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06: Session layer - layer 5 of OSI model


Every sort of communication, even human communication, has a channel through which messages are communicated. Communication between network devices has also a channel, or rather a multitude of channels opened between the sender and receiver so that each kind of data is headed towards its proper destination (software program) on the same host. Session layer, the fifth layer of OSI reference model, is the one that is in control of and accountable for opening, managing, and terminating a session between end-user application processes. The following picture may help you visualize the layer 5 sessions. This is only to remove fuzziness from your head concerning sessions in networking; however, sessions between two devices vary and their functions are much more complex than shown in the picture.


So, now you recognize that for two or more devices connected to the internet to communicate and exchange information, there must be a channel or channels through which data to be conveyed. You also know that any given session is regulated by the session layer whose responsibility is to set up, manage, and tear up sessions between end-user application processes. It is noteworthy that the session layer is not whatsoever concerned with data per se, it is rather associated with the exchange of data. its role is merely to provide a way for devices or nodes to communicate.

Authentication & authorization

Session layer, as any OSI model layer, has a myriad of functions and services in the world of networks. Authentication and authorization are services that are provided by the session layer. To begin with, authentication is a process found in network devices. Its job is to make sure that the person who is going to get access or enter, for example, a web server, is the owner of a given account on the server. Simply put, Authentication is the process of verifying who you are. Logging on to a sever with a username and password is authentication. The main purpose of authentication is security. Without authentication, user's data stored on a server is unsafe on the grounds that everyone can get it from the server.

After that, we have the concept 'authorization'. It is different from authentication, for a user is in front of the process of authorization after being authenticated. Authorization is the process of verifying what a user is entitled to do or access on a given server. To put it another way, it is the process of verifying that you have access to something or what you are allowed to do on given device. For instance, we have a server on which thousands of users store their files ( e.g., videos, pictures, audios, texts...). Each user has its username and password in order to successfully pass the process of authentication and enter the server. Imagine if once a user logs on to the server, he/she can navigate all the data stored on the server; things would, then, be in utter chaos. Each one would take the others' files. For this reason, authorization is configured on every server. It is to avoid intrusion on users' personal data. Therefore, Authorization is crucially instrumental in network devices. Take a look at the following picture to have a clear idea about authentication and authorization.


Dialog control

A successful communication is the one that is well organized and sorted out. Such success in communication is usually achieved by furnishing the setting with various communicative components. Regardless of the channel, a third party in a communicative context may effectively lead to a successful communication by regulating things. The role of this third party starts when two persons are communicating, and they fail to convey their messages. consider this rather simplified example, suppose that two participants are communicating (through a session of course), after a while the two participants speak at the same time, they utter words simultaneously. Undoubtedly, should two persons are speaking concurrently, no one would benefit from, get, or grasp the other's messages. A good resolution is to come up with the third person whose role is coordinate, organize, and dictate rules for communication. This signifies that the two participants would agree to a set of guidelines to use during the conversation, like taking turns in speaking to avoid interrupting each other. This third party in computer network is called 'Dialog control' and 'Dialog separation'.

As for dialog control, it determines whose turn it is to transfer data in a session. In a given open session, a device plays dual roles, which is requesting services and replying with services. Dialog control determines which role they are playing at any given moment. Thus, dialog control is critically significant for data transmission. Importantly, it provides three different modes or ways of communication, which are simplex, half-duplex, and full-duplex. 

First, Simplex is a mode of communication which allows data to flow only in one direction. An example of this mode is the radio. when you are listening to a radio program, you cannot interrupt or respond at that moment, absolutely no, because the radio does not communicate back to the station in any way; you can merely listen. Accordingly, this communication mode is referred to as simplex.

Second, Half-duplex is another mode of communication. Half-duplex mode indicates that communication between two end-points can merely occur in one direction at a time. Unlike Simplex mode, in Half-duplex communication, two devices are allowed to send messages, but not at the same time. The sender transmits data to the receiver until the the last bit, then after it finishes, the the other device ( receiver) can initiate its transmission. That is why this communication mode is also referred to as two-way alternate or TWA. One example of this mode is the police radio. Only one police officer can talk at a time. Besides, Coaxial cables are strictly Half-duplex.
Last but not least, Full-duplex is a communication mode provided by dialog control. This mode is the ideal one for communication for the reason that it allows data to flow simultaneously in two directions. By using this mode, the sender and recipient in communication can send or exchange their files, messages, etc. concurrently. An example of Full-duplex communication is chatting using Facebook. When you are chatting with someone on Facebook, both parties (the sender and receiver) can send and receive messages at the same time. This is the reason why this mode is also called two-way simultaneous or TWS. Ethernet technology is an example of Full-duplex mode; with twisted pair cables, one pair of twisted wires can be used for transmitting and the other pair is used for receiving. Ethernet is assigned comprehensive lessons in this course.


Dialog separation

In addition to dialog control, dialog separation plays a prominent part in data transmission. It is concerned with the initiation, termination, and managing of communication. Consider the following picture.

The picture shows that Computer A sends a set of data, for example, four pieces of data, to computer B. The two first data ( 1 and 2) have been successfully received at computer B, but data number 4 and 5 have not. They have not been received because an error occurred during transmission which led to the loss of data. In this case, computer has to resend the lost data as well as the received ones. Imagine if computer A has already sent thousands of data. It would be a daunting task to send it again; it is time consuming. This problem is solved by providing something called 'dialog separation'. by virtue of it, when an error occurs during transmission, the sender does not need to start sending data from the beginning, it rather start from where it breaks. This is achieved through synchronization. If the session is closed by mistake, the dialog separation has the ability to re-establish the connection and start from where it leaves off.



Before expounding what the above picture signifies, it would be of paramount importance to give an example of analogy. Assume that you are reading a book of more than seven hundred pages, then you stopped at page number 400, after that you have not read the book for a long time, but when you decided to resume reading it, you did not remember which page you stopped at; thus you need to start from the beginning. But if after you have finished reading those pages you put a marker on the last page you read, it would be much easier to resume reading the book from where you stopped.This example is very close to the idea of what the picture shows.

During the transmission of data, dialog separation allows a process to add checkpoints to each set of data. To help the process, each piece of data is assigned a sequence number. after applying all that to the transmitted data, even if the connection suddenly closed, the data would not be lost. The transmission would resume from where it stops in case the session is mistakenly closed. In other words, when an error occurs, the receiver can re-synchronize the state of the session to a previous synchronization point.


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06: Session layer - layer 5 of OSI model Reviewed by BOUFTIRA on 7:52:00 AM Rating: 5

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