Session Layer explained in detail
- Session layer is the 5th layer of OSI model. It establish, maintain as well as Synchronizes among communicating system.
- Session layer is responsible for main two purpose.
- Dialog Control: Its use for communication between two process to take place in either half duplex or full duplex.
1. Half Duplex
- In the Half duplex Both station transmit and receive data but not a same time. in this figure shows when station A send data station B receive data only similarly when Station B send data at that time station A receive data only not send data at a same time.
- In half duplex utilize whole channel at a time by any one station.
- A half duplex mode use when there is no need for communication in both direction at a same time like Walkie-talkie.
2. Full Duplex
- Full duplex: Two devices can send and receive at a same time. And traffic flowing in both directions at a same time.
- In this either link contain two physical separate transmission paths, one for sending and other for receiving or one channel is pided between signals traveling in both direction. Example is telephone network.
- In the ISO protocol dialog management implemented using data token. User may transmit data when it posses token.
- Second is Synchronization: Its add checkpoints to a stream of data. Lets take a example of this suppose you have to transmit 500 pages of data. So if you direct transmit it and some during transmission its crashes so you need to again transmit whole data to overcome this problem we use check point say after 50 pages. Now suppose at 160 pages crash happen you need to transmit only 150-160 number pages not previous one below 150.
- Activity management allow user to delimit data into logical unit called activity. Activities are also using for quarantining, collection all the messages of a multi message exchange together before processing them. The receiving application would begin processes message only after all message arrive. These provide a way to helping insure all or none a set of operation are performed.
- Session layer use number of important protocol like.
- SQL-structured query language.
- RPC –remote procedure call.
- SCP-session control Protocol.
- DNA-digital network Architecture.
- ASP-apple talks Session protocol.
- X-window system.
RPC(Remote Procedure Call):
- Provides a different paradigm for accessing network services. Instead of accessing remote services by ascending and receiving messages, a client invoke services by making a local procedure call.
- When making a remote procedure call:
- The calling environment is suspended, procedure parameters are transferred across the network to the environment where the procedure is to execute, and the procedure is executed there.
- When the procedure _niches and produces its results, its results are transferred back to the calling environment, where execution resumes as if returning from a regular procedure call. The caller to the callee and then back again.