Seven Layers of OSI Model and functions of seven layers of OSI model

International Standards Organization (ISO) proposed the Open Systems Interconnection (OSI) network model. The Open Systems Interconnection (OSI) model breaks down the problems involved in moving data from one computer to another computer.
Open Systems Interconnection (OSI) Seven Layered reference model is only just a reference model. The following image shows the seven layers described in Open Systems Interconnection (OSI) model.

Physical (Layer 1) 

OSI Model, Layer 1 conveys the bit stream - electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

 Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45, hub Network card.

Data Link (Layer 2)

The data link layer is the secound layer of the osI model. It perform four function:

1) Framing

2) Arbitration

3) Physical Addressing

4) Error detection 

5) Identifying the encapsulated data

When the data link layer accepts the data from the network layer, it adds meaningful bits at the begining and the end of the data. these bits are known as the header and trailer respectively. 

The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

  Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay. 

Network (Layer 3)

Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing,internetworking, error handling, congestion control and packet sequencing.

 Layer 3 Network examples include AppleTalk DDP, IP, IPX, Router, Switches.

Transport (Layer 4)

OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

 Layer 4 Transport examples include SPX, TCP, UDP.

Session (Layer 5)

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

 Layer 5 Session examples include NFS, NetBios names, RPC, SQL.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

 Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

Application (Layer 7)

OSI Model, Layer 7, supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on datasyntax are identified. Everything at this layer is application-specific. This layer provides application services forfile transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

 Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP 

DID YOU KNOW....? Two similar projects from the late 1970's were merged in 1983 to form the Basic Reference Model for Open Systems Interconnection standard (the OSI model). It was published in 1984 as standard ISO 7498.