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OSI Networking Model Layer 5


5 - SESSION - Interhost communication (syncs and sessions)


Function: Establishes and maintains communications channels, so program on different devices can establish a link. Often combined with the Transport Layer and may be different for each Industrial Ethernet Protocol.


Data unit: Packets.


Protocols (examples): RPC, SQL, NetBIOS, Appletalk ASP, DECnet SCP, Names Pipes, Mail Slots.


Components: Gateway.


Topics & Methods Explained

Dialog control

Communications on the transmission channel allowed to occur in only one direction. Only one device is allowed to transmit; others only receive. The channel's full bandwidth is always available.


Allows every device to both transmit and receive simultaneously. Every device (normally only two) are required to have two physical or logical transmission channels.


Each device can both transmit and receive, but only one device can transmit at a time. The channel's full bandwidth is available to the transmitting device which can't receive while transmitting.

Session administration

Connection establishment:
Entities recognize each other and agree to communicate through: Verifying user login names and passwords. Establishing connection ID numbers. Agreeing about required services and duration. Etc.


Data transfer:
Maintain the connection or communication and pass messages between two entities through: Actual data transfer. Acknowledgment of data receipt. Resumption of interrupted communications.


Connection release:
Ends a communication session. It can be done by agreement between the two entities, or by an obvious loss of connection (when an expected ACK or NAK is not received).



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