COMPUTER NETWORKS

What is a Computer Network?

A computer network is a group of computers linked to each other that enables the computer to communicate with another computer and share their resources, data, and applications.

Computer Network Types

• LAN(Local Area Network)
• PAN(Personal Area Network)
• MAN(Metropolitan Area Network)
• WAN(Wide Area Network)

LAN(Local Area Network)

• Local Area Network is a group of computers connected to each other in a small area such as building, office.
• LAN is used for connecting two or more personal computers through a communication medium such as twisted pair, coaxial cable, etc.
• It is less costly as it is built with inexpensive hardware such as hubs, network adapters, and ethernet cables.
• The data is transferred at an extremely faster rate in Local Area Network.
• Local Area Network provides higher security.

PAN(Personal Area Network)

• Personal Area Network is a network arranged within an individual person, typically within a range of 10 meters.
• Personal Area Network is used for connecting the computer devices of personal use is known as Personal Area Network.
• Thomas Zimmerman was the first research scientist to bring the idea of the Personal Area Network.
• Personal Area Network covers an area of 30 feet.
• Personal computer devices that are used to develop the personal area network are the laptop, mobile phones, media player and play stations.

MAN(Metropolitan Area Network)

• A metropolitan area network is a network that covers a larger geographic area by interconnecting a different LAN to form a larger network.
• Government agencies use MAN to connect to the citizens and private industries.
• In MAN, various LANs are connected to each other through a telephone exchange line.
• The most widely used protocols in MAN are RS-232, Frame Relay, ATM, ISDN, OC-3, ADSL, etc.
• It has a higher range than Local Area Network(LAN).

Uses Of Metropolitan Area Network:

• MAN is used in communication between the banks in a city.
• It can be used in an Airline Reservation.
• It can be used in a college within a city.
• It can also be used for communication in the military.

WAN(Wide Area Network)

• A Wide Area Network is a network that extends over a large geographical area such as states or countries.
• A Wide Area Network is quite bigger network than the LAN.
• A Wide Area Network is not limited to a single location, but it spans over a large geographical area through a telephone line, fibre optic cable or satellite links.
• The internet is one of the biggest WAN in the world.
• A Wide Area Network is widely used in the field of Business, government, and education.

Examples Of Wide Area Network:

• Mobile Broadband: A 4G network is widely used across a region or country.
• Last mile: A telecom company is used to provide the internet services to the customers in hundreds of cities by connecting their home with fiber.
• Private network: A bank provides a private network that connects the 44 offices. This network is made by using the telephone leased line provided by the telecom company.

Advantages Of Wide Area Network:

Following are the advantages of the Wide Area Network:

• Geographical area: A Wide Area Network provides a large geographical area. Suppose if the branch of our office is in a different city then we can connect with them through WAN. The internet provides a leased line through which we can connect with another branch.
• Centralized data: In case of WAN network, data is centralized. Therefore, we do not need to buy the emails, files or back up servers.
• Get updated files: Software companies work on the live server. Therefore, the programmers get the updated files within seconds.
• Exchange messages: In a WAN network, messages are transmitted fast. The web application like Facebook, Whatsapp, Skype allows you to communicate with friends.
• Sharing of software and resources: In WAN network, we can share the software and other resources like a hard drive, RAM.
• Global business: We can do the business over the internet globally.
• High bandwidth: If we use the leased lines for our company then this gives the high bandwidth. The high bandwidth increases the data transfer rate which in turn increases the productivity of our company.

Internetwork

• An internetwork is defined as two or more computer network LANs or WAN or computer network segments are connected using devices, and they are configured by a local addressing scheme. This process is known as internetworking.
• An interconnection between public, private, commercial, industrial, or government computer networks can also be defined as internetworking.
• An internetworking uses the internet protocol.
• The reference model used for internetworking is Open System Interconnection(OSI).

Types Of Internetwork:

1. Extranet: An extranet is a communication network based on the internet protocol such as Transmission Control protocol and internet protocol. It is used for information sharing. The access to the extranet is restricted to only those users who have login credentials. An extranet is the lowest level of internetworking. It can be categorized as MAN, WAN or other computer networks. An extranet cannot have a single LAN, atleast it must have one connection to the external network.

2. Intranet: An intranet is a private network based on the internet protocol such as Transmission Control protocol and internet protocol. An intranet belongs to an organization which is only accessible by the organization's employee or members. The main aim of the intranet is to share the information and resources among the organization employees. An intranet provides the facility to work in groups and for teleconferences.

Intranet advantages:

• Communication: It provides a cheap and easy communication. An employee of the organization can communicate with another employee through email, chat.
• Time-saving: Information on the intranet is shared in real time, so it is time-saving.
• Collaboration: Collaboration is one of the most important advantage of the intranet. The information is distributed among the employees of the organization and can only be accessed by the authorized user.
• Platform independency: It is a neutral architecture as the computer can be connected to another device with different architecture.
• Cost effective: People can see the data and documents by using the browser and distributes the duplicate copies over the intranet. This leads to a reduction in the cost.

Network Devices

Network devices are required to amplify the signal to restore the original strength of signal and to provide an interface to connect multiple computers in a network. There are many types of network devices used in networking. Some of them are described below

1. Repeater

A repeater is a device that operates only on the physical layer of OSI model. Repeaters have two ports and can connect two segments of a LAN. It amplifies the feable signals when they are transported over a long distance so that the signal can be as strong as the original signal. A repeater boosts the signal back to its correct level. 

2. Hub

Hub is like a repeater with multiple ports used to connect the network channels. It acts as a centralised connection to several computers with the central node or server. When a hub receives a packet of data at one of its ports from a network channel, it transmits the packet to all of its ports to all other network channel. 

3. Gateway

A gateway is an interconnecting device, which joins two different network protocols together. They are also known as protocol converters.

It accepts packet formatted for one protocol and converts the formatted packet into another protocol.

The gateway is a node in a network which serves as a proxy server and a firewall system and prevents the unauthorised access.

4. Switch

It is a small hardware device that joins multiple computers together within one LAN.

Switches work on the data link layer of the OSI model.

It helps to reduce overall network traffic.

Switch forwards a data packet to a specific route by establishing a temporary connection between the source and the destination.

5. Bridge

Bridges serve a similar function as switches. A bridge filters data traffic at a network boundary. Bridges reduce the amount of traffic on a LAN by dividing it into two segments. Traditional bridges support one network boundary, whereas switches usually offer four or more hardware ports. Switches are sometimes called multiport bridges. packets, directing packets to the appropriate locations etc. 

6. Router 

Router is a hardware device which is designed to take incoming packets, analyze the packets, moving the packets to another network, converting the packets to another network interface, dropping the packets, directing packets to the appropriate locations etc. 

7. Modem

Modem is a device that converts digital signal to analog signal (modulator) at the sender's site and converts back analog signal to digital signal (demodulator) at the receiver's end, in order to make communication possible via telephone lines. A MODEM is always placed betweeed a telephone line and a computer. 

8. RJ11 Connector

Connector RJ11 connector is the typical connector used on two pair, four wire handset wiring.

RJ means 'Registered Jack', the physical connector interface that is most often utilised for handset wire terminals.

RJ11 connector wiring comes in two standard assortments UTP or Unshielded Twisted Pair and flat-satin cable or the untwisted. RJ11 connectors are used to terminate phone lines, and are typically deployed with single line POTS (Plain Old Telephone Service) telephone jacks. 

9. RJ45 Connector

 RJ45 stands for Registered Jack-45. It is an eight wire connector. RJ45 connector is used to connect computers onto a Local Area Network (LAN). It is commonly used in telephony applications and networking. It is also used for serial connections. 

10. Ethernet Card

An Ethernet card is a kind of network adapter. These adapters support the Ethernet standard for high-speed network connections via cables.

Ethernet cards are sometimes known as Network Interface Cards (NICs).

Ethernet cards are available in several different standard packages called firm factors. 

OSI Model

• OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.
• OSI consists of seven layers, and each layer performs a particular network function.
• OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered as an architectural model for the inter-computer communications.
• OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.
• Each layer is self-contained, so that task assigned to each layer can be performed independently.

Characteristics of OSI Model:

• The OSI model is divided into two layers: upper layers and lower layers.
• The upper layer of the OSI model mainly deals with the application related issues, and they are implemented only in the software. The application layer is closest to the end user. Both the end user and the application layer interact with the software applications. An upper layer refers to the layer just above another layer.
• The lower layer of the OSI model deals with the data transport issues. The data link layer and the physical layer are implemented in hardware and software. The physical layer is the lowest layer of the OSI model and is closest to the physical medium. The physical layer is mainly responsible for placing the information on the physical medium.

Functions of the OSI Layers

There are the seven OSI layers. Each layer has different functions. A list of seven layers are given below:

1. Physical Layer
2. Data-Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Physical layer

• The main functionality of the physical layer is to transmit the individual bits from one node to another node.
• It is the lowest layer of the OSI model.
• It establishes, maintains and deactivates the physical connection.
• It specifies the mechanical, electrical and procedural network interface specifications.

Functions of a Physical layer:

• Line Configuration: It defines the way how two or more devices can be connected physically.
• Data Transmission It defines the transmission mode whether it is simplex, half-duplex or full-duplex mode between the two devices on the network.
• Topology : It defines the way how network devices are arranged.
• Signals: It determines the type of the signal used for transmitting the information.

Data-Link Layer

• This layer is responsible for the error-free transfer of data frames.
• It defines the format of the data on the network.
• It provides a reliable and efficient communication between two or more devices.
• It is mainly responsible for the unique identification of each device that resides on a local network.
• It contains two sub-layers:
  • Logical Link Control Layer
    • It is responsible for transferring the packets to the Network layer of the receiver that is receiving.
    • It identifies the address of the network layer protocol from the header.
    • It also provides flow control.
  • Media Access Control Layer
    • A Media access control layer is a link between the Logical Link Control layer and the network's physical layer.
    • It is used for transferring the packets over the network.

Functions of the Data-link layer

• Framing: The data link layer translates the physical's raw bit stream into packets known as Frames. The Data link layer adds the header and trailer to the frame. The header which is added to the frame contains the hardware destination and source address.

• Physical Addressing: The Data link layer adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned in the header.
• Flow Control: Flow control is the main functionality of the Data-link layer. It is the technique through which the constant data rate is maintained on both the sides so that no data get corrupted. It ensures that the transmitting station such as a server with higher processing speed does not exceed the receiving station, with lower processing speed.
• Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that is placed to the Data link layer's trailer which is added to the message frame before it is sent to the physical layer. If any error seems to occurr, then the receiver sends the acknowledgment for the retransmission of the corrupted frames.
• Access Control: When two or more devices are connected to the same communication channel, then the data link layer protocols are used to determine which device has control over the link at a given time.

Network Layer

• It is a layer 3 that manages device addressing, tracks the location of devices on the network.
• It determines the best path to move data from source to the destination based on the network conditions, the priority of service, and other factors.
• The Data link layer is responsible for routing and forwarding the packets.
• Routers are the layer 3 devices, they are specified in this layer and used to provide the routing services within an internetwork.
• The protocols used to route the network traffic are known as Network layer protocols. Examples of protocols are IP and Ipv6.

Functions of Network Layer:

• Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical connection between different devices.
• Addresssing: A Network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device on the internet.
• Routing : Routing is the major component of the network layer, and it determines the best optimal path out of the multiple paths from source to the destination.
• Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This process is known as Packetizing. It is achieved by internet protocol (IP).

Transport Layer

• The Transport layer is a Layer 4 ensures that messages are transmitted in the order in which they are sent and there is no duplication of data.
• The main responsibility of the transport layer is to transfer the data completely.
• It receives the data from the upper layer and converts them into smaller units known as segments.
• This layer can be termed as an end-to-end layer as it provides a point-to-point connection between source and destination to deliver the data reliably.

The two protocols used in this layer are:

• Transmission Control Protocol
  • It is a standard protocol that allows the systems to communicate over the internet.
  • It establishes and maintains a connection between hosts.
  • When data is sent over the TCP connection, then the TCP protocol divides the data into smaller units known as segments. Each segment travels over the internet using multiple routes, and they arrive in different orders at the destination. The transmission control protocol reorders the packets in the correct order at the receiving end.
• User Datagram Protocol
  • User Datagram Protocol is a transport layer protocol.
  • It is an unreliable transport protocol as in this case receiver does not send any acknowledgment when the packet is received, the sender does not wait for any acknowledgment. Therefore, this makes a protocol unreliable.

Functions of Transport Layer:

• Service-point addressing: Computers run several programs simultaneously due to this reason, the transmission of data from source to the destination not only from one computer to another computer but also from one process to another process. The transport layer adds the header that contains the address known as a service-point address or port address. The responsibility of the network layer is to transmit the data from one computer to another computer and the responsibility of the transport layer is to transmit the message to the correct process.
• Segmentation and reassembly: When the transport layer receives the message from the upper layer, it divides the message into multiple segments, and each segment is assigned with a sequence number that uniquely identifies each segment. When the message has arrived at the destination, then the transport layer reassembles the message based on their sequence numbers.
• Connection control: Transport layer provides two services Connection-oriented service and connectionless service. A connectionless service treats each segment as an individual packet, and they all travel in different routes to reach the destination. A connection-oriented service makes a connection with the transport layer at the destination machine before delivering the packets. In connection-oriented service, all the packets travel in the single route.
• Flow control: The transport layer also responsible for flow control but it is performed end-to-end rather than across a single link.
• Error control: The transport layer is also responsible for Error control. Error control is performed end-to-end rather than across the single link. The sender transport layer ensures that message reach at the destination without any error.

Session Layer

• It is a layer 3 in the OSI model.
• The Session layer is used to establish, maintain and synchronizes the interaction between communicating devices.

Functions of Session layer:

• Dialog control: Session layer acts as a dialog controller that creates a dialog between two processes or we can say that it allows the communication between two processes which can be either half-duplex or full-duplex.
• Synchronization: Session layer adds some checkpoints when transmitting the data in a sequence. If some error occurs in the middle of the transmission of data, then the transmission will take place again from the checkpoint. This process is known as Synchronization and recovery.

Presentation Layer

• A Presentation layer is mainly concerned with the syntax and semantics of the information exchanged between the two systems.
• It acts as a data translator for a network.
• This layer is a part of the operating system that converts the data from one presentation format to another format.
• The Presentation layer is also known as the syntax layer.

Functions of Presentation layer:

• Translation: The processes in two systems exchange the information in the form of character strings, numbers and so on. Different computers use different encoding methods, the presentation layer handles the interoperability between the different encoding methods. It converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end.
• Encryption: Encryption is needed to maintain privacy. Encryption is a process of converting the sender-transmitted information into another form and sends the resulting message over the network.
• Compression: Data compression is a process of compressing the data, i.e., it reduces the number of bits to be transmitted. Data compression is very important in multimedia such as text, audio, video.

Application Layer

• An application layer serves as a window for users and application processes to access network service.
• It handles issues such as network transparency, resource allocation, etc.
• An application layer is not an application, but it performs the application layer functions.
• This layer provides the network services to the end-users.

Functions of Application layer:

• File transfer, access, and management (FTAM): An application layer allows a user to access the files in a remote computer, to retrieve the files from a computer and to manage the files in a remote computer.
• Mail services: An application layer provides the facility for email forwarding and storage.
• Directory services: An application provides the distributed database sources and is used to provide that global information about various objects.

TCP/IP Reference Model