OSI Model

Open System Interconnection is a standard that describes how message  is transmitted and received between two points in telecommunication.It is developed by ISO( International Organization for Standardization ). It is a seven layer model and are labeled 1 to 7 with layer 1 at bottom(as per X.200 Recommendations).This concept of a seven-layer model was provided by the work of Charles Bachman, Honeywell Information Services.Idea behind this model was that the telecommunication between two points can be divided into layers with each layer adding its own set of special, related functions. The seven Layers are: 1)Physical Layer 2)Data Link Layer 3)Network Layer 4)Transport Layer 5)Session Layer 6)Presentation Layer 7)Application Layer. these seven layers are divided into two groups. The upper four layers are used whenever a message passes from or to a user(known as Host Layers). The lower three layers (up to the network layer) are used when any message passes through the host computer(known as Media Layers) . Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host.

1)Physical Layer:-It is the first layer of OSI model and defines electrical and physical specification for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or fiber optical cable. This includes the layout of pins, voltages, line impedance, cable specifications, signal timing etc.
The Major Functions of Physical Layer are:
i)Establishment and termination of a connection to a communications medium.
ii)Participation in the process whereby the communication resources are effectively shared among multiple users. For example,  flow control.
iii)Modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

2)Data Link Layer:- This is second layer of OSI Model and provides the functional and procedural means to transfer data between network entities.This layer also detects and correct errors that may occur at physical layer.Functions of Data Link Layer are:
i)Link establishment and termination: establishes and terminates the logical link between two nodes.
ii)Frame traffic control: tells the transmitting node to “back-off” when no frame buffers are available.
iii)Frame sequencing: transmits/receives frames sequentially.
iv)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.
v)Frame delimiting: creates and recognizes frame boundaries.
vi)Frame error checking: checks received frames for integrity.
vii)Media access management: determines when the node “has the right” to use the physical medium.

3)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:
i)Routing: routes frames among networks.
ii)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.
iii)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.
iv)Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
v)Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

4)Transport Layer:-It is the 4th layer of OSI model and performs following functions.
i)It provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers.
ii)It controls the reliability of a given link through flow control, segmentation/ de-segmentation, and error control.
iii)This layer keep track of the segments and retransmit those that fail.
iv) It provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.

5)Session Layer:-Session layer is the 5th layer of OSI model.It Provides following functions:
i)It controls the dialogues (connections) between computers.
ii) It establishes, manages and terminates the connections between the local and remote application.
iii) It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures.
iv)This layer is responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session check pointing and recovery, which is not usually used in the Internet Protocol Suite.
v)Inter-Process communication(SIGNUP, SIGKILL, End Process, etc.)  happen on this layer.

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

7)Application Layer:-it is the 7th and the top most layer of OSI model.This layer contains a variety of commonly needed functions:
i)Resource sharing and device redirection
ii)Remote file access
iii)Remote printer access
iv)Inter-process communication
v)Network management
vi)Directory services
vii)Electronic messaging (such as mail)
viii)Network virtual terminals