Understanding the OSI Model - N10-008 CompTIA Network+ : 1.1

Summary

The script discusses the OSI model, a foundational framework in IT for understanding network communication. It covers the seven layers, from physical to application, and their roles in data transmission. Mnemonics and real-world applications are used to illustrate the model's practicality, emphasizing its importance in IT communication and troubleshooting.

Takeaways

• 🌐 The OSI model stands for Open Systems Interconnection Reference Model and is a guideline to describe how data moves across a network.
• 📚 The OSI model is used as a common language among IT professionals to understand and communicate about network data flow.
• 🔍 Despite the OSI protocols not being widely adopted, the OSI model itself continues to be a reference in IT discussions.
• 🔑 The OSI model consists of seven layers, each with its own set of protocols that may vary depending on the network traffic.
• 🧠 A mnemonic to remember the OSI layers is 'All People Seem To Need Data Processing', corresponding to Application, Presentation, Session, Transport, Network, Data Link, and Physical layers.
• 🌟 The Physical layer (Layer 1) deals with the actual cables and physical connections, focusing on the signaling over network connections.
• 🔗 The Data Link layer (Layer 2) is foundational, using protocols like MAC addresses for communication and is often referred to as the 'switching' layer.
• 🌐 The Network layer (Layer 3) is associated with IP addresses and is crucial for routing decisions, also handling data fragmentation.
• 🚛 The Transport layer (Layer 4) is often called the 'post office' layer, managing data delivery using protocols like TCP and UDP.
• 🗣️ The Session layer (Layer 5) manages the start and stop of communication between endpoints, using control or tunneling protocols.
• 🔒 The Presentation layer (Layer 6) handles data formatting for user understanding, including character encoding and data encryption.
• 🖥️ The Application layer (Layer 7) is what users interact with, such as web browsers, and uses protocols like HTTP, HTTPS, FTP, and DNS.

Q & A

What is the OSI model and why is it important in IT?

The OSI model stands for Open Systems Interconnection Reference Model. It's a guideline that describes how data moves across a network, allowing IT professionals to communicate effectively about network processes and issues.

Why did the OSI protocols not become widely adopted despite the OSI model's popularity?

The OSI protocols themselves did not catch on, likely due to competition with other protocols like TCP/IP, which became the standard for network communication. However, the OSI model's framework remains valuable for understanding network communication layers.

How many layers does the OSI model consist of, and what is the purpose of each layer?

The OSI model consists of 7 layers, each with a specific purpose: the physical layer (Layer 1) for signaling and cabling, the data link layer (Layer 2) for local network communication, the network layer (Layer 3) for routing based on IP addresses, the transport layer (Layer 4) for data delivery, the session layer (Layer 5) for establishing communication sessions, the presentation layer (Layer 6) for data representation, and the application layer (Layer 7) for user interactions with applications.

What is a mnemonic that can help memorize the OSI model layers?

The mnemonic 'All People Seem To Need Data Processing' can be used to remember the OSI model layers, with each word's initial corresponding to the layers from top to bottom: Application, Presentation, Session, Transport, Network, Data Link, and Physical.

What are some common issues that might be investigated at the physical layer of the OSI model?

At the physical layer, issues might include problems with cabling, punch-down connections, and signal transmission. Troubleshooting might involve checking cables, running loopback tests, or swapping adapter cards.

How does the data link layer (Layer 2) contribute to network communication?

The data link layer is foundational for network protocols and uses MAC addresses for local network communication. It's often referred to as the 'switching' layer because switches make forwarding decisions based on MAC addresses.

What is the role of the network layer (Layer 3) in routing and data transmission?

The network layer is responsible for routing based on IP addresses and for fragmenting data into smaller pieces to move between different types of networks, such as between an Ethernet and a WAN network.

What protocols are commonly used at the transport layer (Layer 4), and what are their functions?

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are commonly used at Layer 4. TCP ensures reliable data delivery, while UDP is used for faster, less reliable delivery.

How does the session layer (Layer 5) facilitate communication between network endpoints?

The session layer is responsible for establishing, managing, and terminating communication sessions between network endpoints. It uses control and tunneling protocols to initiate data transfer.

What types of data processing occur at the presentation layer (Layer 6)?

The presentation layer is involved in character encoding, data formatting, and encryption of application data. It ensures that data is presented in a form that can be understood by the application layer.

Can you describe the application layer (Layer 7) and its significance in user interactions?

The application layer is the topmost layer of the OSI model and is responsible for providing the interface between the user's application programs and the network. It includes protocols for email, file transfer, and web services, making it critical for user interactions with applications like web browsers and email clients.

How can the OSI model be practically applied using a packet capture tool like Wireshark?

In a packet capture tool like Wireshark, the OSI model can be applied to analyze network traffic by breaking down each frame into its respective OSI layer components. This allows IT professionals to examine the details of network communication at each layer, from physical signaling to application data.

Outlines

00:00

🌐 Understanding the OSI Model

The OSI model, or Open Systems Interconnection Reference Model, is a foundational framework in IT that describes how data moves across networks. It serves as a guideline and a common language among IT professionals. Despite the OSI protocols not being widely adopted, the model itself continues to be relevant, especially in discussions about TCP/IP. The model consists of seven layers, each with its own set of protocols that handle different aspects of data transmission. The layers, from top to bottom, are the application, presentation, session, transport, network, data link, and physical layers. A mnemonic to remember these layers is 'All People Seem To Need Data Processing.' The physical layer, at the bottom, deals with the actual cables and signals, while the application layer, at the top, is what users interact with directly.

05:02

🔗 Deep Dive into the OSI Model Layers

This paragraph delves deeper into the specifics of each OSI model layer. The physical layer (Layer 1) focuses on the cables and connections that facilitate signal transmission. The data link layer (Layer 2) is foundational, using protocols like MAC addresses for network communication. Layer 3, the network layer, is associated with IP addresses and routing decisions. The transport layer (Layer 4) is where protocols like TCP and UDP ensure data delivery, often dealing with data fragmentation. The session layer (Layer 5) manages the start and end of communication sessions, while the presentation layer (Layer 6) handles data formatting and encryption before it reaches the user. Finally, the application layer (Layer 7) is where users interact with applications like web browsers, FTP, DNS, and more. Practical examples and a packet capture application like Wireshark are used to illustrate how these layers work in real-world scenarios.

10:04

📚 Practical Applications of the OSI Model

This paragraph connects the theoretical OSI model with practical applications, demonstrating how each layer interacts in real-world network communications. For instance, when using Google Mail, the application layer (Layer 7) is where users interact with the interface. The presentation layer (Layer 6) encrypts data using SSL, ensuring secure communication. The session layer (Layer 5) establishes and maintains the connection between the user's device and the server. The transport layer (Layer 4) uses TCP to send data, while the network layer (Layer 3) handles IP addresses and routing. Ethernet frames encapsulate the TCP/IP traffic at Layer 2, and physical signals are transmitted at Layer 1. This detailed breakdown helps IT professionals troubleshoot and understand network applications more effectively.

Keywords

💡OSI Model

The OSI Model, which stands for Open Systems Interconnection Reference Model, is a conceptual framework used to understand how data moves within a network. It is central to the video's theme as it provides a guideline for communication between IT professionals. The model is structured into seven layers, each with specific functions, and is used throughout the script to explain different aspects of network communication.

💡TCP/IP

TCP/IP, or Transmission Control Protocol/Internet Protocol, is a set of communication protocols used on the internet. While the OSI protocols did not become widely adopted, TCP/IP has become the standard. In the video, it is mentioned as the primary protocol suite in use today, contrasting with the OSI model's theoretical framework.

💡Protocol

A protocol in the context of the video refers to a set of rules governing the format and transmission of data. Each layer of the OSI model has its own set of protocols that dictate how data is handled at that layer. The term is integral to understanding the video's discussion on how data is structured and managed as it moves through a network.

💡Mnemonic

The mnemonic 'All People Seem To Need Data Processing' is used in the video to help remember the seven layers of the OSI model. This tool aids in memorization and is a practical example of how educational techniques can be applied to complex technical concepts.

💡Physical Layer

The Physical Layer, or Layer 1, pertains to the physical aspects of a network, including the cables and the signaling mechanisms used to transmit data. The video explains that issues at this layer could involve examining cabling and connectors, which is fundamental to ensuring the network's basic operation.

💡Data Link Layer

The Data Link Layer, or Layer 2, is foundational for protocols that build upon it. It involves MAC addresses and is sometimes referred to as the 'switching' layer due to its role in switch-based forwarding decisions. The script uses this term to illustrate local area network communications.

💡Network Layer

The Network Layer, or Layer 3, is associated with IP addresses and routing. It is where devices make forwarding decisions based on these addresses. The video script mentions this layer in the context of routing and the movement of data between different types of networks.

💡Transport Layer

The Transport Layer, or Layer 4, is often likened to a 'post office' because it manages the delivery of data to appropriate destinations within a system. Protocols such as TCP and UDP operate at this layer, which the video script discusses in the context of data delivery and the handling of large web pages.

💡Session Layer

The Session Layer, or Layer 5, is responsible for establishing, managing, and terminating sessions between networked devices. The video script uses this concept to explain the initiation of communication between devices, such as a browser and a server.

💡Presentation Layer

The Presentation Layer, or Layer 6, is concerned with the presentation of data to the application layer. It deals with character encoding and encryption. The script mentions this layer in the context of viewing information on a screen and the encryption of application data.

💡Application Layer

The Application Layer, or Layer 7, is the topmost layer of the OSI model and is where users directly interact with applications such as web browsers and email clients. The video script describes this layer as the one that allows us to see and interact with the data on our screens, using protocols like HTTP and HTTPS.

Highlights

The OSI model is a guideline for describing how data moves across a network, allowing IT professionals to communicate effectively.

Although the OSI protocols did not become widely used, the OSI model itself is still a valuable reference in IT.

The OSI model consists of 7 layers, each with a specific set of protocols that may vary depending on the network traffic.

A mnemonic, 'All People Seem To Need Data Processing', helps to remember the layers of the OSI model.

Layer 1, the physical layer, is about signaling and the cables used for network connections.

Layer 2, the data link layer, is foundational and involves protocols like MAC addresses used in Ethernet networks.

Layer 3, the network layer, is associated with IP addresses and is often referred to as the 'routing' layer.

Layer 4, the transport layer, is likened to a 'post office' and uses protocols like TCP and UDP for data delivery.

Layer 5, the session layer, manages the start and stop of communication between endpoints.

Layer 6, the presentation layer, is responsible for data encoding and encryption before it reaches the user.

Layer 7, the application layer, is the visible layer where users interact with applications like web browsers.

The OSI model is a common language in IT for describing data flow from the beginning to the end of traffic flow.

Practical examples of the OSI model include working with cables at the physical layer and MAC addresses at the data link layer.

A packet capture application like Wireshark can display network communication broken down by OSI layers.

In Wireshark, the detailed view shows each part of communication corresponding to a specific OSI layer.

The OSI model helps IT professionals understand and troubleshoot network applications by separating communication into well-known layers.

Using Google Mail as an example, the OSI model can explain the layers involved in encrypted communication from the browser to the server.

Transcripts

If you work for any amount of time in information technology,

you'll certainly hear someone refer

to a layer of the OSI model.

That stands for the Open Systems Interconnection Reference

Model.

And it's a way to describe the way

that traffic is moving from one part of the network to another.

As the name implies, the OSI model is a model.

It's a guideline.

It's a very broad way to describe the way that data

moves across the network.

And it allows us to communicate with other IT professionals

in a way that we both can understand exactly what we're

talking about.

If

You do any type of research into the OSI model,

you may see references to the OSI protocol suite.

The OSI protocols, themselves, didn't catch on.

And in fact, today, we use primarily TCP/IP.

But the OSI model, which was built around those OSI

protocols, continues to be used as a reference when we

talk to other IT professionals.

The OSI model consists of the 7 layers.

You can see them represented, here,

on the left side of the screen.

And at each layer of the model is a set of protocols.

And these protocols may be very different

depending on the type of traffic going over your network.

You'll use this OSI model for the rest of your career.

It's a common language that we can use within IT

to help describe how data is progressing

from the very beginning of the traffic flow to the very end.

One common way to memorize these different layers of the OSI

model is to start at the top with the application layer

through the presentation, session, transport, network,

data link, and physical layers.

One common way to remember the different layers of the OSI

model is to remember the mnemonic, All People

Seem To Need Data Processing.

And those initials, for each one of those words,

correspond to application, presentation, session,

transport, network, data link, and physical.

Let's start our talk about the OSI model at the very bottom.

This is Layer 1, or the physical layer, of the OSI model.

And although we're talking about the physical part

of the network, a lot of what happens at this Layer 1

is really about the signaling, or the signal,

that's going over the network connections.

For layer 1, we're talking about the cables that you're using,

the fiber runs, or just the ability

to get signal from one part of the network to the other.

If someone tells us that we're having a physical layer

problem, then we'll want to look at the cabling

that we're using.

Make sure that all of our punch-down connections

and connectors are working the way they should.

We may want to run loopback tests.

We may want to check or replace the cables that we're using.

Or perhaps even swap the adapter cards

to make sure that we can resolve these layer 1 problems.

The next layer of the OSI model is Layer 2, or the data link

layer.

This is the layer that is a foundational layer

for the protocols that we will then begin

to stack on top of Layer 2.

You'll find that Layer 2 is sometimes

referred to as the Data Link Control, or DLC layer.

And there will be a series of protocols

that run as DLC protocols.

A good example of this is on Ethernet networks,

where we use the media access control addresses, or MAC

addresses.

We sometimes refer to those as Layer 2 addresses

because they correlate back to the data

link control protocols.

And since switches make their forwarding decisions

based on these MAC addresses, we sometimes will refer to Layer 2

as the "switching" layer.

This image describes communication at Layer 2.

You can see the MAC address of this network interface card.

And the MAC address of this network interface card.

And any communication between those 2,

that is using that MAC address, we

can refer to as a Layer 2 communication.

If we move up 1 layer in the OSI model,

we're at Layer 3, or the network layer.

Sometimes you'll hear this referred to as the "routing"

layer because this is the layer associated with IP addresses.

IP addresses are, obviously, very common on our networks.

And any device that is making forwarding decisions,

based on these IP addresses, is communicating at Layer 3.

This is also the layer where frames

will be fragmented or broken into smaller pieces,

in order to move those between different types of networks.

So if you move between an Ethernet network,

off to a WAN network, and then back to Ethernet,

you may find that the data is being fragmented.

And that fragmentation is occurring at Layer 3.

Layer 4 of the OSI model is the transport layer.

Sometimes, you'll hear this referred to as the "post

office" layer because this is the layer that

describes how data is being delivered

and where it is being delivered into a system.

The protocol is used at Layer 4 are protocols such as TCP,

that is the Transmission Control Protocol, and UDP, which

is the User Datagram Protocol.

This layer is used when you're accessing a web page,

but the web page itself is so large

that you can't send all of the data

across the network in one single frame.

Instead, you have to split it up into separate frames

and send those individual pieces across the network, where

they're put back together on the other side.

Layer 5 of the OSI model, or the session layer,

is designed to start and stop communication between one

endpoint and another.

This is where you would use control protocols,

or tunneling protocols, in order to begin

the communication of data between one device and another.

For example, 1 device may ask, in a browser, can we talk?

And it's communicating that information

to Google, which will then set up a session

and begin transferring information.

OSI layer 6 is the presentation layer.

And just before we're able to view information on our screen,

the presentation layer needs to take this data

and put it into a form that we can understand.

Anything dealing with the encoding of characters,

or encryption of application data, will occur at Layer 6.

This layer is often combined with the application layer

at Layer 7 because the functionality is so closely

associated with our ability to use these applications.

Layer 7 of the OSI model is the application layer.

And this is the layer that we get to see.

As humans using a computer, we go to a browser screen

and start a browser session.

And this information that we're seeing on the screen

is the Layer 7 application data.

If you're transferring a file with FTP,

or performing a name resolution with DNS,

or simply using a browser with HTTP or HTTPS,

you're using an application layer protocol.

Let's now take these very broad concepts with the OSI model,

and try to associate them back to

practical, real-world examples.

Let's look first at Layer 1, or the physical layer.

If you're holding a cable, or you're

looking at an analysis of signal going across the network,

then you're working at the physical layer of the OSI

model.

If you're working with switch forwarding,

or you're examining the MAC address of a device,

then you're working at Layer 2, or the data link layer.

Moving up to layer 3, or the network layer,

we're now working with IP addresses.

So if you're working with routers

that make forwarding decisions based on IP address,

then you're operating at Layer 3.

Layer 4, or the transport layer, deals

with UDP and TCP protocols and the port numbers associated

with TCP and UDP.

Layer 5, or the session layer, is our control layer.

Where tunneling information, or setting up communication,

between one device and another.

Layer 6 of the OSI model, or the presentation layer,

is commonly associated with encryption.

If we're communicating to a web browser over an HTTPS,

or encrypted communication, then we're working at OSI Layer 6.

And lastly, the web browser that we're viewing

and the information that's on our screen

is able to be shown to us, thanks

to OSI Layer 7, or the application layer.

Another practical view of the OSI model

can be seen in a packet capture application.

In this case, I'm using Wireshark to capture

data going across the network.

And there are 3 windows that you can see in this Wireshark

communication.

The view at the top is the summary view.

Each line that's within that summary view

is showing us 1 frame that's going across the network.

At the bottom of the screen is a Hexadecimal and ASCII

representation of that data.

But where we want to focus is the middle window,

which is the detailed window.

In this detailed window, we can see

each part of this communication broken out

into the separate OSI layers for a single frame.

For example, we'll look at this frame number

88 that's currently selected.

Frame 88 shows us that it's using 2,005 bytes on the wire.

And that is describing the traffic

that is being received at Layer 1, or the physical layer.

Just underneath that is a line of information showing

this as an Ethernet II frame.

It shows a source MAC address and a destination MAC address,

which means that it's referring to layer 2,

or the data link layer.

To see layer 3 information, we go

to the next line, which is showing us

internet protocol, or IP.

And we know if there are IP addresses, such as the ones

listed here, then we must be working at Layer

3, or the network layer.

The next line underneath the layer 3 information

is the layer 4 information.

In this case, it's the TCP protocol using a port number

of 18-4-29 as the source.

And the destination port is the HTTPS protocol using port 4-43.

All of those TCP communications are

occurring at Layer 4, or the transport layer.

Not uncommonly, you'll often see layers 5, 6, and 7

grouped together.

And in fact, in Wireshark, we see it grouped together

here as well.

You can see there is the Secure Sockets

layer, which is the final line in this detailed view.

And that particular Secure Sockets

layer, details the session, presentation,

and application layer traffic traversing the network.

Each time you use an application,

or send traffic over the network,

you can describe this communication

in the context of the OSI model.

Let's take for example an application like Google Mail.

We'll log into Google Mail at mail.google.com.

And as we are using that application on our screen,

we're using OSI Layer 7, or the application layer.

If we're sending information back and forth to our browser,

everything being communicated to this mail server at Google

is being encrypted.

And it's being encrypted thanks to the presentation layer

at Layer 6, which is using SSL encryption.

Layer 5 is the session layer, which is linking together

everything above at the presentation and application

layer, to all of the traffic below that

will be traversing the network.

And just below that layer 5 is the layer

for transport information.

We know that this communication is probably

going to use TCP 4-43, which means

that we'll be using the transport

layer to send that TCP data.

There's, of course, IP addresses that

are used between your workstation and the Google

email server.

And all of that communication is occurring

thanks to the IP encapsulation associated with Layer 3,

or the network layer.

All of that TCP and IP traffic is being encapsulated

within Ethernet frames.

And those Ethernet frames are being sent over the network

thanks to Layer 2, or the data link layer.

And lastly, the physical signals to be

able to send these frames from one device to another

are occurring at Layer 1, or the physical layer.

By separating the network communication

into this well-known series of layers,

you're able to communicate with other IT professionals

when you're troubleshooting, examining applications,

or trying to get an understanding of how

applications may be working over your network.