OSI REFERENCE MODEL:

OSI REFERENCE MODEL:

The OSI Reference Model is composed of seven layers, each specifying particular network functions.

Figure:   Seven layers of the OSI model

Physical Layer

The physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a 0 bit.

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

 

Responsibilities:

Physical characteristics of interfaces and medium:

·         The physical layer defines the characteristics of interfaces between the devices and transmission medium.

·         It is also defines the transmission medium

Representation of bit: The physical layer data consist of a stream of bits. To transmitted, data must be encoded into signals.

Data rate:  The physical layers define the number of bits send each second.

Synchronization of bit: the sender and receiver are may not be use same data rate at bit level, the sender and receiver clock must be Synchronized.

Line configuration: The physical layer is concerned with the connection of device to the media.

Physical topology: The physical topology defines with types of network connected to a network. 

Transmission mode: The physical layers also define the direction of transmission between the two devices.

Data link layer:

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

 

Responsibilities:

Framing: The data link layer divides the stream of bits received from the network layer into manageable data unit is called framing

Physical addressing: The data link layer adds a header to the frame to define the sender and/or receiver of the frame. This address is called as MAC adders 

Flow control: The data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error control: The data link layer adds reliability to the physical layer by adding mechanisms to detect and transmit damaged or lost frame.

Access control: When the two or more device is connected to the same link, the 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.

 

– Defines network logical address (not MAC)

– Provide switching and routing facilities

– Determines network address and best path to deliver packets

– Translate logical address into physical address

– This layer responsible for:

1. Addressing

2. Route selection

– If router cannot send data in same size as sent by source then layer 3 divides data into smaller sizes, at receiving end network layer reassembles data.

– Forms Packets

• Routers, ATM switches,

Transport Layer:

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

 

– Used for data transfer between end systems.

– Processes to processes delivery (not source to destination delivery)

– Provides QoS

– Whole message is received in order.

– Converts data into segments.

– Ensures data is delivered error free and in order.                                      

– Flow control: send that amount of data which can be handled by destination. Similarly if data

Packet lost then resend.

– Protocols at layer 4: TCP, UDP and SCTP

Session Layer:

The session layer is responsible for dialog control and synchronization.

 

– Used for dialogue control and synchronization purposes.

– Establishes sessions between systems.

– Dialog control:

– Dialog between two parties for communication to take place in either half or full duplex mode.

– Synchronization:

– Add synchronization points to stream of data.

– If session fails only send that data which was not delivered not whole message.

– E.g. files of 2000MB

Presentation Layer:

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

 

– Concerned with syntax and semantics of information.

– Responsible for translation (data into bits and encoding format), compression, and encryption.

– Translation: data into bits and selecting appropriate encoding technique and changing from sender format to receiver format.

– Compression: Reduce number of bits.

Application Layer:

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

 

– Layer support Software applications to access network.

– Examples: Virtual terminal (Remote desktop), FTP, TFTP, email (SMTP), Directory services, TELNET.