- The OSI Model's Seven layers and its function
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The OSI Model consists of 7 layers that begins with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application).
1) Physical Layer
This is the first lower layer of the OSI model 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. This layers makes use of the Physical medium transmission, „Transmission technique and Data encoding.
2) Data link (MAC) layer
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. „Data Link layer makes use of functions such as Link establishment and termination,„ Frame traffic control, Frame sequencing, Frame acknowledgment, Detects and recovers from errors that occur in the physical layer by re-transmitting non-acknowledged frames and handling duplicate frame receipt, Frame delimiting, Frame error checking,
„Media access management.
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.
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. Some of the functions are, „Routing, Subnet traffic control, Frame fragmentation, Logical-physical address mapping, Subnet usage accounting and,
„Communications Subnet.
4) Transport Layer
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 transport layer provides, Message segmentation, Message acknowledgment, Session multiplexing and Message traffic control. „End-to-end layers
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.
5) Session Layer
„The session layer allows session establishment between processes running on different stations. It provides:
Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
„Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.
6) Presentation Layer
„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:
Character code translation: for example, ASCII to EBCDIC.
„Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
„Data compression: reduces the number of bits that need to be transmitted on the network.
Data encryption: encrypt data for security purposes. For example, password encryption.
7) Application Layer
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:
1) Physical Layer
This is the first lower layer of the OSI model 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. This layers makes use of the Physical medium transmission, „Transmission technique and Data encoding.
2) Data link (MAC) layer
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. „Data Link layer makes use of functions such as Link establishment and termination,„ Frame traffic control, Frame sequencing, Frame acknowledgment, Detects and recovers from errors that occur in the physical layer by re-transmitting non-acknowledged frames and handling duplicate frame receipt, Frame delimiting, Frame error checking,
„Media access management.
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.
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. Some of the functions are, „Routing, Subnet traffic control, Frame fragmentation, Logical-physical address mapping, Subnet usage accounting and,
„Communications Subnet.
4) Transport Layer
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 transport layer provides, Message segmentation, Message acknowledgment, Session multiplexing and Message traffic control. „End-to-end layers
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.
5) Session Layer
„The session layer allows session establishment between processes running on different stations. It provides:
Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
„Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.
6) Presentation Layer
„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:
Character code translation: for example, ASCII to EBCDIC.
„Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
„Data compression: reduces the number of bits that need to be transmitted on the network.
Data encryption: encrypt data for security purposes. For example, password encryption.
7) Application Layer
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:
- „Resource sharing and device redirection
- „Remote file access
- „Remote printer access
- „Inter-process communication
- „Network management
- „Directory services
- „Electronic messaging (such as mail)
- „Network virtual terminals