ignored.

Broadcast systems generally also allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted, it is received and processed by every machine on the network. This mode of operation is called broadcasting. Some broadcast system also support transmission to a subset of a machines, something now has multicasting.

Point-to-point networks: It consists of many connections between individual pairs of machines. To go from the source to the destination, a packet on this type of network may have to first visit one or more intermediate machines. Often multiple routes, of different lengths are possible, so routing algorithm play an important role in point-to-point networks. As a general rule (although there are many exceptions), smaller, geographically localized networks tend to use broadcasting, where larger usually are point-to-point.

Whenever we want two devices – transmitting and receiving device to communicate with each other, we need hardware’s to achieve that.  We would be discussing about the various hardware such as:

1.    Sender and Receiver hardware

2.    Communication devices

3.    Communication channels

 

Sender and Receiver Hardware:

Following hardware are used for handling communication messages, data transfer etc.

Nodes and workstations:

Data communication is done using various communication devices and softwares interconnected for information exchange.  The devices used to communicate a data in communication network are called Workstations.  These workstations may be computer, terminal, printer, telephones and other communication devices.  Each workstation is connected to something called a Data Communication Network Mode.

Multiplexer:

As the name suggests, multiplexing is a form of data transferring which are communication channels and is used for several transmission.  For e.g. the telephone lines that we used for our daily conversation can carry 100’s and even 1000’s of conversations using multiplexing.  In other words multiplexing is a type of network which allows a number of simple, low cost terminals to share each communication lines introduces almost no delay and requires no special computer softwares.

Multiplexing is used in two major ways:

FDM (Frequency Division Multiplexing)

TDM (Time Division Multiplexing)

FDM: By dividing a communication channel into various smaller segments of different frequencies.

TDM: By taking groups of bytes from each sender and send or transmit them over the channel one after another.  Each group of data bytes are tagged at the beginning and end with start and stop bytes.  These by control bytes are then separated at the receiving end and send to their respective places.  This type of multiplexing occurs so fast that the transmission seems continuous.

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