OSI Model

 What is OSI Model?

The OSI (open system interconnection) model, created in 1984 by ISO, is a reference framework that explains the process of transmitting data between computers.

Layers of OSI Model

OSI model has seven layers, which are as follows:

1. The Physical Layer

2. The Data Link Layer

3. The Network Layer

4. The Transport Layer

5. The Session Layer

6. The Presentation Layer

7. The Application Layer

1.  Application Layer – Layer 1 (considering we are on sender side )

• · User work on app layer.
• · App layer provide the interface to user like Gmail.
• · At the very top of the OSI Reference Model stack of layers, we find the Application layer, which is implemented by the network applications. These applications produce the data, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user.

Example: Application – Browsers, Skype Messenger, Gmail, outlook etc.

Note: 1. The application Layer is also called Desktop Layer.

           2. Device or Protocol Use : SMTP

Functions of the Application Layer:

The main functions of application layer are given below.

· Network Virtual Terminal: It allows a user to log on to a remote host.

· FTAM- File transfer access and management : This application allows a user to access file in a remote host, retrieve files in remote host and manage or control files from a remote computer.

· Mail Services : Provide email service.

· Directory Services : This application provides distributed database sources and access for global information about various objects and services.

2. Presentation Layer – Layer 2

The presentation layer is also called the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.

Functions of the Presentation Layer:

· Translation: For example, ASCII to EBCDIC.

· Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.

· Compression: Reduces the number of bits that need to be transmitted on the network.

3. Session Layer – Layer 3 (It will ensure that data is reached at the destination successfully)

This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and ensures security.

Functions of the Session Layer:

· Session establishment, maintenance, and termination: The layer allows the two processes to establish, use and terminate a connection.

· Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.

Note: Synchronization occurs between network nodes to ensure that data streams are received and transmitted correctly.

· Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

For example:-

Let us consider a scenario where a user wants to send a message through some Messenger application running in his browser. The “Messenger” here acts as the application layer which provides the user with an interface to create the data. This message or so-called Data is compressed, encrypted (if any secure data), and converted into bits (0’s and 1’s) so that it can be transmitted.

4. Transport Layer – Layer 4

The transport layer provides services to the application layer and takes services from the network layer. The data in the transport layer is referred to as Segments. It is responsible for the End to End Delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.

At the sender’s side: The transport layer receives the formatted data from the upper layers, performs Segmentation, and also implements Flow & Error control to ensure proper data transmission. It also adds Source and Destination port numbers in its header and forwards the segmented data to the Network Layer.

Functions of the Transport Layer:

· Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.

· Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

Note:

1. Data in the Transport Layer is called Segments.

2. Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls.

3. The transport layer is called as Heart of the OSI model.

4. Device or Protocol Use : TCP(if you want to send the data successfully and don’t want the data loss), UDP(if you want to send the data very fast, don’t care of data loss ) NetBIOS, PPTP

5. Network Layer – Layer 5

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP addresses are placed in the header by the network layer.

Functions of the Network Layer:

· Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.

6. Data Link Layer (DLL) – Layer 6

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL, to transmit it to the Host using its MAC address. The Data Link Layer is divided into two sublayers:

1. Logical Link Control (LLC)

2. Media Access Control (MAC)

· The packet received from the Network layer is further divided into frames depending on the frame size of the NIC (Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header.

· The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.

· The acronym ARP stands for Address Resolution Protocol, which is one of the most important protocols of the Data link layer in the OSI model. It is responsible to find the hardware address (MAC address) of a host from a known IP address.

Functions:

· Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.

· Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.

· Error control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.

7. Physical Layer – Layer 7

The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.