OSI stands for Open Systems Interconnection. It has been developed by ISO – ‘International Organization of Standardization‘, in the year 1984. It is a 7 layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe.
In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.
The 7 Layers of the OSI Model
Physical Layer (Layer 1) :
The physical layer is responsible for the physical cable or wireless connection between network nodes. It defines the connector, the electrical cable or wireless technology connecting the devices, and is responsible for transmission of the raw data, which is simply a series of 0s and 1s, while taking care of bit rate control. It is the lowest layer of the OSI model.
- Functions of a Physical layer are : Bit synchronization, Bit rate control, Physical topologies, and Transmission model. Hub, Repeater, Modem, Cables are Physical Layer devices.
- Protocols supported at Physical Layer are : RS232, 100BaseTX, ISDN, 11.
Data Link Layer (Layer 2) :
At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer. It contains two sub-layers:
- Logical Link Control Layer –
It is responsible for transferring the packets to the Network layer of the receiver that is receiving. It identifies the address of the network layer protocol from the header. It also provides flow control. - Media Access Control Layer –
Media access control layer is a link between the Logical Link Control layer and the network’s physical layer. It is used for transferring the packets over the network.
Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.
- The functions of the data Link layer are : Framing, Physical addressing, Error control, Flow Control, and Access control.
- Protocols supported at Data Link Layer are : RAPA, PPP, Frame Relay, ATM, Fiber Cable.
Network Layer (Layer 3) :
The network layer is responsible for receiving frames from the data link layer. The network layer finds the destination by using logical addresses, such as IP (internet protocol).
- The functions of the Network layer are : Internetworking, Addressing, Routing, and Packetizing.
- Protocols supported at Network Layer are : IP (IPv4, IPv6), ICMP, IGMP, IPsec, X.25 PLP.
Transport Layer (Layer 4) :
Transport Layer is called as Heart of OSI mode. Layer 4 is responsible for end-to-end communication between the two devices. The transport layer is also responsible for flow control and error control. Flow control determines an optimal speed of transmission to ensure that a sender with a fast connection doesn’t overwhelm a receiver with a slow connection. The transport layer performs error control on the receiving end by ensuring that the data received is complete, and requesting a retransmission if it isn’t.
- The functions of the transport layer are : Service-point addressing, Segmentation and reassembly, Connection control (Connection Oriented Service, and Connection less service), Flow control, and Error control.
- Protocols supported at Transport Layer are : TCP, UDP, SCT, PDCCP, SPX.
Session Layer (Layer 5) :
This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security. This is the layer responsible for opening and closing communication between the two devices. The session layer also synchronizes data transfer with checkpoints.
- The functions of the session layer are : Session establishment, maintenance and termination, Synchronization, Dialog Controller.
- Protocols supported at Session Layer are : SAP, PPTP, RTP.
All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as “Application Layer”.
Presentation Layer (Layer 6) :
The presentation layer prepares data for the application layer. It defines how two devices should encode, encrypt, and compress data so it is received correctly on the other end. The presentation layer takes any data transmitted by the application layer and prepares it for transmission over the session layer.
- The functions of the presentation layer are : Translation, Encryption/ Decryption, and Compression.
- Protocols supported at Presentation Layer are : MPEG, ASCH, SSL, TLS.
Application Layer (Layer 7) :
At this layer, both the end user and the application layer interact directly with the software application. This is the only layer that directly interacts with data from the user. Software applications like web browsers and email clients rely on the application layer to initiate communications.
- The functions of the Application layer are : Network Virtual Terminal, File transfer access and management, Mail Services, and Directory Services.
- Protocols supported at Application Layer are : NNTP, DNS, FTP, TELNET, SMTP, HTTP, FTP, POP3, SNMP, NFS, NTP.
Note –
OSI model acts as a reference model and is not implemented in the Internet because of its late invention. Current model being used is the TCP/IP model.
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