OSI Reference Model | 7 layers of OSI Reference Model

 

Introduction of OSI Reference Model

OSI Model developed by ISO (International Organizations for Standardization). OSI stands for Open System Interconnection. There are seven layers in this model. OSI Model is developed after TCP/IP Model. (How to remember all layers in sequence - Watch the video)

Figure: OSI Reference Model

OSI model provides following services: 1-Provides peer-to-peer logical services with layer physical implementation. 2-Provides standards for communication between system. 3-Define point of interconnection for the exchange of information between system. 4-Each layer should perform a well-defined function.

Physical Layer

Physical layer is the lowest layer of the OSI model. Physical layer co-ordinates the function required to transmit a bit stream over a communication channel. The physical layer is responsible for movements of individual bits from one hop (node) to the next. It deals with electrical and mechanical specifications of interface and transmission media. It also deals with procedures and functions required for transmission. Carries the bit stream over a physical media.

Function of Physical Layer:

Physical characteristics of interfaces and media: The design issue of physical layer considers the characteristics of interface between devices and transmission media.

Representation of bits: Physical layer encodes the bit stream into electrical or optical signal.

Data rate: The physical layer defines the duration of a bit which is called as data rate or transmission rate.

Synchronization of bits: The transmission rate and receiving rate must be same. This is done by synchronizing clocks at sender and receiver.

Datalink Layer

The data link layer is responsible for transmitting frames from one node to the next. It transforms the physical layer to a reliable link making it an error free link to upper layer. Data link layer is divided into two sub layers: LLC (Upper Layer), MAC (Lower Layer).

LLC is responsible for Flow control, Error Control, Access Control. MAC is responsible for Physical addressing.

Function of Data Link Layer:

Framing: The frames received from network layer is divided into manageable data units called frames.

Physical Addressing: When frames are to be sent to different LANs, the data link layer adds a header to the frame to define sender to receiver.

Flow Control: When the rate of the data transmitted and rate of data reception by receiver is not same, some data may be lost. So, this layer manages data flow.

Error Control: Data link layer incorporates reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames.

Access Control: When multiple devices are connected to same link, the data link layer determines which device has control over link.

Network Layer

The network layer is responsible for the delivery of packets from the source to destination. It provides host to host delivery. In case of data link layer, packet delivers on the same network. If two different networks are connected then packet is concern with network layer.

Function of Network Layer:

Logical Addressing: DLL implements physical addressing. When a packet passes network boundary. An addressing system is needed to distinguish source and destination, network layer performs this function. The network layer adds a header to the packet of upper layer includes the logical addresses of sender and receiver.

Routing: Network layer route or switch the packets to its final destination in an internetwork.

Transport Layer

The transport layer is responsible for delivery of message from one process to another. The network does the host to destination delivery of individual packets considering it as independent packet. But transport layer ensures that the whole message arrives intact and in order with error control and process control.

Function of Transport Layer:

Port addressing: Computer performs several operations simultaneously process-to-process delivery means specific process of one computer must be delivered to specific process on other computer. The transport layer header therefore includes port address. Network layer delivers packet to the desired computer and transport layer, goes message to the correct process on that computer.

Segmentation and reassembly: Messages are divided into a segment; each segment contains a sequence number which enables transport layer to reassemble at destination.

Connection Control: Transport layer performs connectionless or connection-oriented services with the destination machine.

Flow Control: Transport layer performs end-to-end flow control while data link layer performs it across the link.

Error Control: Error control at this layer is performed on end-to-end basis rather than across the link. The transport layer ensures error free transmission.

Session Layer

The session layer is responsible for network dialog controller i.e., it establishes and synchronizes the interaction between communication system.

Function of Session Layer:

Dialog Control: Communication between two processes take place in either half duplex or full-duplex mode. The session layer manages dialog control for this communication.

Synchronization: Session layer adds synchronization points into stream of data.

Presentation Layer

This layer is concerned for data representation. The presentation layer deals with syntax and semantics of the informing being exchanged. The presentation layer is responsible for translation, compression, and encryption.

Function of Presentation Layer:

Translation: Different computers use different encoding systems. The presentation layer maintains interoperability between the two-encoding system. 

Encryption: Encryption is transforming sender information to non-readable format while transmission. Decryption is reverse process.

Compression: Compression is a technique of reducing number of bits required to represent the data.

Application Layer

Application layer is nearest layer of user. The application layer is providing services to the user. Application layer is responsible for accessing the network by user. It provides user interfaces and other supporting services such as e-mail, remote file access, file transfer, sharing database etc...

Function of Application Layer:

Network virtual terminal: It is a software version of physical terminal that allows a user to log onto a remote host.  

File Transfer, Access and Management (FTAM): FTAM allows user to access files and remote hosts, to retrieve files and to manage files in remote computer.

Mail Services: E-mail forwarding, storage are the services under this category.

Summary of OSI Model

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