OSI Reference Model

The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It provided the standardized framework  of Networking communication. The OSI model consists of seven layers, each corresponding to a specific network function:

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The OSI models are divide into 2 part One is Upper layers and Lower layers see the above Picture for same.

1. Application Layer

Application layer provides platform to send and receive data over the network. All applications and utilities that communicate with network will come in this layer.

For examples: All Browsers, TFTP, SNMP, FTP, Telnet, Mail clients, DNS and DHCP,

2. 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.

This layer can also encapsulate the data, but it’s uncommon as this can be done by lower layers more effectively.

3. The Session Layer

Session layer deals with connections. It establishes, manages, and terminates sessions between two communicating nodes. This layer provides its services to the presentation layer. Session layer also synchronizes dialogue between the presentation layers of the two hosts and manages their data exchange. For example, web servers may have many users communicating with server at a given time. Therefore, keeping track of which user communicates on which path is important and session layer handle this responsibility accurately.

4. Transport Layer

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

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.

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 multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions

Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.

In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

5. 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.
  • Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

6. 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 no frame buffers are available.
  • Frame sequencing: transmits/receives frames sequentially.
  • Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • 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.

7. Physical Layer

Physical layer deals with communication media. This layer receive frame from data link layer and convert them in bits. It loads these bits on actual communication media. Depending on media type these bit values are converted in single. Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines.

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