Session Layer PDF

This is an overview of the session layer pdf, if you want to read full article in best quality in pdf, we have provided download link below.

Session layer is the fifth layer in the OSI (Open Systems Interconnection) Model. This layer is responsible for managing the connection between two machines during the course of communication between them. It is at this layer where it is determined whether all the information for the session has been received or not. And thus, it can decide whether to stop the reception or transmission of data packets. Since we are dealing with data packets, this layer also has built-in error correction and recovery methods.

The Session layer communicates between service requestors and providers. There are certain mechanisms that help control communication sessions. These mechanisms help in establishing, maintaining synchronization, and managing dialogue between the communicating entities.

Within the layering arrangements of the OSI Network architecture, the session layer responds to service requests from the presentation layer and issues service requests to the transport layer.

Thus, the main aim of the Session layer is to manage and synchronizing conversation between two different applications and avoid any premature loss of data stream.

This is an overview of the session layer pdf, if you want to read full article in best quality in pdf, we have provided download link below.

Purpose of the Session Layer:

The Session layer facilitates communications between service requestors, and providers, as already mentioned. Often, this layer helps the upper layers identify and connect to the services that are available on the network.

The session layer could be compared to telephone operators and telephone directory services.  For example,  suppose that you wanted to call a  hotel for room reservations but you do not know the Hotel’s telephone number; you could call a  telephone operator who provides directory services.  That operator uses information provided by other telephone company employees to find the number of the Hotel. Similarly, the Session layer uses logical address information provided by lower layers to identify the server names and addresses that upper layers need.

After you have been told the telephone number, you could ask the telephone operator to place the call for you. The session layer also places the “calls” and initiates conversations (between service providers and requestors).

Various Functions of the Session Layer:

  • Session establishment, maintenance, and termination: The layer allows the two communicating sides to establish, use and terminate a connection.
  • Synchronization: Information transactions at the sender and receiver should be happening at the same time, this is called synchronization. To achieve this, checkpoints are added by this layer. Such checkpoints are called as synchronization points in the data. This helps in error identification, re-synchronizing data properly, and avoiding premature cutting of the ends of messages thereby avoiding data loss.
  • Dialog Controller: With the use of various modes of communication, such as half-duplex or full-duplex, this Layer allows entities to start communication.

The session layer has these works: Dialog control, Session Administration.

Dialog control methods are Simplex, Full Duplex, Half Duplex. Of these, Half Duplex and Full Duplex are used in the Session Layer.

In Simplex dialog permits communication on the transmission channel to occur in one direction only. Only one device transmits at a time and another device(s) receives. Full channel bandwidth is available. In simplex channels transmitting device(s) cannot receive information while receiving device(s) cannot transmit information. This provides a broad coverage area and a large target audience. For instance, commercial television broadcasts use this mode.

This is an overview of the session layer pdf, if you want to read full article in best quality in pdf, we have provided download link below.

In Half Duplex dialog method, only one device transmits at a time and each device can both transmit and receive. Hence, here also the channel’s full bandwidth is available and bi-directional communication can be established. Disadvantages of it are: only one unit transmits at a time; channel not being effectively used while waiting for direction change, and it uses relatively more expensive hardware than simplex. For instance, Police wireless communication devices use this mode.

In the Full Duplex dialog method, every device can both transmit and receive at the same time or simultaneously. Thus, to achieve this, every device ought to have two physical or logical transmission channels, i.e. on for receiving and others for transmitting. Certain disadvantages to it are: hardware concerned becomes more expensive than simplex and half-duplex. Requires more transmission media, and can connect to a limited or exclusive target audience at a time. For instance, modern telephone systems provide Full-Duplex Channels.

This is an overview of the session layer pdf, if you want to read full article in best quality in pdf, we have provided download link below.

Session Administration methods are Connection establishment, Data Transfer, Connection Release.

In Connection Establishment, subtasks are performed so as the entities recognize and communicate with each other. This includes: verification of username and passwords; establishing connection identification numbers; agreeing on serves required for what durations; coordinating acknowledgment numbering and retransmission procedures.

In Data Transfer, maintaining connection and passing of messages between the entities is ensured. Subtasks performed are: actual transfer of data; acknowledgment of receipt of data, and this includes negative acknowledgment when the data is not received; resumption of interrupted communications.

In Connection Release, the task of ending a communication session is carried out. This can be done by wilful acknowledgment of the communication entities, or due to the loss of a connection caused probably by the accidental breakup of communication among other causes. The Communicating entities recognize the loss of connection when they don’t get expected acknowledgment or negative acknowledgment.

This is an overview of the session layer pdf, if you want to read full article in best quality in pdf, we have provided download link below.

The fifth layer of the Open Systems Interconnection (OSI) model is named Session Layer. It controls the connection and interaction between computers. What makes it a vital part of the OSI Model is that it is responsible for two important functions in the computers; authorization and authentication of a connection between two end-user applications.

It establishes the communication between systems and servers through the three communication channels: Simplex, Half Duplex, and Full Duplex. It communicates with the layers above and below it; the transport layer and the presentation layer.

The Session Layer is one of the three ‘upper layers’, as they are called, and with the two other upper layers, it is responsible for the communication of the applications between hosts.

The reason for categorizing these three layers separately from the other four layers is that their role in the model is to build communication channels and they have no knowledge of the process of networking and network addresses.

The most known protocols that are used in Session Layer are Domain Name Server (DNS) Protocol, Lightweight Directory Access Protocol (LDAP) And Network Basic Input/Output System (NetBIOS), Remote Procedure Call Control (RPC), Point to Point Tunneling Protocol (PPTP), Session Control Protocol (SCP), Session Description Protocol (SDP).

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Services provided by the Session Layer:

Management – So many layers and with so many applications and protocols can be confusing especially for the system itself. Session Layer manages the order of the operation in case more than one party is attempting a critical operation at the same time. Opening and closing of a connection or session between end-user applications is also the role of this layer.

Authentication – After a request for a connection is made and the address of the sender has been recognized, it is important to identify the source of the request and find out if it is a legitimate one. In simple words, this is a process of verification of the endpoint from where the request was made.

This is an overview of the session layer pdf, if you want to read full article in best quality in pdf, we have provided download link below.

Authorization – After authentication comes to the authorization, after the verification of your identity it is important that you receive the permit to establish a session and this is the simple process of that which most probably is the simplest function of the entire model.

The failed attempts of communication by the upper layers are fixed by this layer by re-establishing the connection.

To understand the session layer, you need to imagine a live stream of a sports program where video and audio come from two different sources but are synchronized to give viewers a great experience.


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