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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

Simplex:
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.

 

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

 

Half-duplex:
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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