OSI MODEL

                  Developed by representatives of major computer and telecommunication companies beginning in 1983, OSI was originally intended to be a detailed specification of actual  Instead, the committee decided to establish a common reference model for which others could then develop detailed interfaces, which in turn could become standards OSI was officially adopted as an international standard by the International Organization of Standards (ISO).

OSI LAYERS

• Application

• Presentation

• Session

• Transport

• Network

• Data Link

• Physical

     The main concept of OSI is that the process of communication between two endpoints in a telecommunication network can be divided into seven distinct groups of related functions, or layers. Each communicating user or program is at a computer that can provide those seven layers of function. So in a given message between users, there will be a flow of data down through the layers in the source computer, across the network and then up through the layers in the receiving computer. The seven layers of function are provided by a combination of applications, operating systems, network card device drivers and networking hardware that enable a system to put a signal on a network cable or out over WIFI or other wireless protocol).

APPLICATION LAYER

      This is the layer at which communication partners are identified (Is there someone to talk to?), network capacity is assessed (Will the network let me talk to them right now?), and that creates a thing to send or opens the thing received.  (This layer is not the application itself, it is the set of services an application should be able to make use of directly, although some applications may perform application layer 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

 PRESENTATION LAYER

          This layer is usually part of an operating system (OS) and converts incoming and outgoing data from one presentation format to another (for example, from clear text to encrypted text at one end and back to clear text at the other).

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.

SESSION LAYER

         This layer sets up, coordinates and terminates conversations. Services include authentication and reconnection after an interruption. On the Internet,Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) provide these services for most applications.

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.

TRANSPORT LAYER

         This layer manages packetization of data, then the delivery of the packets, including checking for errors in the data once it arrives. On the Internet, TCP and UDP provide these services for most applications as well.

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 (see session layer).

NETWORK LAYERS

        This layer handles the addressing and routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). IP is the network layer for the Internet.

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.

DATALINK LAYER

         This layer sets up links across the physical network, putting packets into network frames. This layer has two sub-layers, the Logical Link Control Layer and the Media Access Control Layer. Ethernet is the main data link layer in use.

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.

PHYSICAL LAYER

            This layer conveys the bit stream through the network at the electrical, optical or radio level. It provides the hardware means of sending and receiving data on a carrier network.

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, and to aid in bit and             frame synchronization. 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:
  
  
  • Will an external transceiver (MAU) be used to connect to the medium?
  • How many pins do the connectors have and what is each pin used for?
• Transmission technique: determines whether the encoded bits will be transmitted by baseband (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
  • How many volts/db should be used to represent a given signal state, using a given physical medium