OSI (Open Systems Interconnection) Network Model

ISO refers to International Organization for Standardization – established on 23rd February 1947 and headquartered in Geneva (Switzerland) is a non-governmental organization dedicated to promote worldwide proprietary, industrial and commercial standards.

OSI refers to Open Systems Interconnection that covers all aspects of network communication. It is a standard of ISO based on layered framework that allows any two different systems to communicate regardless of their underlying architecture.

The Open Systems Interconnection (OSI) model was developed by International Organization for Standardization (ISO) which describes seven layers that computer systems use to communicate over a network. It was the first standard model for network communications, developed in 1983 and first published in 1984 as the ISO standard 7498.

OSI model had two major components- the Basic Reference Model or Seven-layer model given by Charles Bachman, and a set of specific protocols. It is a layered server architecture system in which each layer is connected with the next layer by using interfaces and each interface defines what information and services a layer must provide for the layer above it. Thus, all the seven layers work collaboratively to transmit the data from one layer to another.

The Seven Layers of OSI Model from lowest are: 
1. Physical Layer
2. Data Link Layer 
3. Network Layer 
4. Transport Layer 
5. Session Layer  
6. Presentation Layer
7. Application Layer. 

Physical Layer is the lowest layer in the model.

Organization of Layers: The seven layers are further arranged into three sub groups as follows:

Network Support Layer: Layer 1, 2, and 3 (Physical Layer, Data Link Layer and Network Layer) are called network support layers that mainly deals with electrical specifications, physical connections, transport timing and reliability. In short, they generally deal with the physical aspects of moving data from one device to other.

User Support Layer: Layer 5, 6 and 7 (Session Layer, Presentation Layer and Application Layer) are the user support layers which mainly deal with the intercommunication or interoperability among unrelated software systems.

Intermediate Layer: Layer 4 (Transport Layer) is in the middle and used to link both the network support layers and user support layers.

 1.      Physical Layer
This layer includes the physical equipment involved in the data transfer, such as the cables and switches (connectors, pins, electric current and wireless technology used, etc.). This is also the layer where the data gets converted into a bit stream, which is a string of 0s and 1s. This layer basically provides a physical medium through which bits are transmitted. In short, it is physical layer that determines the transmission speed in a network (i.e. the number of bits sent per second), types of connection between the devices (i.e. point to point or multi-point), physical topology (i.e. mesh, star, tree, bus or ring) and transmission mode (i.e. simplex, half duplex or full duplex).

2.      Data Link Layer
The data link layer provides node-to-node data transfer and defines protocol to establish and terminate a connection between two physically connected devices. It breaks up packets into frames, adds a header that defines the physical address of the sender and the receiver and sends them from source to destination. This layer is also responsible for detecting and correcting errors that may have occurred at the physical layer and imposing a flow control mechanism used to ensure that the data rate at the sender and receiver should be same. 

This data link layer is basically composed of two parts: 

(a) Logical Link Control (LLC), which defines network protocols, performs error checking and synchronizes frames. 

(b) Media Access Control (MAC), which uses MAC addresses to connect devices and define permissions to send and receive data.

3.      Network Layer
If the sender is in one network and the receiver is in some other network then the network layer is responsible for the source to destination delivery. There are two principal functions of Network layer. One is to receive frames from the data link layer, break them into network packets, deliver them to their intended destinations and reassemble the receiving end packets. Secondly, the network layer determines the best path using logical addresses such as the Internet protocol to route the packets to their destination node, based on the addresses found within the frame. Routers are a key component at this layer used to route information where it has to go between networks.

4.      Transport Layer
The Transport layer is responsible for the end to end delivery of the entire message. Segmentation and reassembling take place at this layer. A message is split into manageable units called segments that are reassembled again at the receiver end so that it can be used by the session layer. The transport layer performs error checking to ensure that the entire message is delivered sequentially and without error. It controls the size, sequencing, and eventually the data transfer between source and destination. TCP (Transmission Control Protocol) is one of the most common examples of the transport layer.

5.      Session Layer
Conversations between different computers are managed by the session layer. It acts as a dialog controller,responsible for opening sessions, ensuring that while data is being transferred, they remain open and functional, and closing them when communication ends.During a data transfer, the session layer can also set Checkpoints - if the session is interrupted, devices can resume data transfer from the last checkpoint. The services on the session layer also include authentication and re-connections.

6.              Presentation Layer
The presentation layer (also called syntax layer) formats, translates and prepares data for the application layer based on the syntax or semantics that the application accepts. In other word, it defines how two devices should encode, encrypt, and compress data so it is received correctly on the other end. In short, this layer handles the encryption, decryption and compression required by the application layer.

7.      Application Layer
The application layer gives the user access to the network via web browsers and email clients. It provides protocols that allow software to send and receive information and presents meaningful data to users. The Hypertext Transfer Protocol ( HTTP), File Transfer Protocol ( FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol ( SMTP), and Domain Name System ( DNS) are some examples of application layer protocols.

Self-Check Exercises

1.Which layer changes bits into electromagnetic signals?

Answer: (A) Physical Layer

2. Which among the following is an application layer service?

Answer: (D) All the above

3. Which layer is closest to the transmission medium?

Answer: (A) Physical layer

4.Which layer is used to link the network support layers and user support layers?

Answer: (C) Transport Layer

5. Which layer is responsible for the process-to-process delivery of the entire message?

Answer: (B) Transport Layer

Post a Comment