The Open Systems Interconnection (OSI) can be divided into three subgroups. The first three layers 1, 2, and 3 are the network support layers which deal with the physical aspects of moving data from one device to another device.
The next layers 5, 6, and 7 are the user support layers which allow interoperability among unrelated software systems.
And layer 4 links two subgroups to ensure that what the lower layers have transmitted is in a form that the upper layers can use.
Functions of the Layers in the OSI Model
1. Physical Layer
Coordinates the functions required to carry a bit stream over a physical medium. It deals with the electrical and mechanical specifications of the interface and transmission medium. It defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.
2. Data Link Layer
Takes a raw transmission facility and transforms it into reliable link. It makes the physical layer appear error-free to the upper layer which is the Network layer. It also responsible for other functions such as framing, error control, flow control, physical addressing and access control mechanisms.
3. Network Layer
This layer is responsible for the source-to-destination delivery of a packet possibly across multiple networks. The Data Link Layer oversees the delivery of the packet between two systems on the same network, the network layer ensures that each packet gets from its point of origin to its final destination.
4. Transport Layer
The basic function of the transport layer is to accept data from the session layer, split it into smaller units, pass it to the network layer, and ensure that the bits delivered are the same as the bits transmitted without modification, duplication or loss.
5. Session Layer
The session layer acts as the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.
6. Presentation Layer
This layer is concerned with the syntax and semantics of the information exchanged between two systems. For outgoing messages, it converts data into a generic format for the transmission. For the incoming messages, it converts the data from the generic form to a format understandable to the receiving application. Different computers have different codes for representing data. The specific responsibilities of this layer include Translation, Encryption, and Compression.
7. Application Layer
The application layer enables the user whether human or software to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management and other types of distributed information services. Specific services offered by the Application layer include: Provision of Network Virtual terminals, File transfer, access, and management, mail services, and Directory Services.
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