2.1 The OSI Model: Introduction and Network Support Layers

In the previous tutorial, we summed up the basics of computer networking. Now, we will learn the functioning of networking quite closely, looking into concepts of layered structures and the OSI model. First of all, we need to understand about the layered approach of doing a task.

Layers (or levels) are the various stages of the process through which task is competed. For example, while sending a letter, firstly writer posts it in mailbox, the postman takes it to post office and then the carrier takes the letter. So these are the layers on sender’s part. while the layers on receiver’s end would be in exact opposite sequence. The letter will be taken from carrier to post office. From where, postman would take it to mailbox. From where receiver will get the letter.

The OSI Model

The communication system of the computer and digital world also works in a layered manner. There are several layers through which the message (or data in this case) has to go through providing a systematic and secure way to transmit data. One of such model was introduced in late 1970’s by the International Standard’s Organization (ISO), called the Open Systems Interconnection Model or Simply OSI Model.

The OSI Model consists of 7 layers that work on both sender’s and receiver’s ends in exact opposite sequence. Both end contain their own 7 layers. The sending and receiving of data across the layers is made possible through the presence of interface between the adjacent layers. 2 same layers on different machines also contact through protocols, i.e. peer to peer connection.

7 Layers of OSI Model

The first 3 layers (Physical, data link and network) are the network support layers as they deal with physical aspects of moving a data from one node to another. They address issues like physical addressing, reliability, transport timing etc. These are implemented in both the software and the hardware (except for physical layer, which is mostly hardware). Layer 4 (Transport) is the linking layer. It ensures the proper flow of data between the 2 subgroups (Upper and lower layers). The last 3 layers (Session, Presentation and Application) are the user support layers. They allow inter-functionality among various software systems. These are implemented in the software.

Network Support Layers

  1. Physical Layer

    This layer is responsible for the movements of individual bits from one node to another. It manipulates the functions that involves the transmission of bits over physical medium. The type of encoding and the data rate that will be followed during the transmission is also defined by Physical layer. Physical layer also determines the type of configuration that transmission will follow (i.e duplex  or simplex). It’s mostly installed in hardware form.

  2. Data Link Layer

    The data link layer is responsible for the hop to hop delivery of frames. In fact, this layer is responsible for the process of framing itself, whee it divides the data into various sets (or frames). This layer also controls the flow of the data, error recognition and the access (specially in multi point systems). One the main features of this layer if that it is responsible for the physical addressing. It adds a header to the frame that contains the address of receiver and/or sender.

  3. Network Layer

    The network layer finally ensures the source to destination delivery of data. It helps in logical addressing so that, all the ends points are identified. It also facilitates interconnecting i.e. interconnection between two or more heterogeneous networks. The main task for this layer however is routing. During routing, the layer defines the most optimum path to be traveled by data from source to the destination.

So, in this way the network support layers take care of data transmission and its characteristics. In next tutorial, we will look upon function of other layers and more about models and function of computer networks.

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