THE OSI MODEL

This is the first of the 7 series post; each focusing on the different layers of the OSI model in detail. This post is for anyone who wants to get into understanding the foundation of the internet and the networking framework.

Let’s dive in!

 What is a Networking Model?

The more fundamental concept to begin with is to understand what a network is. Networking is the concept of two entities communicating with each other and network is simply the infrastructure or framework facilitating this process. This definition can be extrapolated to different contexts and it generally means the same with a few exceptions. 

The Networking model talks about different techniques and protocols through which this network infrastructure is built. As technology advances by the hour, there are more complex models and of course the fundamental ones upon which all the other ones are built. To set context to this series, we will be talking about one of the most fundamental networking models, the OSI model and briefly touch upon the TCP/IP model as and when needed.

 In general, for entities to communicate what would be needed? For the sake of explaining the concept, let us consider Humans. What do we need for two humans to communicate? First, would be the two humans (Sender and Receiver); Second, the medium of communication that is air (carrier). Even with one less component, the entire discussion becomes redundant. Likewise, for devices or machines to communicate, there needs to be a sender, a receiver and a carrier. These construct the fundamental blocks of a network.

Let’s Dive deeper..

THE OSI Model a brief overview

The most fundamental networking framework is the Open System Interconnection model introduced in the 1970s. It was a famous model designed, to help create a system capable of communication with another system irrespective of their underlying hardware make. The model merely dictated a set of protocols that systems can adhere to, for easy and structured communication. This model was the first of its kind, in the sense; it allowed the individual systems to have their own underlying architecture and still communicate with other units which may or may not be identical to it. The TCP/IP model followed the OSI model and is currently the most used model and the internet we use is predominantly based on this framework. (More on the TCP/IP model later).

Layers of the OSI

Having defined what the OSI model is, it is now imperative to look at, what these rules and protocols are; and what constitutes the model itself. The OSI model is divided into 7 layers each layer having a role in the networking context. Each of the layers act as a service provider to the layer below; and a user of the service, provided by the layer above. This is quite logical considering the hierarchical way it is defined.

The 7 layers of the OSI model is diagrammatically represented below

As mentioned earlier, each layer has a dedicated responsibility and an important role to play in this communication framework. Let’s look at each of these layers in depth and understand their roles

PHYSICAL LAYER

As the name suggests; the physical layer governs the physical transmission and sees the data outside the system into the network. The physical layer is entrusted with a few responsibilities; responsibilities you would expect the physical data transporter to hold.

 Responsibilities of the Physical Layer

1. Determination of the physical characteristics and encoding:

The physical layer determines the kind of encoding that should happen to the stream of bits coming down from the layers above. Thus, the determination of the transmitting medium and creating that perfect interface to convert bits into transmittable data is the sole responsibility of this layer.

2. Data Rate and synchronization:

The rate at which data is pushed into the stream is very critical in the implementation of any communication channel. The data rate is thus decided by the physical layer(rather configured in the layer). The layer is also responsible for the synchronization; it is important to discuss this property; without synchronization the reception of bits may not be in the correct order; ultimately leading to a failed communication.

Note: Synchronization mismatch, could lead to a lot of system level problems, ultimately leading to the application crashing.

3. Line Configuration and the physical topology:

The physical layer device will be configured depending on whether the communication line is exclusive to itself and its receiver or it being a shared resource. Since we are talking about a network, it only seems sensible to take into consideration, multiple systems communicating with each other using the same infrastructure. The physical layer is to be configured based on these configurations.

4. Transmission mode:

The physical layer also determines the mode of communication to be a simplex, half-duplex or a full duplex communication system. Simplex meaning only facilitating one way communication; half-duplex meaning, the channel accommodates communication only from one end at a particular instance of time; full-duplex meaning facilitating simultaneous two way communication.

Having described what the responsibilities of the physical layer are; we need to understand how some of these responsibilities are achieved. Is there a device or a controller that acts as a physical layer in this network topology? The answer is yes!  We will look at the common networking devices that operate at the physical layer.

Networking Devices

The answer to this question is, there are very few dedicated devices that work on the physical layer with a few exceptions. No networking device today is dedicated to just operate in one layer. They would be handling the functionality of two or three layers depending on where they are positioned in the network.

                Having said this, there are few nameable devices that operate dedicatedly at the physical layer.

1. Hubs
2. Repeaters
3. Cables; wires etc.

HUBS

Hubs are one of the standalone dedicated physical layer devices. It is a device used to connect multiple systems in a network. Let us try to map the responsibilities of the physical layer one to one with the functionality of the HUB.

A pictorial representation is as shown below

This simulation was done on Cisco’s Packet Tracer. Do try it, to explore networking concepts in depth.

Repeaters

Repeaters are yet another physical layer device. The device is meant to simply repeat or amplify the signal it receives through its input port. Very often it is known as a Wi-Fi booster; but there are different kind of repeaters based on the input signals it receives (Analog, Digital etc.).

The repeater like a hub is not an intelligent device; it cannot do any sort of intelligent processing. It merely does signal amplifying, even if it means amplifying noise. A hub can act as repeater as well, by relaying messages to every system in the network. They are not very often used interchangeably.

This brings us to the conclusion of this post. Hope this gives you good insight on the physical layer of the OSI model. Now hopefully you can jump into conversation if it’s about hubs or repeaters!

Cables and Wires

As the name suggests, the physical cables and the pins (ex RS232 serial connectors ); all are also considered to be part of the physical layer.

Miscellaneous Content

This section is to briefly cover up on the term PHY (In embedded and networking context) being used in the industry. The word PHY typically refers to the Ethernet physical layer; they are usually controllers that are specifically used to act as Ethernet transceivers. This is manufactured by famous vendors such as microchip, NXP and more.

The role of the device would be almost similar to the content we just discussed, with a few exceptions. I will save this content for another series!

What’s this noise about TCP/IP vs. OSI?

                Quite often there is always a comparison made between both these models. They are compared based on the actual effectiveness in creating the infrastructure and envisaging a more reliable and easy communication model. The major difference between these models would be that, the OSI is a more generic framework while the TCP/IP would be a protocol driven network communication model. The internet is built on this model!

Hope you enjoyed this post; feel free to leave your comments and reach me in case of any queries.

Thanks for reading!

Please Read through my other content as well!

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