Physical Layer

Physical Layer:

It is the bottom layer of OSI Model. It transmits stream of bits and defines how the data is transmitted over the network and what control signals are used. Its main function is to control how a stream of bits is sent and received over the physical medium. The following function performed by the physical layer.

1. Characteristics of Media: It layer define the characteristics and type of transmission medium.
2. Transmission Mode: The physical layer defines the direction of transmission between two devices simplex, half duplex or full duplex.
3. Data Rate: It define the number of bit that will be sent in each second.

Data Link Layer

Data Link Layer:

It responsible for the reliability of the physical link established at layer 1. The data link layer perform the following function:

1. Framing: The data link layer divides the stream of bits into manageable data units called frames.
2. Flow Control: The rate of sending data can be more than the rate of receiving data. Data link layer imposes flow control mechanism to prevent it.
3. Error Control: The data link layer detects and retransmits damaged lost frames. It also prevents the duplication of frames.
4. Access Control: If two or more devices are connected to the same link, data link layer determines which device has control over the link at a given time.

Network Layer

Network Layer:

It responsible for establishing, maintaining, and terminating network connections. it manages the delivery of data form source to destination.

Network layer determines logical path between sender and receiver. There may be many networks between two computers. This layer manages to send data from source to destination computer

Session Layer

Session Layer:

The session layer establishes, manages, and terminates user connections. A session is an exchange of message between computers. It synchronizes user task.

Synchronization involves the use of checkpoint in data stream. If a failure occurs, only the data from the last checkpoint is retransmitted. Suppose  we want to send 500 pages of data. Checkpoint can be used aster each 50 pages. if there is an error ate page 130, the pages from 101 will be retransmitted. Page from 1 to 100 will not be retransmitted.

Transport Layer

Transport Layer:

It layer controls the flow of data. It ensures that messages are delivered error free. It divides large messages into small packets for efficient transmission. These packets are reassembled, checked for errors and acknowledged at receiving side. If there are errors in transmission, the data is retransmitted.

Presentation Layer:

Presentation Layer:

It perform the following functions

1. Data Reformatting: When two computers exchange data, the data is changed to bit streams before it is transmitted. The computers may use different encoding techniques. The presentation layer at sending computer changes data according to the sender’s format. The presentation layer at receiving computer changes data according to the receiver’s format.
2. Encryption: The presentation layer encrypts data before transmission. It means that the sender transform the original information to another form and sends resulting message over the network. The receiver again transforms the message back to its original form. It is called decryption.
3. Compression: Data compression reduces the number of bits to be transmitted. The presentation layer compresses a large amount of data into small size.

Application Layer

Application Layer:

It is a top most layer of OSI Model. it provides services directly to user applications. It enable the user to access the network. It provides the user interfaces and support for services such as email, remote file access and transfer, shared database management and other types of distributed information services.

1. File Transfer: It allows a user to access, retrieve and manage files in a remote computer.
2. Mail Services: It provides the basis for email forwarding and storage facilities.
3. Directory Services: It provides distributes database sources and access for global information about various objects and services.

OSI Model

OSI Model

OSI stands for Open System Interconnection. An open system is a model that allows any two systems to communicate even if their architectures are different. OSI model was developed by International Standards Organization (ISO) in 1983. It provides logical framework for data communication processes to interact across networks. The standards were created for computer industry to allow different networks.

This model consists of seven layers. Each layer performs a specific functions in a network communication. The layer are divided into two groups:

Upper Layer: The upper layers focus on user applications and how files are represented on computers before transfer.

Lower Layer: The lower layers are concerned with how the communication across a network actually occurs.

Different Layers of OSI Model:

7. Application Layer

6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer