I’ve started writing several posts about networking and then I reminded myself that some of you might have questions about several terms I used or things I always mention in my posts, while I haven’t stopped explaining them yet so far.

Since everything is connected, I will try to cover as much as I can from the networking field so you can understand the rest of the articles. The down side in this process is that I can’t guess the stage of knowledge each of you has reached, that’s why some of the things might look too easy and you will have to skip until you find something interesting for you – this is for those of you “experts”. However, for the average people, you could just take a look, even if you already know what you read because you might find something that you didn’t manage to understand before or you didn’t know there is another way too.

For example, I usually like to read about the same subject from more than 1 source, most of the times 3 versions. In this way, you will find out something from each source, and you are even closer to fully understand what you are looking for.

If you would like to learn about something else, give me a sign and I will prioritize it for a future article.

A network can be as a simple as the connection through a single cable from one computer to another or as complex as a network that spans the globe, like the Internet and this is what I will be trying to cover in this Network Presentation article. There are packets traveling each second from source to destination on each network available in the world.

Devices and media are the physical elements or elements of the network. The hardware can be anything from a PC, switch, cabling to the components that might not be so visible like in the case of the wireless media.

Devices use IP addresses and MAC addresses to identify themselves on the network environment but before learning more about them, let’s first see how a network operates, the layers involved and how network protocols interact.

We will be using 2 layered models, the TCP/IP Model and the OSI Model to actually explain the interaction of the protocols and how packets travel from layer to layer when they are processed.

OSI Model

Description

Application

Is responsible for data output to the user plus encoding and dialog control.

Presentation

Provides common grounds for the representation between application layers.

Session

Provides services to the Presentation layer to organize the dialog between parties and to manage the data exchange.

Transport

Facilitates the communication between different devices on different networks.

Network

Routing – determines the best path possible for devices between networks.

Data Link

Protocol that takes the role of a controller for hardware devices and media on the network.

Physical

It’s formed of the electrical, mechanical, hardware, functional environment and it enables, maintains and disables physical connections for bit transmission on the media to the destination device.

The OSI Model is a reference model that has the primary purpose of helping people understand the functions and the processes involved for all network protocols and services. However, this is not intended to be used as a specification for the development of new protocols and models, also it will not provide a level of detail needed for network services.

There is, however, another network model called the TCP/IP Model, this one being a protocol model, meaning it closely represents the structure of a certain protocol suite. The related protocols inside such a model usually represents all the functionality required for networks to successfully communicate .

You can learn how network communication works from the OSI Model, but the actual protocols that really run inside a network and closely matches live environments for today’s networks can be found inside a protocol model like the TCP/IP Model.

TCP/IP Model

Description

Application

Is responsible for data output to the user plus encoding and dialog control.

Transport

Facilitates the communication between different devices on different networks.

Internet

Routing – determines the best path possible for devices between networks.

Network Access

Protocol that takes the role of a controller for hardware devices and media on the network.

As application layer data is passed down from protocol to protocol until it reaches the bottom and last one (meaning the data gets to be transmitted over the media environment) it gets new information added to the original data, like port numbers at Transport layer, IP addresses at network layer and MAC addresses at data link layer. This process is called encapsulation and the original packet gets information about how to reach its destination and what to do when it reaches it (What a packet does when it reaches a destination ? For example, a packet with a transport layer port 80 should be processed by a browser as it has a destination port of 80 – HTTP).

Network prefix
Let’s imagine we have an email ready to be sent. When you send it, it will pass through the following stages before reaching its destination:

1: each network layer will add its own information to the original packet, addressing and adding additional tags on TCP, for example the CRC checksum to verify if the packet has reached the destination without being corrupted on the way.

2: while it travels through each layer, the data packet is called a Packet Data Unit or PDU, for example at the transport layer it will be called Transport layer PDU, at network – Network layer PDU and so on.

3: the last stage means being transmitted on the media. For example on a wired network, it could be electrical signals sent as “1”s and “0”s.

To understand how these models apply to the network I will give you one last example of protocol or application running at each network layer of those presented. We will be doing this on the OSI model, as the TCP/IP model contains basically the same layout.

Application layer – HTTP is one of the protocols that run at this layer and is responsible for the way that the web server and the web client interact. HTTP governs over the content and formatting of the messages (requests/replies) exchanged between the web client and the web server.

Transport layer – TCP (Transmission Control Protocol) manages the conversations between the web client and the web server. TCP divides the messages into smaller chunks, called segments, before being sent to the destination. The TCP is also responsible for controlling the size and the rate at which the messages are exchanged between the client and the server.

Internet layer – IP (Internet Protocol) is responsible for taking the formatted segments from Transport protocol (for example TCP), encapsulating them into packets, assigning the appropriate addresses while also adding the extra tags needed, and selecting the best path to the destination.

Network Access layer – (Ethernet: LLC & MAC)The protocols that run at this layer are in control of two primary functions: the data link management and the physical data transmission over the media. The Data Link MGMT takes the packets from the IP and formats them to be sent across the media, while the protocols that deal with the physical data transmission govern how the signals are sent over the media and how they are interpreted by the receiving clients.

At the OSI Model we also have the Presentation layer and the Session layer that at TCP/IP model they are embedded inside the Application layer with all the functions described below.

Presentation layer deals with the conversion of the files stored at the web server, to be ready for display at the web client, for example in the Web Browser that runs at Application layer (HTTP).

Session layer is most of the times transparent and the protocols that run at this level for the OSI layer initiate and close the connections between web client and web server.

At OSI Model we also have the Physical layer which describes the LLC part of the Network Access layer for TCP/IP Model, while the Data Link (OSI Model) covers the rest of the functions of Network Access layer.

All the layers match, even if the names differ, the functions are mostly the same except the additional information I provided on what is not the same for both of them.

The protocols I used to describe are just a part of the ones possible to interact at the different layers. I did not present every possible protocol, as in some cases there are too many, depending on needs in most of the cases. The purpose of this article is not to cover everything a network could be, but to describe in a few words the operations behind a network and how it should look like so you have a basic knowledge on how a packet travels.

 

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