|The Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers.The upper layers of the model are responsible for implementing encryption and connection management services with the help of some software whereas the lower layers are responsible for implementing the hardware oriented functions like routing,addressing etc. Each layer has specific functions and handles a unique data format. When two computers communicate, data flows from the application at|
the sending computer down through the protocol stack to the connecting physical medium (i.e. cable, radio, or infrared) and up through the protocol stack to the network-to-user interface (application) at the receiving computer. When data flows from an upper layer to a lower layer, it is converted to the lower layer data format and a lower layer header is added to it. This process is called encapsulation. Conversely, when data flows from a lower layer to an upper layer, it is converted to the upper layer data format and the lower layer header is discarded. Functions of the layers of OSI model are explained below
The Physical Layer :
The physical layer defines the physical characteristics of the interface and its main function is to transmit bits over a medium and to provide mechanical and electrical specifications for the systems in use.Well-known physical layer interfaces for data communication include serial interfaces, parallel interfaces, and the physical specifications for LAN systems such as Ethernet and token ring.
The Data Link Layer :
The data link layer defines the rules for sending and receiving information across a physical connection between two systems.It is responsible for organizing bits into frames and to provide node to node delivery. Data links are typically network segments (not internetworks) and point-to-point links. Data is packaged into frames for transport across the underlying physical network. Some reliability functions may be used, such as acknowledgment of received data. In broadcast networks such as Ethernet, a MAC (Medium Access Control) sublayer was added to allow multiple devices to share the same medium.
The Network Layer provides means for transferring variable length data sequences from a source to a destination via one or more networks and to provide inter-networking, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, fragmentation and reassembly, and report delivery errors.The protocol data unit in use at this level of the OSI model is referred to as a "packet."
Transport Layer :
The main function of transport layer is to provide process to process message delivery and error recovery.The protocol data unit in use at this level of the OSI model is referred to as a "segment."TCP and SPX are transport layer protocols.
The main function of session layer to establish, manage and terminate sessions.The session layer coordinates the exchange of information between systems by using conversational techniques, or dialogs.The protocol data unit in use at this level (and all of the subsequent levels) of the OSI model is referred to simply as "data."
The Presentation Layer :
Protocols at this layer are part of the operating system and application the user runs on a workstation. Information is formatted for display or printing in this layer. In other words, we can say that this layer is responsible for taking the data from applications at the application layer and breaking it down for use on the session layer as well as the reverse. Data encryption and the translation of other character sets are also handled in this layer.
The Application Layer:
This layer is responsible for allowing access to the network and is used to define the range of application that handle file transfers, terminal sessions and message exchange e.g electronic mail. The diagram below shows the flow of data in OSI model.
Why a Layered Model :
As OSI model is a layered model,so question may arise in your mind that why do we use this layered approach model. Following are the reasons that why this model came into existence :
- Reduces complexity
- Standardizes interfaces
- Facilitates modular engineering
- Ensures interoperable technology
- Accelerates evolution
- Simplifies teaching and learning
The following figure shows the summary of the OSI model explained above.This figure shows the data unit at each layer and overview of the function of that layer.
Internet Protocol (IP) corresponds to the third layer i.e Network layer of the OSI model and TCP and UDP correspond to the fourth layer of OSI model i.e Transport layer.Layers at the bottom of the OSI model shows technologies like Ethernet. Layers at the top of the OSI model are represented by application protocols like TCP and UDP.
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