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OSI Session Layer Concepts
The next OSI layer, the Session layer, manages dialogs between two computers by establishing, managing, and terminating communications. As illustrated in Figure 2.7, dialogs can take three forms:
- Simplex dialogs. These dialogs are responsible for only one-way data transfers. An example is a fire alarm, which sends an alarm message to the fire station but cannot (and does not need to) receive messages from the fire station.
- Half-duplex dialogs. These dialogs handle two-way data transfers in which the data flows in only one direction at a time. When one device completes a transmission, this device must “turn over” the medium to the other device so that this second device has a turn to transmit.
- CB radio operators, for example, converse on the same communication channel. When one operator is finished transmitting, he must release his transmit key so that the other operator can send a response.
- Full-duplex dialogs. This third type of dialog permits two-way simultaneous data transfers by providing each device with a separate communication channel. Voice telephones are full-duplex devices, and either party to a conversation can talk at any time. Most computer modems can operate in full-duplex mode.
Costs rise for half and full-duplex operation because the more complex dialog technologies are naturally more expensive. Designers of communications systems, therefore, generally use the simplest dialog mode that satisfies the communication requirements.
Half-duplex communication can result in wasted bandwidth during the intervals when communication is turned around. On the other hand, using full-duplex communication generally requires a greater bandwidth than half-duplex communication.
The Session layer also marks the data stream with checkpoints and monitors the receipt of those checkpoints. In the event of a failure, the sending PC can retransmit starting with the data sent after the last checkpoint, rather than resending the whole message.
Further Information