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For a long time (4 months) it was really complicated to me, not knowing what OSI really is or why does it exist, even though I understood everything about the layers cleary.
First off, you should know that it's just a virtualized understand of physical, software and hardware based technology.
Application layer is the part of technology which works as the name says through Applications.
Applications and what they desire or want to access. If I make an application then I don't really have to invent routers(network layer) or switches(data layer) or cables(physical layer)Cisco "mostly" works on Network and Data Layer as their main selling point is Routers and Switches.
Now knowing this you can read about the OSI layer and you'll understand really easily.
there are seven layer defined in osi refenence model.
1:Physical Layer- mainly work on cabling signal and bit transmission
devices working on this layer like Hub (todays not in use)
2: Data link Layer- mainly work on physical addresses or ethernet addresses
devices working on this layer like Switches and Bridges
3:Network layer- mainly work on logical addresses(Ip addresses) devices working on this layer like Router
4:Transport Layer- work to provide host to host communication
port address is main things for this layer
protocol working on this layer is TCP and UDP
5:Session Layer-mainly provide session tracking of a user Token management and session Tear down.
6:Presentation Layer:-this layer mainly work for compression technique to compress our data
7: Application Layer: this layer is mainly interact with application
like : mail , file transfer etc
The Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application). The layers are stacked this way:
The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:
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:
The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address. This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet). In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.
The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery. The transport layer provides:
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame. The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.
Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.
The session layer allows session establishment between processes running on different stations. It provides:
The 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:
The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
The OSI model defines internetworking in terms of a vertical stack of seven layers. Upper layers of the OSI model represent software that implements network services like encryption and connection management. Lower layers of the OSI model implement more primitive, hardware-oriented functions like routing, addressing, and flow control.
