the application layer provides services for an application program to ensure that effective communication with another application program in a network is possible. The application layer is not the application itself that is doing the communication. It is a service layer that provides these services:
•Makes sure that the other party is identified and can be reached
•If appropriate, authenticates either the message sender or receiver or both
•Makes sure that necessary communication resources exist (for example, is there a modem in the sender's computer?)
•Ensures agreement at both ends about error recovery procedures, data integrity, and privacy
•Determines protocol and data syntax rules at the application level
It may be convenient to think of the application layer as the high-level set-up services for the application program or an interactive user
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The Physical layer defines all the electrical and physical specifications for devices.
This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are:
Establishment and termination of a connection to a communications medium.
The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. This is the layer at which the bridges and switches operate.
The best known example of this is Ethernet.
Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks.
The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer.
The best known example of a layer 3 protocol is the Internet Protocol (IP).
The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable and cost-effective data transfer. This means that the transport layer can keep track of the packets and retransmit those that fail.
The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP).
The Session layer controls the dialogues (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application.
The Presentation layer transforms data to provide a standard interface for the Application layer.
The Application layer provides a means for the user to access information on the network through an application. This layer is the main interface for the user(s) to interact with the application and therefore the network
Some examples of application layer protocols include Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP).
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HowStuffWorks "How OSI Works"
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importance of OSI model:
the osi model bascially just shows that a computer should be divided into differnt sections.. this way every layer is independent... so if i wanted to chage my ethernet cord to a fibre optic cord i should be able to with out all my software stuffing up... if all layers where interdependent upgrading or changeing sections would be next to impossible and computers need to adapt and change..
The OSI Model is a model that illustrates how data communications should take place. It segregates the process into seven groups, called layers. Into these layers are integrated the protocol standards developed by the ISO and other standards organization, including the Institute of Electrical and Electronic Engineers (IEEE), American National Standards Institute (ANSI), and the International Telecommunications Union (ITU), formerly known as the CCITT (Comite Consultatif Internationale de Telegraphique et Telephone). The OSI Model affirms what protocols and standards should be used at each layer. It is modular, each layer of the OSI Model functions with the one above and below it.
i have given the best shot of my answers..
if you still have doubts and not clear ,, then maybe others can help you out who are more knowledgeable and good in explaination