The OSI Model's Seven Layers Defined and Functions Explained

The Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application). The layers are stacked this way:

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data Link
7. Physical

• PHYSICAL LAYER

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:

• Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium. It determines:
• What signal state represents a binary 1
• How the receiving station knows when a "bit-time" starts
• How the receiving station delimits a frame
• Physical medium attachment, accommodating various possibilities in the medium:
• Transmission technique: determines whether the encoded bits will be transmitted by base band (digital) or broadband (analog) signaling.
• Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines
• What physical medium options can be used

• DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides: 

• Link establishment and termination: establishes and terminates the logical link between two nodes.
• Frame traffic control: tells the transmitting node to "back-off" when frame buffers are not available.
• Frame sequencing: transmits/receives frames sequentially.
• Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer
• Frame delimiting: creates and recognizes frame boundaries.
• Frame error checking: checks received frames for integrity.
• Media access management: determines when the node "has the right" to use the physical medium.

• NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides: 

• Routing: routes frames among networks.
• Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
• Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
• Logical-physical address mapping: translates logical addresses, or names, into physical addresses.

• TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. 

The transport layer provides:

• Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
• Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
• Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.

• SESSION LAYER

The session layer allows session establishment between processes running on different stations. It provides: 

• Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
• Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

• PRESENTATION LAYER

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station. 

The presentation layer provides:

• Character code translation: for example, ASCII to EBCDIC.
• Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
• Data compression: reduces the number of bits that need to be transmitted on the network.
• Data encryption: encrypt data for security purposes. For example, password encryption.

• APPLICATION LAYER

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions: 

• Resource sharing and device redirection
• Remote file access
• Remote printer access
• Inter-process communication
• Network management
• Directory services
• Electronic messaging (such as mail)
• Network virtual terminals