Manoj KuMar

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Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards.

An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model.

It was first introduced in the late 1970s. It deals with connecting open system- that is , systems that are open

for communication with other system.

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It has seven layers. The principle that were applied to arrive at the seven layers can be

briefly summarized as follows:◦ A layer should be created where a different abstraction is needed.◦ Each layer should perform a well-defined task.◦ the layer boundaries should be chosen to minimize the

information flow across the interfaces.◦ The number of layer should be large enough that distinct

functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

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7. Application Layer

6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer

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The physical layer is responsible for movement of individual bit from one hop (node) to the next.

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Major duties of the physical layer are as follows :Physical characteristics of interfaces and medium : the purpose of the physical layer is to transport a raw bit stream from one machine to another. Various physical media can be used for the actual transmission.Each one has its own niche in terms of bandwidth, delay, cost, and ease of installation and maintenance.Representation of bits : The physical layer data consists of a stream of bits (sequence of 0’s or 1’s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how 0’s and 1’s are changed to signals).Data rate : The transmission rate-the number of bits sent each second-is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts.

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Synchronization of bits : The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.

Line configuration : The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices.

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Physical topology : The physical topology defines how devices are connected to make a network. 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)A hybrid topology (this is a combination of two or more topologies).

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Transmission mode : The physical layer also defines the direction of transmission between two devices:

Simplex Half-duplex Full-duplex. In simplex mode, only one device can send; the other can only receive. The

simplex mode is a one-way communication. In the half-duplex mode, two devices can send and receive, but not at the same

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

the same time.

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Media is a general term used to describe the data path that forms the physical channel between sender and receiver. Transmission media can be twisted pair wire such as that used for telephone installation, wire media are referred to as Bounded Media and wireless media are sometimes referred to as Unbounded Media.

Bandwidth, noise, radiation and attenuation are considered while using the different transmission media.

Higher bandwidth transmission media supports higher data rates. Attenuation limits the usable distance that data can travel on the media. Noise is

related to electrical signal noise that can cause distortion of the data signal and data errors.

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Radiation is the leakage of signal from the media caused by undesirable electrical characteristic of the transmission media.

The transmission medium is the physical path between transmitter and receiver in a data transmission system. In transmission medium, communication is in the form of electromagnetic waves.

The characteristics and quality of transmission are determined both by the characteristics of the medium and the characteristics of signal.

TRNSMISSION MEDIA

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Guided media/ bounded media

Unguided media/ unbounded media

Twisted pair cable

Co-axial cable

Fiber optics cable

Microwave or radio links

Infrared

Design Factors:◦ Bandwidth : The greater the bandwidth the higher the data rate can be

achieved.◦ Transmission Impairments : such as attenuation, limit the distance (repeater

spacing) for guided media. Twisted pair generally suffer more impairment than co-axial cable.◦ Interference ◦ Number of Receivers

Guided or Unguided Media :◦ Depending on the type of application and geographical situation suitable

guided or unguided media is chosen.◦ For long distance point-to-point transmission guided media are suitable.◦ For long distance broadcasting transmission unguided media are chosen.

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TP is least expensive and most widely used. TP consists of two insulated copper wire arranged in regular spiral pattern. A wire

pair act as a signal communication link. TP may used to transmit both analog and digital signals. For analog signal

amplifiers are required about every 5 to 6 km. for digital signals, repeaters are required every 2 or 3 km.

TP is most commonly used medium for in the telephone network. Compared to other commonly used transmission media, TP is limited in distance, bandwidth and data rate when two copper wire conduct electric signal in close proximity, a certain amount of electromagnetic inference occurs (EMI). This type of inference is called Cross Talk. Twisting the copper wire reduces cross talk.

Twisted pair cable comes in two varieties.◦ Unshielded twisted pair (UTP) cable.◦ Shielded twisted pair (STP) cable.

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UTP is a set of twisted pairs of cable within a plastic sheet. UTP is ordinary telephone wire. This is the least expensive of all the transmission media commonly used for LAN, and is easy to work with and simple to install.

UTP is subject to external electromagnetic inference. Category 3 and category 5 UTP are commonly used in computer networks. UTP can transfer data at 1 to 100 Mbps over a distance of 100 meters. The difference between cat3 and cat5 cable is the number of twists in the cable per unit distance cat5 is much more tightly twisted.

Category 5 : Used in local network. It supports upto 100 Mbps data transmission speed.

Category 4 : It support transmission speed of 16 Mbps and three twist per foot. Category 3 : It supports data transmission speed upto 10 Mbps. Atleast three twist

per feet and used in telephone system. Category 2 : It supports data transmission speed upto 4 Mbps and suitable for voice

data transmission Category 1 : mostly used in telephone system. Cat1 is suitable for voice and low

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CHARACTERISTICS OF UTP :Transmission rate of 10-100 Mbps.UTP is less expensive than FOC and co-axial cable.Maximum cable segment of UTP is 100 meters.UTP cable is very flexible and easy to work.Most susceptible to electrical interference or cross talk.

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ADVANTAGES OF UTP :UTP is easy to terminate.Cost of installation is less.High installed base.DISADVANTAGES OF UTP :It is very noisy.It covers less distance.UTP suffers from interference.

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STP offers a protective sheathing around the copper wire. STP provides better performance at lower data rates. They are not commonly used in networks.

Installation of STP is easy. Special connectors are required for installation. Cost is moderately expensive. Distance is limited to 100 meters for 500 meters. STP suffers from outside interference but not as much UTP.

APPLICATION OF TP CABLES : Twisted Pair can used for both analog and digital signals. Twisted Pair cable are used in telephone network. In LAN, TP wires mainly use for low cost, low performance application.

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It is made up of two conductors that shares the common axis. It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor.

Coaxial cable is used to transmit both analog and digital signals. Data transfer rate of coaxial cable is in between TP and FOC.

Coaxial cable is classified by size (RG) and by the cable resistance to direct or alternative electric currents. RG means Government Rating.

50 ohm, RG-8 and RG-11 for thick Ethernet. 50 ohm, RG-58 used for thin Ethernet. 75 ohm, RG-59 used for cable TV. 93 ohm, RG-62 used for ARC net.

It is relatively inexpensive. The cost for thin coaxial cable is less than STP. Thick coaxial cable must be grounded and terminated.

A typical data rate for today’s coaxial network is 10 Mbps, although potential is higher. It suffers from attenuation.

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CHARACTERISTICS OF CO-AXIAL CABLE :10 Mbps is transmission rate.Maximum cable length for thinnet is 185 meters and thicknet is 500 meters.Flexible and easy to work with thinnet.Ethernet designation to 10 base 2 (thinnet) or 10 base 5 (thicknet).Less expensive than FOC but more expensive than TP.Good resistance to electrical interference.APPLICATION OF CO-AXIAL CABLE :In analog and digital transmission.In telephone networks.In Ethernet LANs.In cable TV network.

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ADVANTAGES OF CO-AXIAL CABLE :Coaxial cable used for both data transmission i.e. analog and digital data transmission.It has higher bandwidth.Easy to handle and relatively inexpensive as compared to FOC.It uses for longer distance at higher data rates.Excellent noise immunity.DISADVANTAGES OF CO-AXIAL CABLE :Distance is limited.Number of nodes connection is limited.Proper connectors and termination is must.

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It is a light pipe which is used to carry a light beam from one place to another place.

Light is an electromagnetic signal and can be modulated by information. since the frequency of light is extremely high hence it can be achieved with excellent reliability.

The modulated light travel along the fiber and at the far end, are converted to an electrical signal by means of a photo electric cell. Thus the original input signal is recovered at the far end.

FOC transmits light signal rather than electrical signals. Each fiber has a inner core of glass or plastic that conducts light. the inner core is surrounded by cladding, a layer of glass that reflects the light back into core.

A cable may contain a single fiber, but often fibers are bundled together in the centre of the cable.

FOC may be multimode or signal mode. Multimode fibers use multiple light paths whereas signal mode fibers allow a single light path and are typically used with laser signaling. It is more expansive and greater bandwidth.

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Plastic Core and Cladding

Glass Core with Plastic Cladding (often called PCS fiber, plastic-clad

silica)

Glass Core and Glass Cladding.

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Plastic fibers have several advantages over glass fibers. Plastic fibers are more flexible and, consequently, more rugged than

glass. They are easy to install, can better withstand stress, are less expensive,

and weight approximately 60% less than glass. The disadvantages of plastic fibers is their high attenuation

characteristics; they do not propagate light as efficiently as glass. Plastic fibers are limited to relatively short runs, such as within a single

building or abuilding complex.

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Fibers with glass cores exhibit low attenuation characteristics.

PCS fibers are slightly better than SCS fibers.

PCS fibers are less affected by radiation and are therefore more attractive to military applications.

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SCS fiber have the best propagation characteristics and they are easier to terminate than PCS fibers.

SCS cables are the least rugged, and they are more susceptible to increase in attenuation when exposed to radiation.

The selection of fiber for a given application is a function of specific system requirements. They are always trade-offs based on the economics and logistics of a particular application.

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Transmission rate of 100 Mbps. Not affected by the electrical interference. Most expensive cable. FOC support cable length of 2 km or more. It supports voice, video and data. It provides most secure media. Commonly used as backbones between buildings and token

ring networks. Not very flexible, difficult to work.

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Wide Bandwidth Low Losses Immune to Cross Talk Interference Immune Lightweight Small Size More Strength Security Long Distance Transmission Environment Immune Safe and Easy Installation

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High Initial Cost :The initial installation or setting

up cost is very high compared to all other systems. Maintenance and Repairing Cost :

The maintenance and repairing of fiber optics systems is not only difficult but expensive also.

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Radio waves have frequencies between 10 KHz and 1 GHz. Radio waves includes following types :

Short wave. Very High Frequency (VHF) television and FM radio. Ultra High Frequency (UHF) radio and television. Radio waves can be broadcast Omni direction. Various kind of

antennas can be used to broadcast radio signals. The power of the radio frequency signal is determined by the antenna and transreceiver (a device that transmit and receive a signal over a medium such a copper, radio waves, or fiber optic cables).

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Radio waves are easy to generate. They can travel long distances. They can penetrate building easily so they are widely used for

communication both indoor and outdoors. Radio waves are omni directional. The properties of radio waves are frequency dependent. At low

frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source.

At high frequencies, radio waves are subject to interference from motors and other electrical equipment.

Low frequency and medium frequency range cannot be used for data transfer because of their very small bandwidth.

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Mobile telephone network uses microwave communication. Wireless LAN. Point-to-point communication between stations. Line –of sight communication.

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Unguided infrared light (wave) are widely used for short range

communication. The remote control used in TV, VCR and stereos all use

infrared communication. They do not pass through solid objects.

An infrared system in one room of a building will not interfere with a

similar system in adjacent rooms.

Infrared light is suitable for indoor wireless.

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

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

The DLL breaks the stream into discrete frames and computes the checksum for each frames.

At the destination the checksum is recomputed. At breaking of bit stream by inserting spaces or time gaps is called framing.

Since it is difficult and risky to count on timing and mark the start and end of each frame, various simple method used for framing are,◦ Character count◦ Starting and ending character, with character stuffing.◦ Starting and ending flags, with bit stuffing.

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Physical addressing. 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.◦ 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. 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.

Flow control mechanism is incorporated which includes a feedback mechanism requesting transmitter a retransmission of incorrect message block.

The most common retransmission technique is known as Automatic Repeat Request.

Retransmission of data in three cases : Damaged frames Lost frames Lost acknowledgements.

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Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames.◦ It also uses a mechanism to recognize duplicate frames. ◦ Error control is normally achieved through a trailer added to the end of

the frame. Access control. 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.

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Improvement to ISO Model Logical Link Control (LLC) sub-layer◦ Manages service access points (logical link)◦ Error and flow control

Media Access Control (MAC) sub-layer◦ Applies directly to network card communication◦ Access control

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Network Interface Card driver

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NETWORKSOFTWARE

NETWORKCARD

NIC Driverfacilitates data transfer

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Hop-to-hop delivery

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

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Logical addressing : The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. 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.

The network layer functions are carried out by adding header to every Network Service Data Unit (NSDU) forming Network Protocol Data Unit (NPDU). The header contains all the information necessary for carrying out functions.

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Routing : 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.

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Source-to- destination delivery

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

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Service-point addressing. Computers often run several programs at the same time. 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. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

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Segmentation and reassembly : A message is divided into transmittable segments, with each segment containing a sequence number. 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 : The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

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Flow control : 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 : Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication).

Error correction is usually achieved through retransmission.

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Process-to-process delivery

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The session layer is responsible for dialog control and synchronization

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Dialog control : The session layer allows two systems to enter into a dialog. 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 : The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. 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.

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

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Translation : The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods.

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

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

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Encryption : 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. Decryption reverses the original process to transform the message back to its original form.Compression : Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

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The application layer is responsible for providing services to the user

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

To do so, the application creates a software emulation of a terminal at the remote host.

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

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

File transfer, access, and management : 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.

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Mail services : This application provides the basis for e-mail forwarding and storage.

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

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At each layer, additional information is added to the data packet An example would be information related to the IP protocol that is added

at Layer 3

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Header Trailer

Data

A general concept of packets serves as a prerequisite tothe understanding of the ISO-OSI model.

Packet arrival information Receiver’s address Sender’s address Synchronization character

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Error correction code◦ Character oriented◦ VRC (Parity Checking)

Packet oriented error correction codes◦ LRC◦ CRC

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