The Transport Layer:
The essential function of the transport layer is to accept data from above, split it up into smaller units if need be, pass these to the network layer, and make sure that the pieces all arrive correctly at the other end. In addition, all this should be done efficiently and in a way which isolates the upper layers from the inevitable modification in the hardware technology. The transport layer also determines which type of service to provide to the session layer, and, eventually, to the users of the network. The generally popular type of transport relation is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. However, one another way to possible kinds of transport service are the transporting of isolated messages, with no guarantee about the order of delivery, and the broadcasting of messages to multiple destinations. The type of service is determined when the connection is recognized. The transport layer is a true end-to-end layer, all way from a source to the destination. In other words, a program on the source machine carries on a discussion with a like program on the destination machine, using message headers and control messages. In lower layers, the protocols are connecting each machine and its immediate neighbours, and not between the ultimate source and destination machines, which may be detached by some routers.
The Session Layer:
The session layer allows users on unlike machines to found sessions stuck between them. Sessions offer a range of services, including dialog control (keeping track of whose turn it is to transmit), token management and synchronization (check pointing long transmissions to allow them to continue from where they were after a crash).
The Presentation Layer:
The presentation layer is troubled with the syntax and semantics of the information transmitted. In order to create it possible for computers with different data representations to communicate, the data structures to be changed can be defined in the conceptual way, with a standard encoding to be used ''''on the wire.'''' The presentation layer handle these abstract data structures and allows higher-level data structures for example banking records to be defined and exchanged.
The Application Layer:
The application layer has a multiplicity of protocols that are regularly needed by users. One mostly used application protocol is HTTP (Hypertext Transfer Protocol), which is a beginning for the World Wide Web. When a browser desires a Web page, it sends the name of the page it wishes to the server using HTTP. The server after that sends the page reversed. Other application protocols can be used for file transfer, electronic mail as well as network news.