OPEN SYSTEM INTERCONNECTION REFERENCE MODEL

The Open System Interconnection (OSI)  Reference Model

The International Standards Organization Open System Interconnection (ISO OSI) reference model first achieved the International standardization of the protocols. ISO developed the standards for connecting systems that are open to communication with other systems.
The OSI reference model has seven layers. The following figure shows the seven-layer OSI reference model:

OSI Seven Layers

The seven layers of the model are listed as follows in the bottom-to-top approach:

• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

Physical Layer
The Physical layer is responsible for the transmission of data over a communication channel or the transmission medium. This transmission medium is the physical path over which the data is transmitted in the form of signals. During a transmission, the Physical layer converts the data into series of bits and places these bits on the transmission medium. This layer is also responsible for specifying the physical structure or topology of the network. Although the Physical layer deals with transmission media, the transmission medium is not specified.


Data Link Layer
The Data Link layer is responsible for the following aspects of communication:

1. Providing unique identification to each node on the network
2. Transforming data bits from the Physical layer into groups called frames
3. Detecting errors that occur during a transmission
4. Managing the flow of data packets or frames

The Data Link layer provides unique identity to each node on the network. It uses the network address, which is hard-coded into the network card of each node for this purpose. Although the Data Link layer is responsible for the detection of transmission errors, it is not responsible for the correction of errors.

The Data Link layer is divided into two sublayers. These are:

1. Media Access Control (MAC): This sublayer helps the nodes on a network to communicate with each other as it provides information about physical address of the node or the MAC address.
   
2. Logical Link Control (LLC): This sublayer establishes and manages a logical link between two communicating devices on the network. It provides error control and flow control within a network

Network Layer
The Network layer is responsible for the following functions:

1. Providing a unique network address to each node on the network
2. Transmitting data across networks
3. Controlling network traffic

The Network layer provides a unique address to each node on a network. The addresses are different from Data Link layer addresses because Data Link layer addresses can only be used for communication within a single local network. However, if a node on a network A, needs to communicate with a node on network B, it would not be able to do so using Data Link layer addresses.

For communication among different networks, a special Network layer-addressing scheme known as logical addressing is used. When two nodes that are located on two separate networks need to communicate with each other, they need the logical address provided by the Network layer.

The Network layer is also responsible for determining all the possible routes to the destination network and selecting the best path to the network where the destination node is located. This process is known as routing.


Transport Layer
The Transport layer is responsible for the following functions:

1. Organizing messages into segments or breaking large segments into smaller segments 
2. Delivering segments to recipients
3. Providing error control

The Transport layer segments and reassembles data. In case the upper layers generate large data packets, the Transport layer breaks the large packets into smaller segments that can be handled by lower layers. Similarly, when lower layers pass small segments to the Transport layer, it combines multiple segments to form large packets. Because the Transport layer deals with segmentation and reassembly, it is also responsible for correctly sequencing the segments so that the entire data can be reconstructed correctly.

The Transport layer is also responsible for the unreliable or reliable delivery of segments with the help of connectionless or connection-oriented services, respectively. The recipient acknowledges a transmission in connection-oriented services. If the sender does not receive an acknowledgement from the recipient within a specific interval, it retransmits the unacknowledged packets. In connectionless services, the sender continues to transmit packets without receiving acknowledgements.

The Transport layer also provides error control services. When a packet is lost during transmission, the Transport layer retransmits it after a specific interval. In addition, the Transport layer adds checksums to the segments before transmission. The recipients use these checksums to determine whether the segment was corrupted during the transmission or not. If the segment is corrupted, the Transport layer retransmits the appropriate segments.


Session Layer
The Session layer establishes, manages, and synchronizes the communication between two communicating nodes. The two nodes can exchange information only after a session has been established between them. In this case, session is defined as a logical connection between the two nodes.

The Session layer can also control the direction in which data flows. Based on this direction of data flow, a session can be any one of the following:

1. Simplex: Only one node can transmit data at a time.
2. Half duplex: One node can transmit while the other node can receive data. However, both nodes cannot transmit data at the same time.
3. Duplex: Both nodes can transmit as well as receive data at the same time.

Presentation Layer
The Presentation layer encodes and decodes data in a mutually agreeable format. As a result, this layer plays an important role in facilitating data exchange between heterogeneous hardware and software platforms that use different data formats. For example, if one node uses one byte to represent a character and the other node uses two bytes to represent a character, the Presentation layer plays a significant role in facilitating successful data exchange between the two nodes.

If required, the Presentation layer also compresses and decompresses data packets. As a result, the network traffic is less, preventing congestion. The Presentation layer also plays an important role in the encryption and decryption of data. This ensures high data security during transmission.


Application Layer
The Application layer provides an interface between the user and the network. It supports a number of software programs and end-user processes that act as a link between the user and the network. As a result, all network applications and protocols reside on this layer. This is the reason for the name Application layer. Some of the common applications and protocols that operate on this layer include e-mail, FTP, and Telnet.