OSI PROTOCOL

PHYSICAL Layer - OSI Model

Physical layer is the lowest layer of all. It is responsible for sending bits from one computer to another. This layer is not concerned with the meaning of the bits and deals with the physical connection to the network and with transmission and reception of signals. This layer defines electrical and physical details represented as 0 or a 1. How many pins a network will contain, when the data can be transmitted or not and how the data would be synchronized.

DATA LINKLlayer-OSI Model

The Data-Link layer is the protocol layer in a program that handles the moving of data in and out across a physical link in a network. The Data-Link layer is layer 2 in the Open Systems Interconnect (OSI) model for a set of telecommunication protocols.
The Data-Link layer contains two sublayers that are described in the IEEE-802 LAN standards:
Media Access Control (MAC)
Logical Link Control (LLC)
The Data-Link layer ensures that an initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully. It also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames.

Network Layer - OSI Model

The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If two computers (system) are connected on the same link then there is no need for a network layer. It routes the signal through different channels to the other end and acts as a network controller.
It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels

FUNCTIONS OF NETWORK LAYER

1)It translates logical network address into physical address. Concerned with circuit, message or packet switching.
2)Routers and gateways operate in the network layer. Mechanism is provided by Network Layer for routing the packets to final destination.
3)Connection services are provided including network layer flow control, network layer error control and packet sequence control.
4)Breaks larger packets into small packets. .

TRANSPORT Layer-OSI Model

The main aim of transport layer is to be delivered the entire message from source to destination. Transport layer ensures whole message arrives intact and in order, ensuring both error control and flow control at the source to destination level. It decides if data transmission should be on parallel path or single path
Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer and ensures that message arrives in order by checking error and flow control.

FUNTIONS OF TRANSPORT LAYER

1)Service Point Addressing : Transport Layer header includes service point address which is port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer
2).Segmentation and Reassembling : A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.
3)Connection Control : It includes 2 types :
Connectionless Transport Layer : Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.
Connection Oriented Transport Layer : Before delivering packets, connection is made with transport layer at the destination machine.
4)Flow Control : In this layer, flow control is performed end to end.
5)Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error 6)Correction is done through retransmission.

Session Layer - OSI Model

Its main aim is to establish, maintain and synchronize the interaction between communicating systems. Session layer manages and synchronize the conversation between two different applications. Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

FUNCTIONS OF SESSION LAYER

1)Dialog Control : This layer allows two systems to start communication with each other in half-duplex or full-duplex.
2)Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged. This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

Presentation Layer - OSI Model

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

FUNCTIONS OF PRESENTATION LAYER

1)Translation : Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
2)Encryption : It carries out encryption at the transmitter and decryption at the receiver.
3)Compression : It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Application Layer - OSI Model

IIt is the top most layer of OSI Model. Manipulation of data (information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring of files, distributing the results to user, directory services, network resource etc.

FUNCTIONS OF APPLICATION LAYER

1)Mail Services : This layer provides the basis for E-mail forwarding and storage.
2)Network Virtual Terminal : It allows a user to log on to a remote host. The application creates software emulation of a terminal at the remote host. Users computer talks to the 3)software terminal which in turn talks to the host and vice versa. Then the remote host believes it is communicating with one of its own terminals and allows user to log on.
4)Directory Services : This layer provides access for global information about various services.
5)File Transfer, Access and Management (FTAM) : It is a standard mechanism to access files and manages it. Users can access files in a remote computer and manage it. They can also retrieve files from a remote computer
.