Notes on Session Layer

In some devices and in some application programs or processes, the first four layers are the physical data, linked, network and transport. These four layers are not sufficient to meet the requirements of specific tasks or specific application programs. For them, some higher layers and some higher level of processing is required. The data has to be more filtered, more polished and more processed, and for that, we require more layers. 

The Session Layer

The session layer is the fifth layer of the OSI model, and it is called the network dialogue controller. It is called a Network dialogue controller because when two devices or two systems or two nodes communicate with each other and form a connection between the computers. For the exchange of data or information, they are undergoing or engaging in a dialogue. 

So that dialogue that indulges in the process of exchange of data is controlled by the session. It establishes, maintains, synchronises and terminates the exchange of data between the two systems that are involved in the communication process. The session layer has two main tasks: the first is dialogue control, second is synchronisation.

Aspects Of Session Layer

The two main aspects that are controlled by the session layer are dialogue control and synchronisation. In dialogue control, what happens is that the session layer it enables gives permission or allows two systems to form a communication link to undergo a dialogue or to exchange data with each other. 

To exchange data among themselves allows them to enter into a dialogue for the exchange of data or information to form a communication channel, so the session layer allows two systems to communicate with each other and form connections between two computers. How they will communicate and for connection between computers will be determined by the mode of communication, which can be either half-duplex or full-duplex mode.

Now the modes of communication, there are three types of modes of communication: simplex mode, half-duplex mode and full-duplex mode. In simplex mode, the communication between the computers is unidirectional in nature. That is data can one device can only transmit data, and the other device can only receive data. 

Next is the half-duplex mode; now duplex has a mode both the devices involved can transmit and receive, but they have to do so at separate time slots, so this is bi-directional in nature. Of course, because both devices can transmit and can send data as well as receive data, they cannot do so at the same time. When one device is transmitting, otherwise the device has to receive, and when the other device is transmitting before, it has to receive.

The session layer is the conversation that has to be kept separate from others to prevent the intermingling of data. So we set up these sessions so that when I’m talking to YouTube, for instance, it doesn’t get mixed in with my talking to Facebook, and so I keep those sessions separate even though they’re all coming from the same computer or the same network as the session layer. 

We’re concerned with three major things: we are setting up our sessions, we are maintaining our sessions, and we’re tearing down our sessions. So setting up – a session at this point, we’re checking our user’s credentials, we’re assigning numbers to the session to identify. We negotiate services between the sessions, and we negotiate who’s going to be sending the data first. Am I going to send it, or as the server is going to send it first, and it’s going to be decided as we set up this session. 

Maintaining a session is fairly quick as well. What we’re going to do here is we’re going to transfer our data, so the session is up. Now we’ve made the connection with Facebook, so now Facebook’s going to send me my newsfeed. If I get a broken connection, they’re going to re-establish a disconnected session, and we’re going to have that acknowledgement and authorisation of the data that’s being sent and received so that way we have another acknowledgement layer as well.

Then we’re going to tear down our session at the end, and this is going to be done due to either mutual agreement or if the other person disconnected. So due to a mutual agreement, this would lead to termination, and the connection between the computers be terminated.

So basically, we have Establishment, management and termination.

Conclusion

If you are interested in the fourth layer, then you should look forward to the remaining layers as well. They would give you the proper knowledge of the subject.