Layer 5: The Session Layer

The Session layer of the OSI model is responsible for managing communications between a sender and a receiver. Some of the communication tasks performed at this layer include:

• Establishing connections

• Maintaining connections

• Synchronizing communication

• Controlling dialog

• Terminating connections

  

Creating a Connection

Creating a connection requires:

• Authenticating through a username and password that are valid on the receiving computer

• Determining which computer will be the sender, which will be the receiver, and which will send first

• Figuring out which type of communication will take place

• Specifying which lower-layer protocols will be used for transmission

Transferring Data and Using Dialog Control

dialog control

Manages the dialog between the sender and receiver. It consists of managing the transfer of data, determining whether an acknowledgment is required, and determining the appropriate responses to the sender.

Data transfer and dialog control consist of:

• The actual transfer of data

• Any acknowledgments that are needed

• Responses to requests that the sender or the receiver makes

There are three types of dialog control at the Session layer. Dialog control services include:

Simplex communication  Specifies that communication is one-way only. Simplex is more common in less complex systems for which two-way communication is not necessary.

Half-duplex communication  Specifies that communication can be two-way but only one channel can communicate at a time. Most computer networks communicate in half-duplex mode.

Full-duplex communication  Specifies that communication can be two-way simultaneously. This is a valuable function found in some recent network devices. Full-duplex should be implemented with servers for improved performance.

Terminating a Connection

After the session is complete, the connection is terminated. This enables other devices to open new sessions. Sometimes termination of the connection will come from the receiver. Other times the sender will terminate the session if it does not hear back from the receiver within a specified time.