OSI Model

Open System Interconnection

(OSI) Model

              The Open Systems Interconnection (OSI) model was developed by the International Standards Organization (ISO) to help standardize communication between computer systems. OSI model is also called as ISO OSI Reference Model. It divides communications into seven different layers, which each include multiple hardware standards, protocols and other types of services.

Application Layer-7

Presentation Layer-6

Session Layer-5

Transport Layer-4

Network Layer-3

Data Link Layer-2

Physical Layer-1

Layers of OSI model

Transmission of data in OSI layers

Layers of Open System Interconnection (OSI) Model

Physical Layer (layer-1)

The physical layer, 1^st or the bottom most layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium.

It describes the electrical/optical, mechanical, and functional interfaces (like cables and connectors) to the physical medium, and carries the signals for all of the higher layers.

Functioning of Physical Layer

Physical layer receives data (bit’s) from data link layer. It converts the received data into bit stream. The data is then transmitted through the medium to the data link layer.

Functions of Physical layer:

Characteristics of media

Defines the characteristics of the interface which is used for connecting the devices, also specifies the type of transmission media whether it is wire, wireless or fiber optic cable.

Encoding

           Encoding is the process of changing bit stream (0s and 3s) into signal. Before transmission, physical layer encodes the signal into electrical or optical form depending upon the media.

Transmission Rate

Defined as number of bits transmitted per second, it specifies the duration of the bit.

Transmission Mode

          Transmission media specifies the direction of the signal flow between the 2 devices. The different types of transmission modes are:

·        Simplex- Communication is done only in one direction 

·        Half Duplex- Communication is done in both the directions, but not at the same time.

·        Full Duplex- Communication is done in done in both the directions at the same time.  

Topology

Describes how the devices are connected to form a network.

Networking Devices for Physical Layer:

·        Hub

·        Repeater

Data Link Layer (Layer-2)

The data link layer is the second layer of OSI model. It handles data transfer between network and physical layer. The data unit at data link layer is called as frames.

Functioning of Data Link Layer

The data link layer receives data from network layer. It adds header and trailer to the data and passes data to physical layer.

At the receiver side, data link layer receives data from the physical layer. It detaches header and trailer from the data and passes data to the network layer.

Functions of Data Link layer:

Framing

The physical layer delivers raw bits from the source to destination.  During transmission, the value of the bits can change.  It is also possible that the number of bits received by the receiver may be different from the number of bits sent by the sender.  To resolve this problem, the data link layer organizes the bits into manageable data units called as frames.

Physical Addressing

Data link layer adds header to the frame which contains the physical address of the sender and/or receiver.  Headers are assigned when the frames are to be distributed to various systems within the network.

Error Control

Another function of the Data Link layer is error control.  Error control detects and corrects errors.  During transmission, if a frame is lost or corrupted, the data link layer retransmits that frame.  It also prevents duplication of frames.  Error control is accomplished using trailer at the end of the frame.

Networking Devices for Data Link Layer:

·        Switch

·        Wireless access point

·        Cable modem

Network Layer (layer-3)

This is the third layer of the OSI model.  The data unit at this layer is known as packet.  There is no need of the network layer if the two communicating devices lie on the same network.  However, when the two devices are connected on different networks, network layer is essential for providing source to destination delivery of packets

Functioning of Network Layer

Network layer receives data from transport layer.  It adds header to the data and passes data to the data link layer.  At the receiver side, network layer receives data from data link layer.  It detaches header from the data and passes data to the transport layer

Functions of Network Layer:

Logical Addressing

The data link layer provides the physical addressing which is useful for a local network.  When the packet is destined for a device outside the network, we require other addressing scheme to identify source and destination.  Network layer adds header to the data that includes the logical address (IP address) of the source and destination.  It is a 32-bit address that uniquely identifies the device connected to the network.

Routing

              Routing is a process wherein a proper path is defined for the packets to reach the destination.  Routing can be of two types, static or dynamic.  In static routing, the route to be followed by a packet is already set by the network administrator.  In dynamic routing, the route to be followed by a packet is decided at the time of transmission of the packet.  The route can be changed based on the available traffic on the network.

Internetworking

             Internetworking means connecting two or more computer networks together.  The Internet is the best example of internetworking.  There are different types of networks that exist in the real world such as LAN, MAN and WAN.  These networks are interconnected using various networking devices such as bridges, routers and gateways.

Networking Devices of Data Link Layer:

·        Router

·        Brouter

Transport Layer (Layer-4)

             This is the fourth layer of the OSI model.  The data unit at this layer is known as segment.  The network layer deals with source-to-destination delivery of individual packets and does not recognize any relationship between them.  On the other hand, the transport layer ensures that the entire message reaches in order and handles error control and flow control at the source-to destination level.

Functioning of Transport Layer

Transport layer receives data from the session layer.  It divides the data into segments with each segment having its own header and passes these segments to the network layer.  At the receiver side, transport layer receives packets from the network layer.  It detaches headers from them and then reassembles the segments into original message and passes that message to the session layer.

Functions of Transport Layer

Service-point Addressing

             There may be number of programs running at the same time on a computer.  For this purpose, source-to-destination delivery should also include process-to-process delivery.  Data must be transferred from a specific process (running pr) in the header.  Using these port address, transport layer delivers the packets to the correct process.

Segmentation and Reassembly

             A message is divided into segments with each segment having a sequence number.  When the message arrives at the destination, with the help of these sequence numbers, the transport layer reassembles the message correctly. Sequence number also enable transport layer to determine and replace the segments at were lost in the transmission.

Connection Control

             Transport layer provides either connection-oriented or connectionless service.  In connection-oriented service, transport layer establishes a connection with the transport layer at the destination before transmitting segments and terminates the connection when all the data is transferred.  In connectionless service, transport layer treats each segment as individual packet and transmits it to the transport layer at destination.

Error Control

              Transport layer also performs error checking.  Error control takes place end to end.  Transport layer at the sending machine ensures that the entire message is delivered at the receiving machine without an effort.  Segments are retransmitted to achieve error correction. 

Networking Devices for Transport Layer:

·         Gateway

Session Layer (Layer-5)

This is the fifth layer of the OSI model. This layer establishes, manages, synchronizes land terminates connection between the computers. It also sets up, coordinates and terminates interactions and dialogs between communicating devices. It provides either half duplex or full duplex service.

Functioning of Session Layer

Session layer receives data from the presentation layer. It adds header and synchronization bits to the data and passes data to the transport layer. At the receiver side, session layer receives data from transport layer. It detaches header and synchronization bits from the data and passes data to the presentation layer.

Functions of Session Layer:

Dialog Control

The session layer is responsible for setting up sessions between communicating devices.  It allows two devices to enter into dialog (communication process).  These dialogs can take place either in half-duplex or full duplex mode.  Dialog control is implemented using a data token.  The user that has the token is only allowed to send the data.  When the user is done with the operation, the token is passed on to the next user.

Synchronization

At the session layer, checkpoints (synchronization bits) are added into a stream of data to synchronizer the sessions.

Networking Devices for Session Layer:

·        

Presentation Layer (Layer-6)

This is the sixth layer of the OSI model. This layer deals with syntax and semantics of the data exchanges between two devices. It transforms the data in the format that will be accepted by the application layer.

Functioning of Presentation Layer

Presentation layer receives data from application layer. It adds header to the data and passes data to the session layer. At the receiver side, the presentation layer receives data from session layer. It detaches header from the data and passes data to the application layer.

Functions of Presentation Layer:

Translation

The running programs on the computer exchange data in the form of character strings and numbers. Before transmitting, the information should be converted to bit format. 

The presentation layer handles interoperability between different encoding systems used by different computer systems. 

At the sending device, presentation layer changes the information from its sender-dependent format into a common formation that will be accepted by the receiving device.   

At the receiving side, presentation layer changes the information from the common format into its receiver-dependent format. 

Encryption

The presentation layer also adds security to data by encrypting it.  Encryption is a process in which the data is converted into a form that prevents unauthorized users from reading. Encryption allows a system to carry sensitive and private information.  The encrypted data is decrypted by the presentation layer at the receiving end.  Decryption is a process opposite to encryption.  It converts the encrypted message back to its original form.  

Compression

           Data compression refers to a process of encoding data using less number of bits.  This process reduces the number of bits used to represent the information.  It is important while transmitting multimedia information such as text, audio and video.   Presentation layer is responsible for performing data compression.

Networking Devices for Presentation Layer:

·        Gateway

Application Layer (Layer-7)

This is the seventh layer of the OSI model.  It provides the means for the User to access information on the network using an application. It supports services such as electronic mail, remote file access and transfer and shared database management.

Functioning of Application Layer

Functions of Application Layers:

Network Virtual Terminal

             It is a software version of physical terminal. Using this software user can log on to computer that is remotely connected

File Transfer, Access and Management (FTAM)

            Using this application, user can access files in remote host

Mail Services

Using this application e-mails can be forwarded to another device. Message handling system (MHS) is the OSI protocol used to transfer mail over the internet.

Directory Services

A directory is the source of information about user, organization, programs and files, which are managed by directory services.

Networking devices of Application layer:

·        Gateway