Computer Networks Definition and History

Computer Networks

Computer networks means, an interconnected set of autonomous computers. The term autonomous means that the computers can function independent of each other’s. However, the computers can exchange information with each other through the communication network system.

Computer networks are emerged as a result of convergence of two technologies computer and communication.

 History 

One of the most significant developments was the ARPANET (Advanced Research Project Agency Network).The ARPANET was start with only four nodes as an experimental network in 1969. The University of California at Los Angeles, the Stanford Research Institute, University of California at Santa Barbara, and the University of Utah were connected as the beginning of the ARPANET network using 50 kbit/s circuits. It has subsequently grown into a network of several computers spanning across the world. Some of the present day concept like packet switching evolved from the ARPANET project. One of the

problems with early computer networks was the bandwidth. In the later years, use of satellite communication and use of fiber optics allowed high-speed data communication over large distance. Today, computer networks are the core of modern communication.

Some Properties of computer Networks

·        Permit sharing of files, data, and other types of information

·        Facilitate communications

·        Share network and computing resources

Network Technologies

There is no generally accepted taxonomy into which all computer networks fit, even though there exist two general classification that are transmission technology and scale.

Network Transmission Technology

Classification of networks based on transmission technology

Based on the transmission technology computer networks are classified into two.

1. Broadcast Networks

2. Point-to-Point networks 

Broadcast Networks

Broadcast network have a communication channel that is share by all the nodes in the network. BUS network is an example of broadcast network.

     BUS network

 All the nodes on the network receive the messages sent by any machine in broadcast network. An address field of the message determines the intended recipient. Upon receiving a message, the node checks the address field of the message. If the message is intend to itself then process the packet otherwise it simply ignore the packet.

Broadcasting refers to a method of transferring a message to all recipients simultaneously. Some broadcast system also support multicasting. Multicasting means send data to a subset of nodes in the network.

Broadcasting

 Multicasting

                                       

Point-to-point Networks

In the case of point-to-point network, there exist many connections between individual pairs of nodes. A transmitted packet may visit some intermediate node before they actually reach its destination. There may multiple routes between two nodes so routing algorithms play an important rule in point to point networks.       

The connection between two nodes either one of the following

1. Dedicated

2. Shared

In the case of dedicated connection, there exist a dedicated connection between sender and receiver. Three types of dedicated connections are there

a.     simplex

b.    half duplex

c.     duplex

Simplex connections only allow the transmission of data in one direction. Radio and TV are examples. In contrast, links that can be used in either direction, but only one way at a time, like a single-track railroad line are called half-duplex links. CBZ radio is a half-duplex device. Links that can be use in both directions at the same time, like a two-lane road, are call full-duplex links. Point-to-point transmission with one sender and one receiver is sometimes call unicasting.

Shared devices share the transmission link by using the technique known as multiplexing

Network Technologies Based on Scale

Classification of Networks based on scale

Based on scale networks are classified into

1.    Local area network(LAN)

2.    Metropolitan area network(MAN)

3.    Wide area network(WAN)

Local Area Network (LAN)

A local area network (LAN) is a computer network that interconnects computers in a limited area such as a home, school, or office building using network media. LAN is normally private owned. These are use to exchange information and to share resources (both hardware and software). The defining characteristics of LANs, in contrast to other networks, include their size, transmission technology and topology.

The most common LAN topology is bus, ring and star.

Metropolitan Area Network

A metropolitan area network (MAN) is a computer network that usually spans a city or a large campus. It may be a single network or it may be means of connecting a number of LANs into larger network so that resources may be share. Cable TV network is an example of MAN.

There exist a standard for MAN, that is DQDB (Distributed Queue Dual Bus) or IEEE 802.6

Wide Area Network (WAN)

A Wide Area Network is a network that covers a large geographical area may comprise a country, continent or even the whole world. WAN allows transmission of data and information across a large geographical area. 

The Internet

The Internet collection of networks or network of networks that consists of millions of private, public networks, of local to global scope, that are linked by a broad array of electronic, wireless and optical networking technologies. The networks are connecting through suitable hardware and software in seamless manner.

The basic difference between WAN and Internet is that WAN is owned by single organization while Internet is not so.

Network Topology Bus, ring and star

Network Topology

The term topology refers to the way a network is laid out, either physically or logically. Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. We will briefly discuss various topologies of LAN.

Bus Topology

Bus topology is an implementation of broadcast networks. In bus topology, a single long cable acts as a backbone to link all devices in the network.

Bus topology                           

The two endpoints of the common transmission cable or medium are usually terminated with a device called a terminator that exhibits the characteristic impedance of the transmission medium.  At any one instance, one node is the master and is allowed to transmit, rest are required to cease from sending. Bus networks use carrier sense multiple access (CSMA) to deal with multiple senders.

Advantages

·        Ease of installation

·        Cheap and simple

·        Uses less cables

Disadvantages

·        Difficulty in fault tolerance and reconfiguration.

·        A break in the cable stops all transmissions.

Ring Topology

   In ring topology, each node has a dedicated point-to-point connection only with the two nodes in either side of it. A message or signal is passed along the ring in one direction, from device to device, until it reaches its destination.

Ring Topology

Advantages

·        Easy to install and reconfigure

·        Fault isolation is simplified

·        Perform better than bus topology under heavy load

 Disadvantages

·        Unidirectional traffic

·        A break in the ring can disable the entire network

Star Topology

Star networks are the most common LAN topology. In a star topology, each device has a dedicated link inly to a central controller, usually the hub. All data exchange between devices take place through hub.

Star topology

 Advantages

·        Better performance

·        Isolation of devices

·        Benefits from centralization

Disadvantage of the star topology is that if the central controller fails, the entire network goes down with it.

Network Topology Tree and Mesh

Tree topology

In tree topology  a central 'root' node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central 'root' node. Means nodes in a tree are link to central hub that controls the traffic to the network but not every device plugs directly to the central controller. Most of the devices are connect to some secondary hub, which in turn connected to central hub.

Tree topology

A very good example of tree topology is the cable TV network.

Advantages

·        Scalable

·        More manageable (because, of different levels of network).

Disadvantages

·        Maintenance may b an issue

Mesh Topology

In mesh network topology every nodes has a dedicated connection to every other nodes. The dedicated connections in the mesh topology only caries the traffic between two devices it connects. A fully connected mesh topology has n(n-1)/2 physical channels to link n devices. To manage this much links very device on the network must need (n-1) input output ports.

Mesh topology

Advantages

·        Secure

·        Robust

·        Eliminate traffic problems

Disadvantages

·        Many ports are needed in each device

·        Installation and reconfiguration are difficult

·        Expensive due to huge cabling