OSI or the open System Interconnection

Has been outlined by International Organization for Standardization (ISO) to facilitate communication of heterogeneous hardware or software platforms with each other with the help of following seven layers of functions with their associated controls: -
Layer 1 or Physical Layer is a hardware layer which specifies mechanical features as well as electromagnetic features of the connection between the devices and the transmission. Network topology is a part of this layer.
Layer 2 or Data Link Layer is also a hardware layer which specifies channel access control method and ensures reliable transfer of data through the transmission medium.
Layer 3 or Network Layer makes a choice of the physical route of transmission of say, a message packet, creates a virtual circuit for upper layers to make them independent of data transmission and switching, establishes, maintains, terminates connections between the nodes, ensures proper routing of data.
Layer 4 or Transport Layer ensures reliable transfer of data between user processes, assembles and disassembles message packets, provides error recovery and flow control. Multiplexing and encryption are undertaken at this layer level.
Layer 5 or Session Layer establishes, maintains and terminates sessions (dialogues) between user processes. Identification and authentication are undertaken at this layer level.
Layer 6 or Presentation Layer controls on screen display of data, transforms data to a standard application interface. Encryption, data compression can also be undertaken at this layer level.