• The collection of interconnected computers is called a computer network.
• Two computers are said to be interconnected if they are capable of sharing and exchanging information.
• Resource Sharing
• Cost Factor
• Communication Medium
Resource Sharing means to make all programs, data and peripherals available to anyone on the network irrespective of the physical location of the resources and the user.
Reliability means to keep the copy of a file on two or more different machines, so if one of them is unavailable (due to some hardware crash or any other) them its other copy can be used.
Cost factor means it greatly reduces the cost since the resources can be shared
Communication Medium means one can send messages and whatever the changes at one end are done can be immediately noticed at another.
1969 - First network came into existence ARPANET (ADVANCED RESEARCH PROJECT AGENCY NETWORK) MID 80’S - NSFNET (NATIONAL SCIENCE FOUNDATION NETWORK)
• Internet is the network of networks.
Switching techniques are used for transmitting data across networks.
Different types are :
• Circuit Switching
• Message Switching
• Packet Switching
• Circuit switching is the transmission technology that has been used since the first communication networks in the nineteenth century.
• First the complete physical connection between two computers is established and then the data are transmitted from the source computer to the destination.
• When a call is placed the switching equipment within the system seeks out a physical copper path all the way from the sender to the receiver.
• It is must to setup an end-to-end connection between computers before any data can be sent.
• The circuit is terminated when the connection is closed.
• In circuit switching, resources remain allocated during the full length of a communication, after a circuit is established and until the circuit is terminated and the allocated resources are freed.
• In this the source computer sends data or the message to the switching circuit which stores the data in its buffer.
• Then using any free link to the switching circuit the data is send to the switching circuit.
• Entire message is sent to the destination. It reaches through different intermediate nodes following the “store and forward” approach.
• No dedicated connection is required.
• Packet switching is a more recent technology than circuit switching.
• Packet switching introduces the idea of cutting data i.e. at the source entire message is broken in smaller pieces called packets which are transmitted over a
network without any resource being allocated.
• Then each packet is transmitted and each packet may follow any rout available and at destination packets may reach in random order.
• If no data is available at the sender at some point during a communication, then no packet is transmitted over the network and no resources are wasted.
• At the destination when all packets are received they are merged to form the original message.
• In packet switching all the packets of fixed size are stored in main memory.
• Data is transmitted over copper wires, fiber optic cable, radio and microwaves. the term 'media' is used to generically refer to the physical connectors, wires or devices used to plug things together.
o unshielded twisted pair (UTP)
o shielded twisted pair (STP)
o coaxial cable (thinnet, thicknet)
• Fiber optic
• Radio & microwave
• These cables consist of two insulated copper wires twisted around each other in a double helix.
• Twisting of wires reduces crosstalk which is bleeding of a signal from one wire to another.
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
STP offers greater protection from interference and crosstalk due to shielding. But it is heavier and costlier than UTP.
1. In local telephone communication
2. For digital data transmission over short distances upto 1 km
• Easy to install and maintain
• Low weight
• Suitable for small (Local) Networks
• Not suitable for long distance due to high attenuation.
• Low bandwidth support.
• Low Speed