OSI Model

The OSI model was designed to promote interoperability by creating a guideline for network data transmission between computers and components that have different hardware vendors, software, operating system, and protocols.The OSI model consists of seven layers. Each Layers performs a specific function and then passes on the result to another layer.

Understanding Data Transfer using OSI model.

Application Layer: The highest layer; defines the manner in which applications interact with the network including databases, e-mail, and terminal-emulation programs using Application layer protocols similar to Lightweight Directory Access Protocol (LDAP), Simple Mail Transfer Protocol (SMTP), and telnet etc.
Presentation Layer: This layer is responsible for the formatting and code conversion of data being passed up to the Application layer.In this layer, character sets are converted (e.g., from ASCII to Unicode or EBCDIC) and data is encrypted. Data may also be compressed in this layer. Anything that is done to the data on the transmitting device must be undone on the receiving device. Presentation Layer is responsible for recognizing file types in an incoming data stream and performing any massaging to the dta to make a file presentable to the Application protocol.
Session Layer: It defines how two computers establish, synchronize, maintain and end a session. Its functions such as security authentication, connection ID establishment, data transfer, acknowledgements, and connection release take place here.
Transport Layer: It is responsible for checking that the data was delivered error-free. It is also used to divide a message that is too long into smaller segments and, in the reverse, take a series of short messages and combine them in to one longer segment.This layer also handles logical address/name resolution. Additionally, this layer can send an acknowledgement that it got the data packet.
Network Layer: It is responsible for logical addressing and translating logical addresses into physical addresses.A little-known function of the network layer is prioritizing data. Nobody is hurt it an e-mail message is delayed a fraction of a second but delaying audio or video data a fraction of a second could be disastrous to the message. This prioritization is known as quality of service (QoS). This layer controls congestion, routes data, build and tear down packets. Routers and manageable switches operates on this layer. Routing protocols (RIP, OSPF, EIGRP, BGP etc)are being implemented on this layer.

Data-Link Layer: It takes raw data from the physical layer and gives it a logical structure, known as a frame. in the opposite direction of flow, the Data Link layer hands frames down to the Physical layer for bit-level encoding onto the networking medium.Frames include information about where the data is meant to go, which device on the local link sent the data, and the overall validity of the bytes sent. It also controls functions of logical network topologies, physical addressing as well as data transmission synchronization and connections. Non-manageable switches and bridges operate at this layer.
Physical Layer: Physical layer is responsible for controlling the functional interface, such as transmission technique, encoding scheme, cable specifications, pin layout, and connector.
                 Physical layer concept involve a network component that is tangible or measurable. For example, when a protocol at the physical receives information from the upper layers, it translates all the data into signals that can be transmitted on a transmission medium.

Data Flow in a Network