Chapter 2 Data Communication and Networking

 UNIT 2. Networking

Communication system

The communication system is a system which describes the information exchange between two points. 

The process of transmission and reception of information is called communication. 

The communication system is a system model that describes a communication exchange between two stations, transmitter, and receiver. 

Signals or information passes from source to destination through a channel. It represents a way in which the signal uses it to move from a source toward its destination.

Basic elements of communication

• Sender (source) which creates the message to be transferred.
• Medium which carry message.
• Receiver (sink) which receives the message.
• Data: The information or message to be carried.
• Protocol: Rules to carry the data.

Telecommunication

The word ‘tele’ means large distance and communication means sharing of ideas views and information with other individuals or machine. So, telecommunication is the process of communicating or transferring of data at a long distance.

Data communication

The use of computer to send and receive the data from one place to another is known as data communication.

Modes of communication:

Based on the flow of direction of communication. Communication has 3 modes.

Simplex mode of communication: 

It is unidirectional mode of communication on which sender always sends the data and receiver receives it. Devices connected to this mode either send or receive data at one time. In this mode receiver never sends the data. For example, radio, television

Half duplex Mode of communication: 

It is the bidirectional mode of communication. Data travels in both directions, but only one direction at a time. It can alternatively send or receive data. For example: Walkie-Talkie, pager, Fax Machine

Full duplex mode of communication: 

It is the bidirectional mode of communication in which both sender and receiver can send and receive data simultaneously.  For e.g., mobile phones, telephone etc.

Transmission Media

Transmission media / communication media

Transmission media are means through which data travel form source to destination. Transmission media are from often called communication media or communication channel. As we know the basic component of communication are sender (source), communication media and receiver (sink / destination) which means communication media are used to transmit data. In other word, the physical channel or pathway through which data travel from one place to another or from one computer to another is called transmission media or communication channel.

Depending upon the cost, requirement and reliability, communication media are classified into:

A) Guided / bounded / wired media

Those type of transmission media in which data are restricted to travel inside a physical wire are known as guided media. In the type of transmission media data have to travel in a closed path. There are different types of guided media.

1)Twisted pair cable: 

 A pair of insulating wires are twisted together in twisted pair cable. It is easy to install and not more costly than other type of cable. There are two types of twisted pair cable.

They are twisted to reduce the electromagnetic interference (EMI). This type of cable has greater transmission and fewer chance of error in transmission. It is the most common type of transmission media used in telephone system and in most of the LAN. Twisted pair offers some significant benefits. It is cheaper, lighter, thinner, more flexible, and easier to install than coaxial cable or fiber optics. Twisted pair cable is found in one pair, two pair and four pair. The media connector used for 4 pair cable is RJ-45. 

i) UTP (Unshielded Twisted Pair) It is the type of twisted pair cable in which inner pair of conducting wire are not shielded. Since the pair of wire are not shielded they are immune to EMI (Electro Magnetic Interference). It is cheaper and cannot transfer data for longer distance compared to STP. It is the most common twisted pair cable. E.g. CAT5

ii) STP (Shielded Twisted Pair) It is the rare type of twisted pair of cable in which inner pair of conducting wire are shielded. In order to reduce EMI. They are expensive and can be used for longer distance of transmission. E.g., lBM type 1.

Advantages

1. It is cheaper than other cables.
2. It is light and thin. So, it is flexible for LAN.
3. It can travel data in short distance with higher bandwidth.

Disadvantage 

1. It is only used for short distance transmission.
2. It can be affected by electrical and magnetic field.
3. It is slower type of transmission media compared to other cables.

Co-axial cable:

Coaxial cable is high-capacity cables look like television cable. It contains two wore inner layer and outer layer. Inner layer is covered with an insulating material. The outer layer has a conducting material in the form of mesh (fishnet like structure) which is again covered with insulating layer. It supports high bandwidth and protect from interference.

 It is one of the most common television broadcasting transmission media that carries data signal of higher frequency and at higher speed than twisted pair cable. Co-axial cable has large bandwidth and better reliability which means it can handle large volume of data at high speed. The other advantages includes that it can carry signal over significant speed. Co-axial cable has inner layer of conducting wire covered with an insulator which is surrounded by a conducting mesh and finally covered with a jacket. Co-axial cable used BNC connector.

Advantages:

• It is faster and reliable than twisted pair cable.
• It can transfer data over medium range of distance.

Disadvantages

• It is not appropriate for relatively larger distance.
• It is expensive than twisted pair cable.
• It is rarely used in computer network.

Fiber Optics cable: 

A fiber optics cable is made up of glass or plastic material to transmit data in the form of light. This cable is different than other cables as it plays different set of rules that means data are transferred in the form of photons (light) instead of electricity (electron). Since there is no electricity, it is completely immune to Electromagnetic Interference (EMI). Hence data are transferred in a very high speed with better reliability & less error. It has enormous bandwidth and can carry signal for long distance. It is also more source than other cable.

Advantages:

• It has higher bandwidth that means it can handle large volume of data.
• This medium can be used for long distance transmission.
• It is the most secured and error free transmission medium.

Disadvantages

• It is one of the expensive types of transmission media.
• It is not used for short distance transmission.
• Highly qualified and technical manpower are required to operate on fiber optics.

[Note: Coaxial cable use BNC (British Naval connector) as a media connector where as fiber optics use ST Connector as its media connector]

Unguided media / Unbounded / wireless media  

Those types of transmission media in which data doesn't have to travel in a close path. Data are not restricted to travel inside physical wire instead they travel through air medium which are known as unbounded media. For eg; microwave, radio wave, satellite communication, infrared etc

1) Microwave: It is the type of transmission media which are used for longer distance. This type of wave has higher frequency than radio wave hence it can handle large amount of a data. Microwave signal cannot pass obstacles like hills, buildings, and others. So, it is necessary that microwave transmission (sender and receiver) must be in line of sight (LOS). The transmitting station must be in visible contact with receiving station.

2) Satellite: They are the microwave transmission system in space. It is used as amplifier or repeater that is used to receive information from one location on the earth, repeats the data and send it to one or more receiving location on the earth. The communication is carried out through uplink and downlink. To make communication possible in even longer distance multiple satellites are used. Satellites are set in geo-stationary orbit directly over the equator which notates in synchronization to the earth hence it looks stationary from any point on the earth.

3) Radio wave: It is the type of wireless transmission medium that con operate on single as well as multiple frequency band. In this type of transmission medium sender and receiver station must not be in the line of sight.

4) Infrared: It is wireless transmission technology that use red-light (below the visibility of human eye) to transmit information. Like fibre optics, information infrared use light for communication. They are commonly used for TV remote. They are used for closed area using line of sight communication. That means it cannot penetrate the wall or any obstacle. It cannot be used in open area or outside the building because sun ray also contains infrared in it which may interfere the communication.

5) Bluetooth: It is wireless medium that allow various device to communicate. It can transfer audio, video, or any multimedia contents. It is cost effective and better than infrared. It forms a PAN which doesn't require an internet connection. Works on radio waves. (Specially on the 2.4 GHz.)

6. Wi-fi technology

It stands for Wireless Fidelity. It uses radio waves to provide network connectivity. A wi-fi network is established using a wireless adapter to create hotspots areas in the area of a wireless router that is connected to the network allows users to access internet services. Typically used to access internet on portable device like smartphone, tablets, or laptop. It provides access to a local network of connected device.

Computer Network

The group of interconnected computers and other hardware by means of transmission media to communicate and share resources is known as computer network.

Services provided by computer network.

• Print services: It include a sharing of printers.
• Message service: it includes sharing of message like e-mail.
• File service: it includes sharing of files.
• Application services: it includes sharing of application programs accessed by users.
• Database of services: it helps in managing, modifying and distributing the data.

Advantages of computer network

1. Hardware sharing: Using computer network expensive hardware devices can be shared among multiple number of computers. For example, in cyber one printer can be shared among multiple number of computers which help to save money as well time.
2. Software sharing: Similarly, software also can be shared among computer n a network which helps to access application program from any remote location.
3. Centralized control and management: Network provides the centralized control and management that means every other computer in a network are centrally connected with server which controls and monitor all the activities within the network. This helps to ensure security.
4. Speedy and fast effective communication: Since all the computer in a Network are interconnected with each other, one can sends and receives messages within the computer in just few seconds.
5. Backup and Recovery: Since all the computers are connected with server, it helps to maintain backup at periodic interval. that means keeping extra copy so that data and the formation can be recover when lost accidentally or intentionally.
6. Flexible Access: In a network a computer can easily access files from other computer he/she has authority or privilege to access/use.
7. Work group computing: Network allows different user to work in group that means; different user can work on o single task by dividing workload.

Disadvantage of computer network

1. Virus Transfer: Since computer Network allows data and information to be transferred, still there remains the possibility of virus transfer and other malicious program.
2. Less reliable: Since computer in a network are interconnected with each other performance of one computer may affect performance of another computer. That means if one computer starts malfunctioning in a network than it may affect other computers in a network.
3. Security: Computer network is a group of computers interconnected with each other. That means, one computer in a network can access data and information from another computer. Since there may be also possibility of unauthorized access there exist security issue in a network.
4. Expensive: Creating a network among different computer is itself a complex task or job. We may need different inter networking devices and others special devices and other to establish a network. Buying all the devices, cables and equipment may be expensive.
5. Need Technical Manpower: Since establishing a network is a technical job, we need a technical manpower to establish, and trouble shoot a network. Hiring technical manpower may be expensive.

Components of computer network.

Client (workstation): Client means user. The computer where the client works is workstation. It requests or utilizes the resources of the server. Clients are the computer in which user run applications.

Server: it is the main computer which response or provides services to other computers. It is a powerful computer having higher processing and storage capacity. Operating system like Linux, Unix, windows NT server, Novell NetWare etc.

Types of servers

• Non dedicated server: it controls and provides services to the workstation as well as can be used as a workstation. For e.g.: windows NT server, Advanced 2000 server etc. are non-dedicated server.
• Dedicated server: it controls and provides services to the workstation but cannot be used as workstation. It is a powerful computer. For e.g.: Novell NetWare, UNIX is dedicated server.

• Dedicated server	Non-dedicated server
  Acts as server ONLY	Acts as server as well as workstation
  Uses specialised software	Uses general software
  Very high-quality hardware	Standard quality hardware
  Runs 24/7	Runs periodically
  Huge backup plan	May be no backup
  Handles massive clients	Handles limited clients
  E.G.:Novell netware server, UNIX	E.G.:Windows 2000 server

• NIC (network Interface Card) A network interface card (NIC) is a hardware component without which a computer cannot be connected over a network. It is a circuit board installed in a computer that provides a dedicated network connection to the computer. It is also called network interface controller, network adapter or LAN adapter. It is also known as Ethernet card. It helps to ensures reliable connection between workstation and server.

Type of Network

Based on geographical area covered

1. LAN 2. MAN 3. WAN

1) LAN (Local Area Network):

 LAN stands for Local Area Network. It is the type of Network which covers small geographical area such as room, building, school, university etc. Since it covers small area, are interconnected with guided or wired media such as twisted pair cable, coaxial cable, or fiber optic cable. It is one of the fastest networks in term of transmission speed among all other. Since, they relate to wires they have higher reliability. Some of the implementation of LAN can be in computer lab, cyber, network between different department.

Advantages of LAN

• LANs are relatively easy to manage, even for non-technical users.
• It is cheaper to establish.
• Data transmission is faster than MAN and WAN.
• LANs are more secure than MANs and WANs
• It is easier to establish, manage es of the network and operate.

Disadvantages of LAN

• It is limited only to a small area.
• It can connect a smaller number of computers comparatively.
• Cannot be used as distributed network.

2) MAN (Metropolitan Area Network):

 MAN stands for Metropolitan Area Network. It covers an entire city, district, or village. MAN connects two or more than two LAN together. Big companies like banks, and many organizations have their branches or sub offices in a city or in neighboring city use MAN to connect with each other. The communication media in MAN can be guided as well as unguided. This network use bridge, repeater, router, switch etc. to establish a connection. The data transmission speed of MAN is faster compared to LAN and slower to thot of LAN. Media like microwave, radio wave, infrared etc. are used to connect devices in a MAN.

Advantages of MAN

• MANs can cover a larger geographic area than LANs
• It can connect to many computers than LAN.
• We can use guided as well as unguided type of transmission media.

Disadvantages of MAN

• It is expensive to set up then LAN.
• Transmission speed slower compared to LAN.
• It is complex to establish, manage and provides security.

3) WAN (Wide Area Network):

 It stands for Wide Area Network, which can be extended to the whole world. It is also known as the network of networks. They are used to connect LANs together so that users and computers in one location can communicate with users and computer in another location. WAN is unlimited. It is the slowest data communication among all of three as it covers entire country, continents or even a whole world. These types of networks use unguided media such as microwave, radio wave, satellites and even fiber optics sometimes to communicate in larger distance. The best example of WAN is Internet which is a public network whereas Intranet is a private network earn by an organization. 

Advantage of WAN

• It covers larger geographical area than LAN and MAN.
• It can connect large number of computers compared to LAN and MAN.
• Using WAN communication can be done over a large distance.

Disadvantage of WAN

• It is expensive to establish, manage and operate.
• It is the slowest type of network compared to that of LAN and MAN.
• Highly qualified manpower is required to establish and run this type of network

Comparison of LAN, MAN and WAN

Feature	LAN	MAN	WAN
Geographic coverage	Building or campus	City or region	Global
Data transmission speed	High (10 Mbps to 10 Gbps)	Moderate (10 Mbps to 1 Gbps)	Low (1 Mbps to 100 Mbps)
Cost	Low	Moderate	High
Management complexity	Low	Moderate	High
Security	High	Moderate	Low
Scalability	Limited	Moderate	High
Resource sharing	Easy	Moderate	Difficult
Transmission media	Guided/wired	Wired/wireless	Often Wireless
Reliable	Highly	Reliable	Less reliable

Based on Network architecture.

1. Centralized computing model
2.  Client server model
3.  Peer-to-Peer model

Network Architecture/model

Network model means how the computers are interacted and communicate in the network.

Centralized computing Network

A centralized computing network is a type of network where all processing and data storage is done on a single, central server. This server is responsible for all tasks, including running applications, managing data, and providing access to users. All other devices in the network are connected to the central server and rely on it for all their computing needs. 

Benefits of a centralized computing network:

• Reduced cost: By centralizing all computing resources, organizations can save money on hardware, software, and maintenance costs.
• Improved security: Centralized servers can be more easily secured than individual devices, as all access and data are controlled by a single point.
• Simplified management: Centralized systems are easier to manage and maintain than distributed systems.
• Increased scalability: Centralized systems can be easily scaled to accommodate a growing number of users or devices.

Drawbacks of a centralized computing network:

• Single point of failure: If the central server fails, it can bring down the entire network.
• Performance bottlenecks: If there is a large number of users or devices accessing the network, the central server can become overloaded and performance can suffer.
• Limited flexibility: Centralized systems can be less flexible than distributed systems, as they are not as easily adapted to changing needs.
• Security risks: If the central server is compromised, all of the data on the network can be exposed.

Peer-to-peer architecture

 A peer-to-peer (P2P) network is a decentralized network in which each computer acts as both a client and a server. This means that each computer can share its resources, such as files, printers, and processing power, with other computers on the network. All the jobs and task are carried out by themselves without having authority from other computer. Hence, they are known as peers. Each computer in a network behave itself as a client as well as server. This type of architecture can be used to small work group where security may not be of greater issue. 

Advantages

• Easy to setup
• It is cheaper than Client-Server architecture.
• Easy to troubleshoot.
• All computers have equal rights.
• It is suitable for homes, office and small organization.

Disadvantages

• Data security is very poor in this type of architecture.
• Data recovery and backup is difficult.
• They are less secure than the client-server model.
• The expansion of the network is limited.
• It is slow because of heavy loads.
• It takes more time to install the required S/W.
• It is not appropriate for large scale organizations.

 Client server Architecture: 

In this architecture, the computer is connected to the device called server. Server controls every client. The client must request the server for the data. The server requests, controls and responds to the client. The client utilizes the services provided by the server. Each workstation or client on the network shares the resources of the server computer. The server computer has high capacity, high speed, and large memory capacity. In this architecture user having client computer can access server only when he/she has right to do so. That means, unauthorized people cannot access the server. On this network the client must request the server for the data instead of carrying it out by themselves.

Advantages 

• Centralized administration is possible through this network.
• It is appropriate for large organizations.
• Data recovery and backup process is easier.
• It covers distance area (Large area)
• It is more secure.
• It reduces traffic occur in Network.
• It is a more efficient type of network.

Disadvantages 

• Overload on server
• Skilled manpower is needed for maintenance.
• If the server fails, the whole network will be disturbed.
• It is expensive due to the use of a dedicated server.
• It is complex to establish and manage.
• Experienced administrator is required to operate.

Network Topology / LAN Topology

The physical layout or a geographical orientation of the computer in a network is known as a network topology. There are many different ways for setting up LAN. Therefore, topology is a physical arrangement in which computers or devices in a network are interconnected with each other. People may choose different topologies due to different requirement of the user. This requirement may be high speed of data transfer, data security, cost etc.

Types of Network Topology.

1) Bus Topology:

 The type of arrangement of computers in which all the computers of network are connected to a single cable or trunk in a linear way is known as bus topology. The linear cable is connected with the terminator at its end so that data will not be able to escape. The position of the server is not fixed in this type of topology. The signal pass through the bus in both direction and can be received by all other nodes. All the nodes in this type of topology have equal access to the bus and there is no discrimination.

Advantages

• It is easy to set up.
• It requires less no. of cable so it is not expensive.
• It is easy to implement and extend.
• Use of terminator doesn’t allow the data to escape.

Disadvantages:

Since every computer in a network are connected to the bus, if the cable fails to work then whole network is affected.

• It depends on the single cable.
• Collision occurs.
• Its performance decreases when no. of computers is added.
• It doesn't cover a large geographical area.
• Fault finding and troubleshooting is difficult.

2) Ring topology: 

A computer arrangement in which all the computers or network are connected to a single cable or trunk in a circular way is known as ring topology. Here, each computer acts as a repeater which accepts the incoming signal and regenerates it before passing it to the next one. One data may flow either in clockwise or in anti-clockwise direction. If any breakthrough occurs the whole network collapses. Guided media like twisted pair cable co-axial cable are used for data transmission.

Advantages

• Increase of computer has minimal impact on performance.

• All computers have equal access.

• Each node on the ring has a repeater, so it can be expanded to a greater distance.

• Speed is high as data travel in only one direction.

Disadvantages

• It depends on the single cable.

• Adding and removing computers disturbs the whole network.

• Changing network structure is very difficult.

• It cannot cover large geographical areas.

• Rewriting the program is required, while adding and removing the computer.

3) Star Topology

A topology in which all the computers and servers are connected to a centrally located device called hub or switch in the form of star is known as star topology. So that adding and removing extra computers is easy. Moreover, it provides an excellent platform for troubleshooting and reconfiguration. In this type of topology breakdown in any medium won’t affect the entire network. The commonly used transmission media in this type of topology is twisted pair cable. It is one of the most popular and practical types of network topology.

Advantages

• It is easy to add or remove computers in this topology.
• If one computer or links fail, the other computer is not affected.
• Computers can be added or removed easily without affecting the network.
• It is one of the most reliable topologies.

Disadvantage

• It depends on the central device, so if it fails the whole network fails.
• More number of cables or required.
• It is more expensive.
• It depends upon a central device so if it fails to work it affects the whole network.
• It may be expensive as we require maximum number of cables and devices.

4) Mesh Topology: 

A topology in which all the computers are connected with one another forming a mesh is called mesh topology. It is the only true point to point design. This design is not very practical. This design is one of the most impractical types as it is difficult to install and reconfigure. It requires maximum amount of cable. As the number of cable increases, difficulty of the installation increases simultaneously.

Advantages

• Data transmission is faster in this type of topology.
• Since devices are connected point to point, it is one of the reliable network topology.
• It is a more reliable network.
• Communication is faster between any two computers.
• Identifying faults and isolating them is easier.

Disadvantages

• It is expensive as it uses the maximum number of cables.
• This type of network topology is difficult and complex in structure.

5) Hybrid topology: 

This type of topology is the combination of two or more than two type of topology. In this topology both star and bus topology are connected with each other.A hybrid topology is a network formed by combining two or more different fundamental network topologies, such as star, bus, ring, or mesh. It's essentially a mix-and-match approach to network design that aims to leverage the strengths of each individual topology while mitigating their weaknesses.

Benefits:

• Enhanced Performance: Combines the strengths of different topologies to optimize performance.
• Scalability: Easily expands by adding more devices or subnetworks.
• Cost-Effective: Utilizes existing infrastructure and minimizes cabling costs.
• Resilient: Fault tolerance is improved due to redundancy in certain topologies.
• Specific Applications: Ideal for complex networks with diverse needs

Drawbacks

• Increased Complexity: Design and troubleshooting can be more challenging.
• Compatibility: Requires careful consideration of compatible hardware and protocols.
• Management: May require specialized tools and expertise for effective management

OSI Reference 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.

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.
  
  
  

Physical layer

• The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices.
• The physical layer contains information in the form of bits. 
• The main functionality of the physical layer is to transmit the individual bits from one node to another node.
• When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together

Functions of a Physical layer:

·         Bit synchronization(management): The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.

·         Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.

·         Physical topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.

·         Transmission mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

 

• Signals: It determines the type of the signal used for transmitting the information.

Data-Link Layer

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address. 
The Data Link Layer is divided into two sublayers:  

• 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

Functions of the Data Link Layer

·         Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.

·         Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.

·         Error control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.

·         Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.

·         Access control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

 Network Layer

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP addresses are placed in the header by the network layer. 

Functions of the Network Layer 

·         Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.

·         Logical Addressing: To identify each device on Internetwork uniquely, the network layer defines an addressing scheme. The sender & receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

·         Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical connection between different devices.

·         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 provides services to the application layer and takes services from the network layer. The data in the transport layer is referred to as Segments. It is responsible for the End to End Delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.

Functions of the Transport Layer 

·         Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.

·         Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

Services Provided by Transport Layer 

1.      Connection-Oriented Service

2.      Connectionless Service

 

1.      Connection-Oriented Service: It is a three-phase process that includes

·         Connection Establishment

·         Data Transfer

·         Termination/disconnection

In this type of transmission, the receiving device sends an acknowledgment, back to the source after a packet or group of packets is received. This type of transmission is reliable and secure.

2.      Connectionless service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service.

 

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.

Session Layer

This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

Functions of the Session Layer

• Session establishment, maintenance, and termination: The layer allows the two processes to establish, use and terminate a connection.

• 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.

• Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

 

Presentation Layer

The presentation layer is also called the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. 

Functions of the Presentation Layer

·         Translation: For example, ASCII to EBCDIC.

·         Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.

·         Compression: Reduces the number of bits that need to be transmitted on the network. Data compression is very important in multimedia such as text, audio, video.

 

7) Application Layer

At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications. These applications produce the data, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. This layer provides the network services to the end-users.

Example: Application – Browsers, Skype Messenger, etc. 

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.

 

OSI Model in a Nutshell

Layer No	Layer Name	Responsibility	Information Form(Data Unit)	Device or Protocol
7	Application Layer	Helps in identifying the client and synchronizing communication.	Message	SMTP
6	Presentation Layer	Data from the application layer is extracted and manipulated in the required format for transmission.	Message	JPEG, MPEG, GIF
5	Session Layer	Establishes Connection, Maintenance, Ensures Authentication, and Ensures security.	Message	Gateway
4	Transport Layer	Take Service from Network Layer and provide it to the Application Layer.	Segment	Firewall
3	Network Layer	Transmission of data from one host to another, located in different networks.	Packet	Router
2	Data Link Layer	Node to Node Delivery of Message.	Frame	Switch, Bridge
1	Physical Layer	Establishing Physical Connections between Devices.	Bits	Hub, Repeater, Modem, Cables