The Session layer:

Establishes and maintains connections.
Manages upper-layer errors.
Handles remote procedure calls.
Synchronizes communicating nodes.

Allows applications on connecting systems to establish a session.
Provides synchronization between communicating computers.

The Session layer comes into play primarily at the beginning and end of a transmission. At the beginning of the transmission, it makes known its intent to transmit. At the end of the transmission, it determines if the transmission was successful.

In a simplex transmission,only one device transmits at a time-all other devices receive.
Simplex transmission is like a one-way road. Travel is only allowed in one direction.

Full-duplex transmission uses two communication channels so that devices (usually two) on each end of a transmission can transmit and receive data at the same time.
Full-duplex transmission is like a two-lane road. Travel is permitted in both directions, each direction of travel in a designated lane.

Half-duplex transmission is a compromise between simplex and full-duplex. In half-duplex mode, there is only one channel. However, both devices can transmit and receive by sharing the channel. When one device transmits data, the other device receives. Citizen's band radio and most LAN data transmission use this mode.

Half-duplex transmission is similar to a one-lane bridge on a two-lane road. Travel over the bridge is permitted in both directions, but not at once. You must wait until traffic traveling in the opposite direction clears the bridge before you can pass over the bridge.

Three Steps of Managing the Session

Establishing the connection. - This involves negotiating the communication.
Transferring the data. - This maintains the connection
Releasing the connection. - This ends the session.

For more information on Networking Basics, the OSI Model and Server Administration:
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