Hosts ~ above a network require to connect with every other. This is no as straightforward as that sounds. That requires various parts that hardware and also software job-related together. They should speak the exact same ‘language’.

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The various languages the network master speak are called protocols. Protocols room a defined set of communication rules, that are constructed into network software and also hardware.

An example of network hardware is the Network interface Card or NIC. An example of network software application is the NIC driver.



A single host will certainly use countless different protocols to achieve different tasks. Usually, number of protocols and also systems are offered together. An example of this is once you send one email. The email client and server interact with a protocol dubbed SMTP. However SMTP only explains how letter works. They also usage protocols like TCP to make sure that the blog post arrives, IP addresses to understand where to send the message, and Ethernet to transform the article to electric signals.

It deserve to be tricky come understand exactly how all these parts fit together. To aid with that, we deserve to use the OSI model. This is a theoretical model that breaks the network components into layers. Girlfriend may have actually heard that terms choose ‘layer-3 switch’. The ‘layer 3’ component is referring to the layer in the OSI model.

Each layer represents component of the network ~ above a host. The stack starts at the bottom with the physics layer and goes best up to the applications layer. Each of the seven layers communicates through the layer above and the great below.

The advantage of this is that an application, up in layer 7, does not require to know anything around how the physics network. The application simply does that is job and passes information to the layer below. The same is true in reverse. A NIC will get bits that information, yet it doesn’t care at all about the application that uses the data. The just focuses on receiving the bits and passing them increase the stack.

As us look deeper, friend will see that we don’t usage the OSI model to describe specific protocols. That a an approach of showing just how information in the network is handled, independant that the certain protocols in use.

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How the Model Works

The OSI model consists of seven layers. Here is a rapid overview from the peak down.

(7) Application – The components of the network stack the communicates with the application. This includes network APIs

(6) Presentation – This layer equates the data between the application and also the network

(5) Session – Builds and maintains a session in between application processes

(4) Transport – bring away a present of data, and breaks it right into segments. This permits multiplexing top top the network (allowing an ext than one traffic circulation at once)

(3) Network – provides addressing and also routing services

(2) Data Link – Handles shipment of data in between two nodes associated to a physics medium

(1) Physical – Transmits and also receives data throughout a physical medium. This may be electrical, light, or radio signals

That’s a lot to remember! If it helps, try to use a mnemonic to remember the layers. This is one the you deserve to use:


Please do Not litter Sausage Pizza Away

It’s a bit silly, yet that’s what renders it memorable.


You might an alert in the diagram on the right, the the layers space grouped into Lower Layers and Upper Layers. The lower layers (layers 1-4) space where most of the network materials live, and also where network engineers traditionally spend most of your time. The top layers (layers 5-6) are greatly application related, and where developers and also application professionals traditionally spend an ext time.


Data Flow

When an application needs to access the network, the starts in ~ layer 7. Data is pass down with the layers towards layer 1, with each great performing important functions. Once data is obtained by a host, that is obtained at layer-1 and works its means back as much as layer 7, wherein the application is waiting.

When details is moving down the layers, some layers will encapsulate the data. Encapsulation is the process of adding much more and an ext information. An example of this is at layer 3, where a destination IP address is commonly added. Adding extra details like this aids in the delivery and also handling that the original data.

Picture a item of data as a block the ones and also zeroes. During encapsulation, more data may be included to the prior and/or the back of this block. Data added to the prior is dubbed a header, if data included to the end is dubbed a trailer.

When information is got by a host, the process is reversed. Headers and trailers are gotten rid of as the data moves back up the layers until ultimately the original details is left because that the application to use.

You deserve to see this procedure in the diagram below:



There room a couple of interesting things we have the right to see here. First, we see that each layer communicates through the layer above and the layer below. Lock don’t communicate with any type of other layer.

Second, a layer on one host will logically communicate through the very same layer top top the second host. The application layer, for example, communicates v the application layer on the far host. It doesn’t know any of the details that the other layers handle. This have the right to be a little complicated to know right now, yet it will certainly make feeling over time.


Breaking it Down

Key point: Data is broken into chunks, and passes with the layers 

When an application demands to accessibility the network, it’s no overly concerned with how the data is formatted. Every it is worried around is even if it is the application on the receiving host deserve to understand it.

But this would not work-related well because that a network. A network demands data to be broken into manageable chunks. There room a couple of reasons for this. Imagine that you’re delivering a huge amount of data, and there’s a problem. If some of the data doesn’t do it, you need to start again. However, if the data is damaged into controllable chunks, just the chunk that went bad needs to it is in resent.

On height of this, remember that there are many applications which can want network access. If the network is bound up through one big transfer, every little thing else will need to wait. But, with manageable chunks the data, applications can take transforms accessing the network. An ext important data can even be prioritised end less necessary data.

Generally, each chunk of data is called a Protocol Data Unit (PDU). Each layer has tendency to have actually its own name because that the data it functions with:

Layer 1 has actually bitsLayer 2 works through framesLayer 3 has packetsLayer 4 will use Segments or Datagrams, depending on how the data is used

We’ll now start to acquire a deeper understanding of what every layer does.




Layer 7 – Application

An interface between the application and also the network

The Presentation layer, AKA the ‘syntax’ or ‘translation’ layer, is used to transform data between the application and also the network. Some instances of as soon as this would certainly be used incorporate encryption, compression, graphics handling, and character conversion.

In a nutshell, it styles the data right into something that the rest of the class can quickly use. This makes it feasible for the data to be handled correctly, even if the basic protocols change.

The sending out host styles the data therefore it deserve to be understood on the network. The receiving organize converts the data ago to a style that the application have the right to use.


Layer 5 – Session

Here’s where the fun begins! We’re outside of the upper layers, and also into the actual networking! The transport layer is used to transport data throughout the network without errors.

Part that its task is come segment the traffic right into smaller blocks. This is completely transparent come the upper layers. When all the block are obtained at the other end, the recipient deliver layer reassembles the blocks right into the original data. This renders the flow of traffic appear to it is in a currently of data.

The most usual protocols used at class 4 room TCP and UDP. They’re substantial topics on their own, so we won’t discuss them in depth here. Both TCP and also UDP use port numbers to identify which traffic flow each block of data belongs to. TCP and also UDP both add resource and location port numbers, in addition to other information, to the data segment in the form of a header.

Using port numbers makes it straightforward to track each flow, which subsequently makes it straightforward to handle various flows in various ways. Because that example, if one circulation is overwhelming the recipient, it have the right to ask the sender come ‘back off’ top top this flow. Network devices can additionally prioritise one flow over another, or block a certain flow entirely.

Another job of this class is multiplexing. Multiplexing method that we have the right to have much more than one circulation on the network in ~ once. It is a major advantage of breaking up the data into manageable blocks. Each circulation can take turns sending out their block of data. This means, because that example, that you can receive mail while you are looking the web.


Layer 3 – Network

The network layer is among two feasible places that we usage addressing. Most commonly, this is wherein we use IP addresses. It’s no surprising then, that this is likewise the layer where routing occurs.

At layer-3, a item of information is dubbed a packet. Routing occurs once a packet needs to relocate from one network come another. The yes, really routing (or web traffic forwarding) in between networks is excellent by a router.

To achieve its purpose, among the points that the network layer walk is add another header. This header includes the resource and location IP addresses, together with other information around the packet. The headers included by layers 3 and also 4 are not usually adjusted as the packet moves about the network. There are technologies favor NAT that space an exemption to this, but we won’t acquire into lock here.

As we’ll watch soon, layer-2 have the right to only take care of a block that data up to a preferably size. So, component of the network layer’s project is to make certain that the packet is not as well big. If a packet is too big, it will certainly be broken into smaller chunks, dubbed fragments. They are reassembled when they reach the location host. Every fragment has actually its own set of headers.

Layer-3 is not forced to be reliable. This means that it does not have to implement any type of error checking favor the transfer layer does. This is due to the fact that other layers currently do error checking.


Layer 2 – Data Link

Data demands to it is in transferred between nodes on the very same network segment. This might be a situation where 2 hosts room on the same segment, or a hold is sending out data to a router. This is what the Data link layer is every about.

But gadgets don’t simply throw a handful of bits at each other. Lock a little bit smarter than that. Instead, layer-2 develops a logical link in between the endpoints. This makes it possible to have actually hosts linked to a switch, fairly than directly linked to each other.


Just choose layer-3, the data connect layer offers addressing. One of the layer-2 address species that you will come throughout regularly is the MAC address. This comes from the address that is stamped onto her network card. As soon as layer-3 overcome a packet come layer-2, an additional header is added, which consists of these addresses. This is no longer dubbed a packet; that is now dubbed a frame.

The network is usually made up of smaller networks that are joined together. For example, friend may have a key office with a network, a branch office through a network, and also a WAN network connecting castle together. These networks may likewise use different connection types, such together Ethernet, WiFi, and also PPP. That is among the factors that we need the data attach layer.

It will develop a channel of communication across one part of the network, for example from a organize to the router. Then a different layer-2 connection will be provided to acquire from the head office router to the branch office router, i beg your pardon uses various cabling types. Another connection is then used to obtain from the router come the location host. This is quite various to layers 3 and also 4, which only thinks around the ultimate source and destination hosts.


The data attach layer is damaged into two sub-layers. The Logical connect Control (LLC) layer is provided to obtain data native layer 3 and also hand it come the Medium access Control (MAC) sublayer. LLC’s main job is to translate the class 3 protocol (most typically IP, yet that’s not always the case) into something the the MAC sublayer can work with.

The MAC layer to add the headers and also trailers come the packet to develop the frame. It will certainly also include information that have the right to be offered for error convey an handling.



Layer 1 – Physical

Defines the physical specifications the the network

The major job the the physical layer is obvious. It demands to ‘get the data top top the wire’ (or fibre or WiFi together the situation may be). It encounters the life stream the ones and zeros.

So, what this method is that the physics layer is interested in physical details. These include radio frequencies, pins, voltages, and also how the bits are put on the physical medium. This method that it demands to work out how to encode the information. For example, which electric or radio states stand for a one or a zero? Is the signal digital or analogue? how do we understand when a framework starts and ends?

There are a few other devices beyond network cards, cables, and radio frequencies that fit in here too. These are ‘dumb’ devices which need to not readjust the data. These encompass hubs, repeaters, and also media converters.


TLDR – The Important Bits

These work the OSI design is just a model. It’s used for training and also understanding just how parts the the network to the right together. It’s consisted of of 7 layers, extending every network role from physical to the application.

When data is sent out from or come at a host, it functions its way through these layers. Each layer has actually their own job come do, i m sorry may incorporate addressing, error handling, and reformatting the data.

A huge stream of data is damaged into controlled blocks the data, which can each be tackled individually. In a process called encapsulation, these blocks have extra information included to them, in the form of headers and trailers.

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Each great communicates directly with the layer above and the class below, as well as logically connecting with the corresponding layer top top the remote host.