Introduction of Computer Networks


Data Communication:When we communicate,when we share information.It can be local or remote.Local communication occur face to face while remote communication take place over distance.Data communication are the exchange of data between two devices via the some form of transmission medium such as wire cable.

Characteristics Of Data Communication

The Data communication must have major three fundamental characteristics:

  • Delivery
  • Accuracy
  • Time Line

1) Where Delivery means system must delivered data to correct destination. Data must be received by the intended device.

2) Accuracy mean data delivered in accurately. Means that data should not be altered during transmission.

3) Time line means data should be delivered in time. When data in form of video audio is transfer as they produced at same time to other location is called real time transition.

Types Of Data Communication

There ore two types of data communication

  • Serial communication
  • Parallel communication

Serial   communication

In telecommunication and computer science, serial communication is the process of sending data one bit at one time, sequentially  on a single wire, over a communication channel or computer bus. Serial is  a common communication protocol that is used by many devices. Serial communication has become the standard for intercomputer communication. Serial communication is used for all long-haul communication and most computer networks its save the costs of cable. Serial communication is a popular means of transmitting data between a computer and a peripheral device such as a programmable instrument or even another computer. its also easy to established and no extra devices are used because  most of  computers have one or more serial ports.Examples isR-232,Universal Serial Bus,R-423,PCI Express.

Parallel communication

Parallel communication is fast method of communication. in Parallel transmission transmit the data across a parallel wire. These Parallel wires are flat  constituting multiple, smaller cables. Each cable can carry a single bit of information . A parallel cable can carry group of data at the same time. In telecommunication and computer science, parallel communication is a method of sending several data signals over a communication link at one time.  Examples is Industry Standard Architecture(ISA),Parallel ATA,IEEE 1284,Conventional PCI.

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  • For synchronous data transfer, both sender and receiver access the data according to the same clock. Therefore, a special line for the clock signal is required. A master(or one of the senders) should provide clock signal to all the receivers in synchronous data transfer mode. Synchronous data transfer supports very high data transfer rate.
  • For asynchronous data transfer, there is no common clock signal between the senders and receivers. Therefore, the sender and the receiver first need to agree on a data transfer speed. This speed usually does not change after data transfer starts. The data transfer rate is slow in asynchronous data transfer..

Data Flow Communication between two devices can be simplex, half-duplex, or full-duplex:

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In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.

In half-duplex mode, each station can both transmit and receive, but not at the same time. : When one device is sending, the other can only receive, and vice versa . The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.

In full-duplex mode,data transmission means that data can be transmitted in both directions on a signal carrier at the same time. For example, on a local area network with a technology that has full-duplex transmission, one workstation can be sending data on the line while another workstation is receiving data. Full-duplex transmission necessarily implies a bidirectional line (one that can move data in both directions).

Network:A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.

Type of Connection: A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For visualization purposes, it is simplest to imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same link at the same time.

There are two possible types of connections:



a)Point-to-Point: A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links. When you change television channels by infrared remote control, you are establishing a point-to-point connection between the remote control and the television’s control system.

b)Multipoint: A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection.

Network Topology:Network topology is the arrangement of the various elements of a computer or biological network. Essentially it is the topological structure of a network, and may be depicted physically or logically. Physical topology refers to the placement of the network’s various components, inducing device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design.

Devices on the network are referred to as ‘nodes.’ The most common nodes are computers and peripheral devices. Network topology is illustrated by showing these nodes and their connections using cables.

Factors to be taken into consideration while choosing a Network topology:

1)  Scale of your project (in terms of number of components to be connected).
2)  Amount of traffic expected on the network.
3)  Budget allotted for the network i.e. amount of money you are willing to invest.
4)  Required response time

Types of Network Topology:

1)Bus Topology

2)Ring Topology

3)Star Topology

4)Mesh Topology

5)Tree Topology

1)Bus Topology: In networking a bus is the central cable — the main wire — that connects all devices on a local-area network (LAN). It is also called the backbone. This is often used to describe the main network connections composing the Internet.  Bus networks are relatively inexpensive and easy to install for small networks. Ethernet systems use a bus topology.A signal from the source is broadcasted and it travels to all workstations connected to bus cable. Although the message is broadcasted but only the intended recipient, whose MAC address or IP address matches, accepts it. If the MAC /IP address of machine doesn’t match with the intended address, machine discards the signal. A terminator is added at ends of the central cable, to prevent bouncing of signals. A barrel connector can be used to extend it.

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Advantages of Bus Topology

  1. It is cost effective.
  2. Cable required is least compared to other network topology.
  3. Used in small networks.
  4. It is easy to understand.
  5. Easy to expand joining two cables together.

Disadvantages of Bus Topology

  1. Cables fails then whole network fails.
  2. If network traffic is heavy or nodes are more the performance of the network decreases.
  3. Cable has a limited length.
  4. It is slower than the ring topology.

2)Ring Topology:All the nodes are connected to each-other in such a way that they make a closed loop. Each workstation is connected to two other components on either side, and it communicates with these two adjacent neighbors. Data travels around the network, in one direction. Sending and receiving of data takes place by the help of TOKEN.

Token Passing: Token contains a piece of information which along with data is sent by the source computer. This token then passes to next node, which checks if the signal is intended to it. If yes, it receives it and passes the empty to into the network, otherwise passes token along with the data to next node. This process continues until the signal reaches its intended destination.
The nodes with token are the ones only allowed to send data. Other nodes have to wait for an empty token to reach them. This network is usually found in offices, schools and small buildings.

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Advantages of Ring Topology

  1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data.
  2. Cheap to install and expand

Disadvantages of Ring Topology

  1. Troubleshooting is difficult in ring topology.
  2. Adding or deleting the computers disturbs the network activity.
  3. Failure of one computer disturbs the whole network.

3)Star Topology: In a star network devices are connected to a central computer, called a hub. Nodes communicate across the network by passing data through the hub.

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Advantages of Star Topology

1)  As compared to Bus topology it gives far much better performance, signals don’t necessarily get transmitted to all the workstations. A sent signal reaches the intended destination after passing through no more than 3-4 devices and 2-3 links. Performance of the network is dependent on the capacity of central hub.
2)  Easy to connect new nodes or devices. In star topology new nodes can be added easily without affecting rest of the network. Similarly components can also be removed easily.
3)  Centralized management. It helps in monitoring the network.
4)  Failure of one node or link doesn’t affect the rest of network. At the same time its easy to detect the failure and troubleshoot it.

Disadvantages of Star Topology

1)  Too much dependency on central device has its own drawbacks. If it fails whole network goes down.
2)  The use of hub, a router or a switch as central device increases the overall cost of the network.
3)   Performance and as well number of nodes which can be added in such topology is depended on capacity of central device.

4)Mesh Topology:In a mesh network, devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network.

There are two types of mesh topologies:

Advantages of Mesh Topology

  1. Each connection can carry its own data load.
  2. It is robust.
  3. Fault is diagnosed easily.
  4. Provides security and privacy.

Disadvantages of Mesh Topology

  1. Installation and configuration is difficult.
  2. Cabling cost is more.
  3. Bulk wiring is required.

5)Tree Topology:Tree Topology integrates the characteristics of Star and Bus Topology. Earlier we saw how in Physical Star network Topology, computers (nodes) are connected by each other through central hub. And we also saw in Bus Topology, work station devices are connected by the common cable called Bus. After understanding these two network configurations, we can understand tree topology better. In Tree Topology, the number of Star networks are connected using Bus. This main cable seems like a main stem of a tree, and other star networks as the branches. It is also called Expanded Star Topology

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Advantages of Tree Topology

  1. Extension of bus and star topologies.
  2. Expansion of nodes is possible and easy.
  3. Easily managed and maintained.
  4. Error detection is easily done.

Disadvantages of Tree Topology

  1. Heavily cabled.
  2. Costly.
  3. If more nodes are added maintenance is difficult.
  4. Central hub fails, network fails.

6)Hybrid Topology:A hybrid topology is a type of network topology that uses two or more other network topologies, including bus topology, mesh topology, ring topology, star topology, and tree topology.

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Hybrid network topology has many advantages. Hybrid topologies are flexible, reliable, have increased fault tolerance. The new nodes can be easily added to the hybrid network, the network faults can be easily diagnosed and corrected without affecting the work of the rest of network. But at the same time hybrid topologies are expensive and difficult for managing.

Types of Network:

1)LAN:A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet.A LAN typically relies mostly on wired connections for increased speed and security, but wireless connections can also be part of a LAN. High speed and relatively low cost are the defining characteristics of LANs.the maximum span of 10 km.

2)WAN:A wide area network, or WAN, occupies a very large area, such as an entire country or the entire world. A WAN can contain multiple smaller networks, such as LANs or MANs. The Internet is the best-known example of a public WAN.

3)MAN:A metropolitan area network (MAN) is a hybrid between a LAN and a WAN. Like a WAN, it connects two or more LANs in the same geographic area. A MAN, for example, might connect two different buildings or offices in the same city. However, whereas WANs typically provide low- to medium-speed access, MAN provide high-speed connections, such as T1 (1.544Mbps) and optical services.
The optical services provided include SONET (the Synchronous Optical Network standard) and SDH (the Synchronous Digital Hierarchy standard). With these optical services, carriers can provide high-speed services, including ATM and Gigabit Ethernet. These two optical services provide speeds ranging into the hundreds or thousands of megabits per second (Mbps). Devices used to provide connections for MANs include high-end routers, ATM switches, and optical switches.


A Personal Area Network (PAN) is a computer network used for communication among computer devices, including telephones and personal digital assistants, in proximity to an individual’s body. The devices may or may not belong to the person in question. The reach of a PAN is typically a few meters. PANs can be used for communication among the personal devices themselves (intrapersonal communication), or for connecting to a higher level network and the Internet .

5)Campus Area Network – This is a network which is larger than a LAN, but smaller than an MAN. This is typical in areas such as a university, large school or small business. It is typically spread over a collection of buildings which are reasonably local to each other. It may have an internal Ethernet as well as capability of connecting to the internet.

6)Storage Area Network – This network connects servers directly to devices which store amounts of data without relying on a LAN or WAN network to do so. This can involve another type of connection known as Fibre Channel, a system similar to Ethernet which handles high-performance disk storage for applications on a number of professional networks.