When two or more networks are connected,they become an internetwork or internet.Individual networks are joined into internetworks by the use of internetworking devices.These devices,which include routers and gateways are discussed in details in the internetworking devices category.
The term internet(lowercase i) should not be confused with the Internet(uppercase I).The first term used to mean an interconnection of networks.The second is the name of a specific worldwide network.

A typical diagram if INTERNETWORK is shown in the figure below:-

Today when we speak of networks,we are generally referring to three primary categories.They are:-
1.Local are networks
2.Metropolitan area networks and
3.wide area networks.

Into which category a network falls is determined by its size,its ownership,the distance it covers and its physical architecture.

Local Area Network(LAN)
A local area network(LAN) is usually privately owned and links the devices in a single office,building or campus.Depending on the requirements of an organization and the type of technology used,a LAN can be as simple as two PCs and a printer in someone's office or it can extend throughout a company and include voice,sound and video peripherals.
A typical diagram of LAN is shown in the figure below:-







LANs are designed to allow resources to be share between personal computers or workstations.The resources to be shared can include hardware(eg,a printer),software(eg,an application program) or data.A common example of a LAN found in many business environments,links a work group of task-related computers.For EXAMPLE-engineering workstations or accounting PCs.One of the computers may be given an large capacity disk drive and become a server to the other clients.Software can be stored on this central server and used as needed by the whole group.
In addition to size,LAN's are distinguished from other types or networks by their transmission media and topology.In general,a given LAN will use only one type of transmission medium.The most common topologies are BUS,RING and STAR.
Traditionally,LAN's have data rates in the 4 to 16 Mbps range.Today,however,speeds are increasing and can reach 100 Mbps with gigabit systems in development.

Metropolitan Area Network(MAN)

A metropolitan area network(MAN) is designed to extend over an entire city.It may be a single network such as a cable television network,or it may be a means of connecting a number of LANs into a larger network so that resources may be shared LAN to LAN
as well as device to device.For example,a company can use a MAN to connect the LANs in all of its offices throughout a city.
A typical diagram of MAN is shown in the figure below:-



A MAN may be wholly owned and operated by a private company or it may be service provided by a public company,such as a local telephone company.Many telephone companies provide a popular MAN service called Switched Multi-megabit Data Services(SMDS)

Wide Area Network(WAN)

A wide area network(WAN) provides long distance transmission of data,voice,image and video information over large geographical areas that may comprise a county,a continent or even the whole world.
A typical diagram of WAN is shown in the figure below:-

A data communication system is made up of five components.They are:-
1.Message
2.Sender
3.Receiver
4.Medium
5.protocol

A typical data communication system components is shown in the figure below:

1.Message:The message is the information(data)to be communicated.It can consist of text,numbers,pictures,sound or video- or any combination o these.

2.Sender:The senderis the device that sends the data message.It can be a computer,workstation,telephone handset,video camera and so on.

3.Receiver:The receiveris the device that receives the message.It can be a computer,workstation,telephone handset,television and so on.

4.Medium:The transmission mediumis the physical path by which a message travels from sender to receiver.It can sonsist of twisted pair wire,coaxial cable,fiber-optic cable,laser or radio waves(terrestrial or satellite microwave).

5.Protocol:A protocolis a set of rules that govern data communication.It represents an agreement between the communication devices.Without a protocol,two devices may be connected but not communicating,just as a person speaking French cannot be understood by a person who speaks only Japanese.

Data communication is the exchange of data(in the form of 0s and 1s) between two devices via some form of transmission medium(such as a wire cable).Data communication is considered local if the communicating devices are in the same building or a similarly restricted geographical area and is considered remote if the devices are father apart.
For data communication to occur,the communication devices must be part of a communication system made up of a combination of hardware and software.The effectiveness of a data communication system depends on three fundamental characteristics:-

1. Delivery:The system must deliver data to the correct destination.Data must be received by the intended device .2.Accuracy:The system must deliver data accurately.Data that have been altered in transmission and left uncorrected are unusable.
3.Timeliness:The system must deliver data in timely manner.Data delivered late are useless.In case if video,audio and voice data,timely delivery means delivering data as they are produced in the same order that they are produced and without significant delay.This type of delivery is called real-time transmission.