OSI MODEL

• OSI stands for Open System Interconnection.
• OSI is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application of another computer.
• OSI consists of seven layers, and each layer performs a particular network function.
• OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered as an architectural model for the inter-computer communications.
• OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.
• Each layer is self-contained, so that task assigned to each layer can be performed independently.
CHARACTERISTICS OF OSI MODEL

The OSI model is divided into two layers: Upper layers and Lower layers.

• The upper layer of the OSI model mainly deals with the application related issues, and they are implemented only in the software.
• The lower layer of the OSI model deals with the data transport issues.
1. PHYSICAL LAYER
• The Physical layer is the foundation of computer networks.
• It deals with the actual hardware and physical aspects that enable data to move from one device to another.
• Layer 1 is the first and lowest layer.
• The Physical layer makes sure that data can be sent and received over the network by converting it into electrical, optical, or radio signals.
• Physical layer focuses on the physical components and infrastructure that enable data transmission.
2. DATALINK LAYER
• The main function of this layer is to make sure data transfer is error free from one node to the another, over the physical layer.
• Transmitting and receiving data frames sequentially is managed by this layer.
• This layer sends and expects acknowledgements for frames received and sent respectively. Resending of non-acknowledgement received frames is also handled by this layer.
• This layer established a logical layer between two nodes and also manages the frame traffic control over the network. It signals the transmitting node to stop, when the frame buffers are full.
• Protocols are Frame relay, Address Resolution Protocol (ARP), Point-to-Point Protocol (PPP), etc.
• Devices are Switch and Bridge.
3. NETWORK LAYER
• It divides the outgoing messages into packets and assembles the incoming packets into messages for higher levels.
• It routes the packet through different channels from one node to other.
• It acts as a network controller. It manages the Subnet traffic.
• It decides which route should the data packets take.
• Protocols are Internet Protocol version 4 (IPv4), Internet Protocol version 6 (IPv6), Internet Control Message Protocol (ICMP), Internet Protocol Security (IPSec).
• Device used is Router.
4. TRANSPORT LAYER
• It decides whether data transmission should be on parallel path or single path.
• Functions such as Multiplexing, Segmenting or Splitting on the data are done by this layer.
• It receives messages from the Session layer above it, convert the message into smaller units and passes it on the Network layer.
• Transport layer can be very complex, depending upon the network requirements.
• Protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP).
5. SESSION LAYER
• The Session layer is for establishing, managing, and terminating sessions or connections between devices.
• Its primary function is to make sure that when data exchanges between devices - it’s organized and structured.
• It sets up, maintains, and ends conversations or sessions between applications on different devices.
• In very simple terms, it manages the dialog between applications.
• Protocols are Point-to-Point Tunneling Protocol (PPTP), Session Initiation Protocol (SIP), Session Announcement Protocol (SAP).
6. PRESENTATION LAYER
• Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.
• While receiving the data, presentation layer transforms the data to be ready for the application layer.
• Language (syntax) can be different between the two communicating systems. Under this condition presentation layer plays a role of translator.
• It performs Data compression, Data encryption, Data conversion etc.
• Protocols are Multipurpose Internet Mail Extensions (MIME), Secure Sockets Layer (SSL).
7. APPLICATION LAYER
• The topmost layer is the Application layer.
• In this layer the user interacts with services like websites, email, and more. It’s the user interface that you see and use daily.
• The Application layer is responsible for data formatting and conversion, making sure that data is shown in a format that the application can understand.
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