Subject Name: Data Communication and Networking

Subject code   : SAZ6B/ SAE6A/ SEU6D

Syllabus Unit I: Introduction to Data Communication, Network, Protocols & standards and standards organizations - Line Configuration - Topology - Transmission mode - Classification of Network - OSI Model - Layers of OSI Model.

                                                        PART-A

1. What are the goals of a network?

2. What is an interface?

3. What do you mean by bit stuffing?

4. Distinguish between LAN and WAN

5. What is a Network?

6. Name the characteristics that determine the effectiveness of data communication system.

7. What are the two types of line configuration?

8. What do you mean by Data Communication?

9. Define Network.

10. List the seven layers of an OSI model.

11. What do you mean by Data Communication?

12. What is LAN?

13. What is the use of Microwaves?

                                                    PART-B

1. Explain the design issues of network layer.

2. Explain briefly the five components of a data communication system.

3. Explain any two types of network topology.

4. Explain the five components of a data communication system.

5. Explain the LAN and MAN Networks with a neat Diagram.

6. Discuss about Twisted-Pair Cable.

7. Write a short note on protocol and standards.

8. Explain the LAN and MAN networks, with neat diagrams.

9. Explain the five components of a data communication system.

10. What are the advantages and disadvantages of optical fiber?

                                                  PART-C

1. Explain the various classification of network in detail.

2. Discuss in detail about OSI model.

3. Explain the various network topologies with neat diagrams.

முன்நோக்கி செல்லும் போது கனிவாயிரு. ஒருவேளை பின்நோக்கி வரநேரிட்டால் யாராவது உதவுவார்கள்.

சலித்துக் கொள்பவன் ஒவ்வொரு வாய்ப்பிலும் உள்ள ஆபத்தைப் பார்க்கிறான். சாதிப்பவன் ஒவ்வொரு ஆபத்திலும் உள்ள வாய்ப்பினைப் பார்க்கிறான்.

1. What are the goals of a network?

ü  The main goal of networking is "Resource sharing" to make all programs, data and equipment available to anyone on the network without the regard to the physical location of the resource and the user.

ü  A second goal is to provide high reliability by having alternative sources of supply.

2. What is a Link?

ü  At the lowest level, a network can consist of two or more computers directly connected by some physical medium such as coaxial cable or optical fiber. Such a physical medium is called as Link.

3. What do you mean by bit stuffing?

ü  Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data, so that the receiver does not mistake the pattern 0111110 for a flag.

4. Distinguish between LAN and WAN

ü  LAN (Local Area Network) is a computer network covering a small geographic area, like a home, office, school, or group of buildings.

ü  One LAN can be connected to other LANs over any distance via telephone lines and radio waves.

ü  LAN covers 100 m.

ü  The network is spread to a very small location.

ü  WAN (Wide Area Network) is a computer network that covers a broad area (e.g., any network whose communications links cross metropolitan, regional, or national boundaries over a long distance)

ü  Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites.

ü  WAN covers more than 100 m.

ü  The network is spread world wide

5. Define Network. (Or) What is a Network?

ü  A network is a set of devices (often referred to as nodes) connected by communication links.

ü  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.

6. Name the characteristics that determine the effectiveness of data communication system.

7. What are the two types of line configuration?

ü  Line configuration. The physical layer is concerned with the connection of devices to the media.

ü  In a point-to-point configuration, two devices are connected through a dedicated link.

ü  In a multipoint configuration, a link is shared among several devices.

8. What do you mean by Data Communication?  

ü  When we communicate, we are sharing information. This sharing can be local or remote. Local communication usually occurs face to face, while remote communication takes place over distance.

ü  The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance.

ü  Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable

9. List the seven layers of an OSI model.

The layers of OSI

ü  Physical Layer

ü  Data Link Layer

ü  Network Layer

ü  Transport Layer

ü  Session Layer

ü  Presentation Layer

ü  Application Layer

10. Which layers are network support layers?

ü  Physical Layer

ü  Data link Layer and

ü  Network Layers

11. Which layers are user support layers?

ü  Session Layer

ü  Presentation Layer and

ü  Application Layer

12. What is LAN?

ü  A local area network (LAN) is usually privately owned and links the devices in a single office, building, or campus.

ü  A LAN can be as simple as two PCs and a printer in someone’s home office; or it can extend throughout a company and include audio and video peripherals.

ü  Currently, LAN size is limited to a few kilometers.

13. What is the use of Microwaves?

Microwaves

ü  Electromagnetic waves having frequencies between I and 300 GHz are called microwaves.

ü  Microwaves are unidirectional.

ü  When an antenna transmits microwave waves, they can be narrowly focused.

ü  This means that the sending and receiving antennas need to be aligned.

ü  The unidirectional property has an obvious advantage.

ü  A pair of antennas can be aligned without interfering with another pair of aligned antennas.

14. What is a node?

ü  A network can consist of two or more computers directly connected by some physical medium such as coaxial cable or optical fiber.

ü  Such a physical medium is called as Links and the computer it connects is called as Nodes.

எல்லாத் துன்பங்களுக்கும் இரண்டு மருந்துகள் உள்ளன. ஒன்று காலம், இன்னொன்று மெளனம்.

15. What are the key elements of protocols?

The key elements of protocols are

ü  Syntax

It refers to the structure or format of the data that is the order in which they are presented.

ü  Semantics

It refers to the meaning of each section of bits.

ü  Timing

Timing refers to two characteristics: When data should be sent and how fast they can be sent.

16. What is point-point link?

ü  A point-to-point connection provides a dedicated link between two devices.

ü  The entire capacity of the link is reserved for transmission between those two devices.

ü  Most point-to-point connections use an actual length of wire or cable to connect the two ends.

ü  Example: 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.

Possible questions and answers for university exams Five marks questions and ten mark questions

1.      Write fundamental characteristics of DATA COMMUNICATIONS.

o   When we communicate, we are sharing information. This sharing can be local or remote. Local communication usually occurs face to face, while remote communication takes place over distance.

o   The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance.

o   Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.

o   The effectiveness of a data communications system depends on three fundamental characteristics: delivery, accuracy and timeliness.

ü  Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.

ü  Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.

ü  Timeliness: The system must deliver data in a timely manner. Data delivered late are useless.

அதிகம் பேசாதவனை உலகம் அதிகம் விரும்புகிறது. அளந்து பேசுபவனை அதிகம் மதிக்கிறது. அதிகம் செயல்படுபவனையே கைகூப்பித் தொழுகிறது.

2.      Explain briefly the five components of a data communication system.

3.      Explain the five components of a data communication system.

A data communications system has five components

ü  Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

ü  Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

ü  Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

ü  Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

ü  Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

4.       Write a short note on protocol and standards.

In computer networks, communication occurs between entities in different systems.

For communication to occur, the entities must agree on a protocol. A protocol is a set of rules that govern data communications.

A protocol defines

What is communicated?

How it is communicated?

When it is communicated?

The key elements of a protocol are Syntax, Semantics, and Timing.

Syntax:

ü  The term syntax refers to the structure or format of the data, meaning the order in which they are presented.

ü  For example, a simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself.

Semantics:

ü  The word semantics refers to the meaning of each section of bits.  How are a particular pattern to be interpreted, and what action is to be taken based on that interpretation? For example, does an address identify the route to be taken or the final destination of the message?

Timing:

ü  The term timing refers to two characteristics: when data should be sent and how fast they can be sent.

ü  For example, if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and some data will be lost.

Standards

ü  Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunications technology and processes.

ü  Standards provide guidelines to manufacturers, vendors, government agencies, and other service providers to ensure the kind of interconnectivity necessary in today's marketplace and in international communications.

ü  Data communication standards fall into two categories: de facto (meaning "by fact" or "by convention") and de jure (meaning "by law" or "by regulation").

ü  De facto: Standards that have not been approved by an organized body but have been adopted as standards through widespread use are de facto standards.

ü  De facto standards are often established originally by manufacturers who seek to define the functionality of a new product or technology.

ü  De jure: Those standards that have been legislated by an officially recognized body are de jure standards.

5.      Write data flow in Networks.

Simplex:

ü  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.

ü  Example: Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output.

ü  The simplex mode can use the entire capacity of the channel to send data in one direction.

Half-Duplex

ü  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

ü  The half-duplex mode is like a one-lane road with traffic allowed in both directions.  Example: Walkie-talkies and CB (citizens band) radios are both half-duplex systems.

ü  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.

Full-Duplex

ü  In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously the full-duplex mode is like a two-way street with traffic flowing in both directions at the same time.

ü  In full-duplex mode, signals going in one direction share the capacity of the link: with signals going in the other direction.

ü  One common example of full-duplex communication is the telephone network.

ü  When two people are communicating by a telephone line, both can talk and listen at the same time.

அறிவு ஒன்றுதான் அச்சத்தை முறிக்கும் அரிய மருந்து. அறிவை வளர்த்துக் கொண்டால் எல்லாவிதமான பயங்களும் அகன்றுவிடும்.

6.      Explain any two types of network topology.

ü  The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another.

ü  There are four basic topologies possible: Mesh, Star, Bus, and Ring.

a)     Mesh Topology:

ü  In a mesh topology, every device has a dedicated point-to-point link to every other device.

ü  The term dedicated means that the link carries traffic only between the two devices it connects.

ü  To find the number of physical links in a fully connected mesh network with N nodes, we first consider that each node must be connected to every other node.

Advantages:

ü  First, the use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.

ü  Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.

ü  Third, there is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it.

Disadvantages

ü  A mesh is related to the amount of cabling and the number of I/O ports required.

ü  First, because every device must be connected to every other device, installation and reconnection are difficult.

ü  Second, the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate

Example:

ü  One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

b)     Star Topology:

ü  In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub.

ü  The devices are not directly linked to one another.

ü  If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device

பழமையைப் பற்றி ஒன்றுமே தெரியாமல் புதுமையைச் சிறப்பாகப் படைக்க முடியாது.

Advantages

ü  A star topology is less expensive than a mesh topology.

ü  This factor also makes it easy to install and reconfigure.

ü  Other advantages include robustness. If one link fails, only that link is affected. All other links remain active.

ü  This factor also lends itself to easy fault identification and fault isolation.

Disadvantage

ü  In a star topology, is the dependency of the whole topology on one single point, the hub.

ü  If the hub goes down, the whole system is dead.

ü  The star topology is used in local-area networks (LANs)

c)     Bus Topology:

ü  Bus Topology the preceding examples all describe point-to-point connections.

ü  A bus topology, on the other hand, is multipoint.

ü  One long cable acts as a backbone to link all the devices in a network Nodes are connected to the bus cable by drop lines and taps.

ü  A drop line is a connection running between the device and the main cable.

ü  A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core.

Advantages

ü  Advantages of a bus topology include ease of installation.

ü  Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths.

ü  A bus uses less cabling than mesh or star topologies.

ü  In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub.

ü  In a bus, this redundancy is eliminated.

ü  Each drop line has to reach only as far as the nearest point on the backbone.

செயல் புரியாத மனிதனுக்கு தெய்வம் ஒருபோதும் உதவி செய்யாது.

Disadvantages

ü  Disadvantages include difficult reconnection and fault isolation.

ü  A bus is usually designed to be optimally efficient at installation.

ü  It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in quality.

ü  A fault or break in the bus cable stops all transmission, even between devices on the same side of the problem.

ü  The damaged area reflects signals back in the direction of origin, creating noise in both directions. Bus topology was the one of the first topologies used in the design of early Local Area Networks.

ü  Ethernet LANs can use a bus topology

d)     Ring Topology:

ü  In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it.

ü  A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater.

ü  When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

ü  A ring is relatively easy to install and reconfigure.

Advantage

ü  Each device is linked to only its immediate neighbors. To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices).

ü  Fault isolation is simplified.

Disadvantage

ü  Unidirectional traffic can be a disadvantage.

ü  In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.

ü  This weakness can be solved by using a dual ring or a switch capable of closing off the break.

ü  Ring topology was prevalent when IBM introduced its local-area network Token Ring.

7.      Explain the OSI Network architecture specifying the functions of each layer.

LAYERS IN THE OSI MODEL

வாசிப்புப் பழக்கம் என்பது அருமையான ருசி, அழகான பசி. ஒரு முறை சுவைக்கப் பழகிவிட்டால் அது தொடர்ந்து வரும்.

The functions of each layer in the OSI model

Physical Layer:

“The physical layer is responsible for movements of individual bits from one hop (node) to the next.”

ü  The Physical Layer coordinates the functions required to carry a bit stream over a physical medium.

ü  It deals with the mechanical and electrical specifications of the interface and transmission medium.

ü  It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur.

ü  Figure shows the position of the physical layer with respect to the transmission medium and the data link layer.

                    

The physical layer is also concerned with the following:

Physical characteristics of interfaces and medium:

ü  The physical layer defines the characteristics of the interface between the devices and the transmission medium.

ü  It also defines the type of transmission medium.

Representation of bits:

ü  The physical layer data consists of a stream of bit (sequence of 0s or 1s) with no interpretation.

ü  To be transmitted, bits must be encoded into signals--electrical or optical.

Data rate:

ü  The transmission rate-the number of bits sent each second-is also defined by the physical layer.

Synchronization of bits:

ü  The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level.

ü  In other words, the sender and the receiver clocks must be synchronized.

Line configuration:

ü  The physical layer is concerned with the connection of devices to the media.

ü  In a point-to-point configuration, two devices are connected through a dedicated link.

ü  In a multipoint configuration, a link is shared among several devices.

Physical topology:

ü  The physical topology defines how devices are connected to make a network.

ü  Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies).

Transmission mode:

ü  The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

ü  In simplex mode, only one device can send; the other can only receive. The simplex mode is a one-way communication.

ü  In the half-duplex mode, two devices can send and receive, but not at the same time.

ü  In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time.

ரகசியத்தை வெளிப்படுத்தியவனுக்கும், துக்கத்தை வெளிப்படுத்தாதவனுக்கும் மனதில் நிம்மதி இருக்காது.

Data Link Layer:

ü  The data link layer transforms the physical layer, a raw transmission facility, to a reliable link.

ü  It makes the physical layer appear error-free to the upper layer (network layer).

“The data link layer is responsible for moving frames from one hop (node) to the next.”

Other responsibilities of the data link layer include the following:

Framing:

ü  The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

Physical addressing:

ü  If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame.

ü  If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

Flow control:

ü  If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error control:

ü  The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames.

ü  It also uses a mechanism to recognize duplicate frames.

ü  Error control is normally achieved through a trailer added to the end of the frame.

Access control:

ü  When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

Network Layer:

“The network layer is responsible for the delivery of individual packets from the source host to the destination host.”

வாழ்வின் வெற்றி என்பது ஒரு மனிதன் பின்பற்றும் சத்தியத்தைப் பொறுத்தது.- ஸ்ரீ அன்னை

ü  The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links).

ü  Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination.

ü  If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery

Other responsibilities of the network layer include the following: 

Logical addressing:

ü  The physical addressing implemented by the data link layer handles the addressing problem locally.

ü  If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems.

ü  The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

Routing:

ü  When independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination.

ü  One of the functions of the network layer is to provide this mechanism.

Transport Layer:

“The transport layer is responsible for the delivery of a message from one process to another.”

ü  The transport layer is responsible for process-to-process delivery of the entire message.

ü  A process is an application program running on a host.

ü  Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets.

ü  It treats each one independently, as though each piece belonged to a separate message, whether or not it does.

ü  The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.

Other responsibilities of the transport layer include the following:

Service-point addressing:

ü  Computers often run several programs at the same time.

ü  For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other.

ü  The transport layer header must therefore include a type of address called a service-point address (or port address).

ü  The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

Segmentation and reassembly:

ü  A message is divided into transmittable segments, with each segment containing a sequence number.

ü  These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

Connection control:

ü  The transport layer can be either connectionless or connection oriented.

ü  A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.

ü  A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets.

ü  After all the data are transferred, the connection is terminated.

Flow control:

ü  Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

Error control:

ü  Like the data link layer, the transport layer is responsible for error control.

ü  However, error control at this layer is performed process-to process rather than across a single link.

ü  The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication).

ü  Error correction is usually achieved through retransmission.

நேர்மையும் நல்லெண்ணமும் இருக்கின்றபோதெல்லாம் இறைவனின் உதவியும் உள்ளது.- ஸ்ரீ அன்னை

Session Layer:

“The session layer is responsible for dialog control and synchronization.”

ü  The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes.

ü  The session layer is the network dialog controller.

ü  It establishes, maintains, and synchronizes the interaction among communicating systems.

Specific responsibilities of the session layer include the following:

Dialog control:

ü  The session layer allows two systems to enter into a dialog.

ü  It allows the communication between two processes to take place in either half duplex (one way at a time) or full-duplex (two ways at a time) mode.

Synchronization:

ü  The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

ü  For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent. Figure  illustrates the relationship of the session layer to the transport and presentation layers.

நீங்கள் சில ஆண்டுகளுக்கு முன்பு எதை விதைத்தீர்களோ அதைத்தான் அறுவடை செய்கிறீர்கள்.

Presentation Layer

“The presentation layer is responsible for translation, compression, and encryption.”

ü  The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.

Specific responsibilities of the presentation layer include the following:

Translation:

ü  The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on.

ü  The information must be changed to bit streams before being transmitted.

ü  Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods.

ü  The presentation layer at the sender changes the information from its sender-dependent format into a common format.

ü  The presentation layer at the receiving machine changes the common format into its receiver-dependent format.

Encryption:

ü  To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network.

ü  Decryption:

ü  Reverse the original process to transform the message back to its original form.

ü  Compression:

ü  Data compression reduces the number of bits contained in the information.

ü  Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

ஒவ்வொரு மனிதனும் விதக்கிறாள். ஒருவன் வாய்ச் சொற்களால் விதக்கிறான். இன்னொருவன் செயல்களால் விதைக்கிறான்.

Application Layer:

“The application layer is responsible for providing services to the user.”

ü  The application layer enables the user, whether human or software, to access the network.

ü  It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

ü  Figure shows the relationship of the application layer to the user and the presentation layer of the many application services available, the figure shows only three:

ü  XAOO (message-handling services),

ü  X.500 (directory services), and file transfer, Access, and Management (FTAM).

ü  The user in this example employs XAOO to send an e-mail message.

Specific services provided by the application layer include the following

Network virtual terminal:

ü  A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host.

ü  To do so, the specific service provided by the application layer e application creates a software emulation of a terminal at the remote host.

ü  The user's computer talks to the software terminal which, in turn, talks to the host, and vice versa.

ü  The remote host believes it is communicating with one of its own terminals and allows the user to log on.

நீ தனிமையில் இருக்கும் போது உனக்கு என்ன தோன்றுகிறதோஅது தான் உன் வாழ்கையை தீர்மானிக்கும்- சுவாமி விவேகானந்தர்

File transfer, access, and management:

ü  This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

Mail services:

ü  This application provides the basis for e-mail forwarding and storage.

Directory services:

ü  This application provides distributed database sources and access for global information about various objects and services.

உன் வாழ்க்கையின் எந்த ஒரு நாளில் உன் முன்னால் எந்தப் பிரச்சினையையும் நீ சந்திக்காமல் முன் செல்கிறாயோ, அப்பொழுது தவறான பாதையில் நீ பயணிக்கிறாய் என்று அறிவாய்.- விவேகானந்தர்