Understanding the OSI Model - CompTIA Network+ N10-009 - 1.1

Summary

The video introduces the OSI (Open Systems Interconnection) model, a conceptual framework for understanding how data travels across networks. It outlines the seven OSI layers, from the physical layer (Layer 1) to the application layer (Layer 7), explaining their roles in network communication. Each layer has specific functions, such as data transmission, routing, and application interaction. The video also emphasizes the importance of the OSI model for IT professionals to communicate effectively and troubleshoot network issues by referencing specific layers.

Takeaways

• 📚 The OSI (Open Systems Interconnection) model is a conceptual framework used to describe how data moves through a network, not a protocol suite itself.
• 🌐 The OSI model has seven layers: Application (7), Presentation (6), Session (5), Transport (4), Network (3), Data Link (2), and Physical (1).
• 🔄 The OSI model helps IT professionals communicate effectively, as it provides a common language for understanding data transmission across networks.
• 🔌 OSI Layer 1 (Physical) deals with the physical transmission of signals, such as cables, fibers, or wireless connections.
• 🖧 OSI Layer 2 (Data Link) is often referred to as the MAC address layer and is responsible for communication between devices on a network, like switches.
• 📡 OSI Layer 3 (Network) handles routing and forwarding traffic using IP addresses. It's where routers operate and data fragmentation occurs.
• 📦 OSI Layer 4 (Transport) deals with transporting data between devices, using protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
• 🔗 OSI Layer 5 (Session) manages communication sessions between devices, including starting, stopping, or tunneling sessions.
• 🔐 OSI Layer 6 (Presentation) handles data formatting, encryption, and decryption, ensuring information is readable and secure.
• 💻 OSI Layer 7 (Application) is the top layer where users interact with applications, such as HTTP, DNS, and other network-based services.

Q & A

What is the OSI model and what is its purpose?

The OSI model, or Open Systems Interconnection Reference model, is a conceptual framework used to describe the process data takes as it traverses networks. It provides a broad overview of how data moves through systems but is not a protocol suite itself.

How does the OSI model relate to the TCP/IP protocol suite?

The OSI model can be applied to many different protocols, including TCP/IP. While most protocols used today are based on TCP/IP, the OSI model is useful for describing how various protocols operate within a network.

What is the significance of understanding the OSI model in IT communications?

Understanding the OSI model allows IT professionals to communicate clearly about network operations. Terms like 'OSI Layer 7' or 'OSI Layer 4' are universally understood in the industry, regardless of the specific organization.

What is the role of Layer 1 in the OSI model?

Layer 1, or the physical layer, deals with the physical transmission of signals over network cables, fibers, or wireless connections. Issues at this layer often involve problems with cables, fibers, or wireless interference.

What is the function of Layer 2, the data link layer?

Layer 2, the data link layer, facilitates communication between two devices on a network. It is associated with MAC addresses, which are physical addresses on network adapters. This layer is often referred to as the 'switching layer' due to the role switches play in forwarding traffic based on MAC addresses.

How does Layer 3, the network layer, function?

Layer 3, the network layer, is responsible for routing and IP addressing. Routers use this layer to forward traffic based on destination IP addresses, and this is also where fragmentation of data packets occurs to fit different network frames.

What are the key responsibilities of Layer 4, the transport layer?

Layer 4, the transport layer, is responsible for ensuring the reliable transmission of data across the network. Common protocols at this layer include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol), which manage the segmentation and reassembly of data.

What happens at Layer 5, the session layer?

Layer 5, the session layer, manages communication sessions between devices. It is responsible for establishing, maintaining, and terminating connections or sessions between devices.

What is the role of Layer 6, the presentation layer?

Layer 6, the presentation layer, ensures that data is presented in a usable format. This layer handles tasks such as character encoding and encryption/decryption, making data readable for applications.

What protocols operate at Layer 7, the application layer?

Layer 7, the application layer, is where users interact with applications. Common protocols operating at this layer include HTTP, HTTPS, DNS, FTP, and POP3. This is the layer where network applications like web browsers and email clients operate.

Outlines

00:00

🔍 Introduction to the OSI Model

This paragraph introduces the OSI model, which stands for Open Systems Interconnection Reference model. It outlines how data traverses networks, serving as a broad framework for understanding network communication. While most modern protocols use TCP/IP, the OSI model is still relevant and can describe many protocols across its seven layers. The paragraph emphasizes that this model helps IT professionals communicate effectively, as terms like 'Layer 7' or 'Layer 4' are universally understood in the industry.

05:03

📡 Overview of OSI Layers 1 and 2

The focus shifts to Layer 1 (Physical) and Layer 2 (Data Link) of the OSI model. Layer 1 concerns physical signals, cables, and fibers, while Layer 2 manages communication between devices using MAC addresses. This layer is also associated with network switches, which use MAC addresses to forward traffic. Troubleshooting at these layers involves checking physical connections and ensuring network devices operate properly.

10:06

🌍 OSI Layer 3: The Network Layer

This paragraph explains Layer 3, the Network Layer, which focuses on routing and forwarding traffic based on IP addresses. Routers operate at this layer, determining the next hop for traffic and managing issues like IP addressing and subnet masks. The text also covers how Layer 3 fragments data into smaller pieces to fit different network requirements.

🚚 OSI Layer 4: The Transport Layer

Layer 4, the Transport Layer, is introduced as responsible for ensuring data is correctly transmitted from one device to another. Protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) handle breaking data into smaller chunks and reassembling them on the other side. The Transport Layer ensures reliable communication between devices.

🔄 OSI Layers 5 to 7: Session, Presentation, and Application

This paragraph discusses OSI Layers 5, 6, and 7. The Session Layer (Layer 5) manages starting, stopping, and maintaining communication sessions. The Presentation Layer (Layer 6) is responsible for character encoding, encryption, and ensuring data is properly formatted. Finally, the Application Layer (Layer 7) is where users interact with applications like HTTP, DNS, or email clients, providing the interface for user activities.

🖥️ Real-world Applications of the OSI Model

Here, the OSI model is connected to real-world scenarios. Layer 1 deals with physical hardware (cables, fibers), Layer 2 with MAC addresses and Ethernet frames, Layer 3 with IP addresses and routing, Layer 4 with ports (e.g., TCP and UDP), Layer 5 with session management, Layer 6 with encryption (SSL/TLS), and Layer 7 with application-level interactions. The paragraph explains how each layer functions in a practical network setup.

🔬 Analyzing Network Data with Wireshark

This section focuses on using Wireshark, a network analysis tool, to examine data packets. It explains how network traffic is broken down into OSI layers in Wireshark: from physical signals (Layer 1) to MAC addresses (Layer 2), IP addresses (Layer 3), TCP ports (Layer 4), session encryption (Layer 5), and application-specific data (Layer 7). The tool allows users to investigate how data is transmitted and received on the network.

🧑‍💻 Breaking Down a Network Frame Using the OSI Model

This paragraph walks through a specific example of a network frame captured in Wireshark, breaking it down according to the OSI layers. Layer 1 handles physical transmission, Layer 2 contains Ethernet frames and MAC addresses, Layer 3 covers IP addresses, Layer 4 involves TCP ports, and Layers 5–7 encompass the session, encryption, and application data. It provides a concrete illustration of how the OSI model applies to real network traffic.

Keywords

💡OSI Model

The OSI Model, or Open Systems Interconnection model, is a framework used to understand how data travels across a network by dividing the process into seven layers. Each layer has a specific function, from the physical transmission of data to the application a user interacts with. The video explains that the OSI Model helps IT professionals describe and troubleshoot network communication by referencing specific layers, such as Layer 1 (Physical) or Layer 7 (Application).

💡Layer 1 (Physical Layer)

The Physical Layer, or Layer 1, is the lowest level in the OSI Model. It deals with the actual hardware involved in transmitting data, such as cables, fibers, and wireless signals. The video emphasizes that problems at this layer often involve issues with physical connections, such as bad cables or wireless interference, and require tests like loopback tests to troubleshoot.

💡Layer 2 (Data Link Layer)

The Data Link Layer, or Layer 2, is responsible for transferring data between two directly connected devices. It is closely associated with MAC (Media Access Control) addresses, which are unique identifiers for network interfaces. In the video, this layer is described as the 'switching layer' because switches use MAC addresses to forward data to the correct destination.

💡Layer 3 (Network Layer)

The Network Layer, or Layer 3, handles routing, determining the best path for data to travel from one device to another across different networks. It uses IP addresses to identify the source and destination of data. The video explains that when problems occur with IP addressing or routing, they are often traced back to this layer.

💡Layer 4 (Transport Layer)

The Transport Layer, or Layer 4, ensures that data is delivered reliably from one device to another. It uses protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) to break down data into smaller packets and reassemble them at the destination. The video refers to this layer as the 'post office' of the network because it handles the packaging and delivery of data.

💡Layer 5 (Session Layer)

The Session Layer, or Layer 5, manages communication sessions between devices. It handles the setup, maintenance, and termination of sessions, ensuring that data is exchanged smoothly. The video explains that when a session needs to be initiated, restarted, or closed, Layer 5 is responsible, making it essential for maintaining continuous data flow.

💡Layer 6 (Presentation Layer)

The Presentation Layer, or Layer 6, ensures that data is in a readable format for the application layer by handling encryption, decryption, and character encoding. The video points out that this layer is responsible for translating data into a format that humans can understand, especially when dealing with encrypted web traffic using SSL/TLS protocols.

💡Layer 7 (Application Layer)

The Application Layer, or Layer 7, is where users interact with the network through applications like web browsers or email clients. It deals with protocols such as HTTP, HTTPS, and DNS. The video emphasizes that when users encounter issues with applications, they are often related to Layer 7, as this is where the visible interaction with the network occurs.

💡MAC Address

A MAC Address (Media Access Control Address) is a unique identifier assigned to network interfaces for communication on a physical network. The video explains that MAC addresses are crucial for Layer 2 communication, as switches use them to forward data to the correct device. It is also highlighted that MAC addresses are associated with the hardware, not the operating system.

💡TCP/IP

TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of protocols used for communication over the internet. Although the OSI model is a theoretical framework, TCP/IP protocols align with its layers. The video mentions that while the OSI model is not tied to any specific protocol suite, TCP/IP is widely used today and maps well to the OSI model, particularly at Layer 3 (Network) and Layer 4 (Transport).

Highlights

The OSI model provides a broad overview of how data traverses a network, rather than detailing specific protocols.

The OSI model is not a protocol suite but can apply to many different protocols, including TCP/IP, which is widely used today.

OSI Layer 1, the Physical layer, deals with the actual physical connections and signals, like cables and wireless signals.

Layer 2, the Data Link layer, is responsible for MAC addresses and is often referred to as the switching layer because switches operate at this layer.

Layer 3, the Network layer, is known as the routing layer, and routers use this layer to determine how to forward traffic using IP addresses.

Layer 4, the Transport layer, handles the transport of data using protocols like TCP and UDP, breaking large data into smaller pieces for transmission.

Layer 5, the Session layer, manages sessions between devices, including initiating, maintaining, and terminating communication sessions.

Layer 6, the Presentation layer, is responsible for formatting, encrypting, and decrypting data before it is displayed to the user.

Layer 7, the Application layer, involves the interaction between the user and the application, with protocols like HTTP, FTP, and DNS operating here.

Ethernet frames, MAC addresses, and switches are examples of technologies that operate at OSI Layer 2, the Data Link layer.

At OSI Layer 3, IP addresses and routing mechanisms are key, including the forwarding of traffic across multiple networks.

The Wireshark network analyzer can decode network data by breaking it into OSI model layers, helping identify issues at each layer.

The transport protocols like TCP and UDP operate at Layer 4, ensuring reliable or fast data delivery depending on the protocol used.

SSL and TLS encryption are associated with OSI Layer 6, encrypting data to secure communications before it's transmitted over the network.

Problems at different OSI layers, such as Layer 1 physical issues or Layer 7 application issues, can be identified and solved using network tools like Wireshark.

Transcripts

We'll start our Network+ course with a broad discussion

of something we refer to as the OSI model.

This is referring to the Open Systems Interconnection

Reference model, and it's a model

we use in it to describe the process that data takes

as it traverses our networks.

The OSI model is not designed to be a detailed description

of this data but instead is intended

to describe a broad overview of how data traverses our systems.

We're also not describing the OSI protocol suite.

Indeed, most of the protocols we use today are based on TCP/IP,

but the OSI model is one that we can

apply to many different protocols,

and it works perfectly with the TCP/IP protocols

that we use today.

Also keep in mind that there are many different protocols that

might operate at an individual layer of the OSI model.

So as we step through an individual layer, keep in mind

there might be tens or even hundreds

of protocols that might exist and work

at that particular layer.

By understanding this OSI model, we

can converse with other people in information technologies

in a way that we would all understand.

It doesn't matter if they're working inside

of your own company or with a different organization.

When you mention OSI layer 7 or OSI layer 4,

everyone understands what that means

and what effect it has on the overall operation

of the network.

If we start at the top of the OSI layer, or OSI layer 7,

and work our way down to OSI layer 1,

we have layer 7 being the application layer.

Layer 6 is the presentation layer.

Layer 5 is the session layer.

Layer 4 is transport.

Layer 3 is network.

Layer 2 is the data link layer.

And layer 1 is the physical layer.

There are many different mnemonics

that people use to memorize this,

but if you start at the top and work your way down,

you can take the first letter of each one of these layers,

and you might have a term such as All People Seem To Need Data

Processing.

That's Application, Presentation, Session,

Transport, Network, Data link, and Physical.

Let's start our conversation of the OSI model

at the bottom of the OSI model, or layer 1.

We refer to this as the physical layer

because it's describing the physical signals

that we send through the cable and fibers on our network.

At this layer, we really don't have many protocols to speak of,

because we're really just talking

about getting a signal from one part of the network to another.

When we refer to a physical layer problem with the network,

we are referring to the OSI layer 1, physical layer.

This means that we might have a bad cable, a bad fiber.

There might be interference on a wireless network.

And it may require us to do a series of tests

to determine if we are really able to get

a signal across this particular wire or this particular fiber

so that we can keep this network running.

A lot of the troubleshooting that you do at layer one

is running loopback tests, testing cables and fibers,

and checking different adapter cards and other devices

to make sure they're working properly on the network.

One step up from the physical layer

is layer 2, the data link layer.

This is the fundamental layer that's

used to communicate between two devices on the network.

We often refer to this as the MAC address layer

because that is the Data Link Control layer, or DLC layer,

that is commonly associated with the network cards that

are in our devices.

And most of the time, these are ethernet adapters or wireless

adapters, and we refer to that physical address on that device

as the data link control address or the MAC address.

In this case, MAC address is not referring

to an operating system.

It's referring to the Media Access Control address.

So any time you hear someone talk about a MAC address,

they are referring to the hardware

address of that particular adapter card.

Since the network switches that we use on our network

determine how to forward traffic based on the destination MAC

address, this is a layer that we often refer to as the switching

layer.

So any time we're referring to a MAC address,

a problem with a switch being able to operate,

or anything else that may be associated

with this particular hardware address,

we're referring to OSI layer 2, the data link layer.

The next layer up is OSI layer 3, or the network layer.

We often refer to this as the routing layer

because this is the layer that routers use to determine how

to forward traffic.

And they are specifically looking at the destination IP

address in order to determine what

the next hop might be for traffic traversing the network.

This is also the layer that we're

able to fragment these frames into multiple pieces,

especially if we're sending it across a network that

may require smaller frames than what is on our local network.

So we can cut those frames up into smaller pieces

to be able to fit them through the network

and then put those pieces back together on the other side.

Any time we're referring to a problem relating

to IP addressing, subnet masks, anything related to an IP

address or anything about routing,

then we're probably referring to layer 3, the network layer.

Layer 4 is the transport layer.

And as the name implies, we're referring to the ability

to transport information from one device to another.

You might also refer to this as the post office layer

because this is responsible for getting

your letter or your information from one side of the network

to the other.

The protocols that are often used and operate at layer

4 of the OSI model is TCP--

this stands for Transmission Control Protocol-- and UDP,

or User Datagram Protocol.

These two protocols are commonly responsible for getting

all of the information within our IP packets from one device

to the other.

In many cases, this involves taking a large amount of data,

putting it into smaller pieces to be

able to get it across the network,

and then putting those pieces back together on the other side.

Before we can send that information

from one side of the network to the other,

we may need to create a session so that a device is

able to receive that data.

Layer 5 is the session layer, and it

provides communication management between point A

and point B. Anything relating to the initiation of a session,

stopping the session, or restarting the session

can commonly be associated with that layer

5 session layer communication.

If an application is using some type of control protocol

or your tunneling information within existing data,

then you're probably using OSI layer 5.

OSI layer 6 is responsible for putting all of this data

into a format that we will eventually

see with our human eyes.

This refers to character encoding, application encryption

and decryption, and it's often combined and discussed

in conjunction with the application layer at layer 7.

Layer 6, or the presentation layer,

is the layer that is commonly in operation just prior to us

seeing this data on our screen.

And the top layer of the OSI model

is OSI layer 7, or the application layer.

This is the layer that we see on our screen.

So any time that we are interacting with an application,

we are operating at layer 7 of the OSI model.

Common applications that would operate at OSI layer 7

are HTTP and HTTPS, FTP, DNS, POP3,

and thousands of other application protocols.

Well, that's a broad overview of the OSI model, but how do

we fit things in the real world into each of those layers?

Let's start down at the bottom with layer 1,

or the physical layer.

When we refer to cables, fiber optic connections,

the signal going across those connections,

or a wireless network, then we're

operating at OSI layer 1, or the physical layer.

When we refer to OSI layer 2 or the data link layer,

then we're talking about ethernet frames, MAC addresses,

or addresses referred to as Extended Unique Identifiers,

or EUI.

There are EUI-48 addresses and EUI-64 addresses, for example.

And as we described earlier, any time we're

referring to the switching process,

we are referring to OSI layer 2.

At OSI layer 3, or the network layer,

we're referring to IP addresses.

So anything that has an IP address or a subnet mask

or referring to the way that routers forward

traffic all happen at OSI layer 3.

If you're referring to a TCP port or a UDP port,

then we're operating at OSI layer 4,

which is the transport layer.

At OSI layer 5, or the session layer,

we're using control protocols to start and end

different sessions.

Or we may be tunneling traffic using protocols

that operate at OSI layer 5.

We often associate the encryption of application data

with OSI layer 6, or the presentation layer.

So if you're communicating to a website

and you're using SSL or TLS to be able to encrypt and decrypt

that data, that process is occurring at OSI layer 6.

And at OSI layer 7, or the application layer,

is where you are interacting with the application itself.

Anything that you're seeing on the screen,

the application itself or any messages from the application,

are often referred to as layer 7 information.

If you wanted to drill down a little bit further

into the OSI model, you might want

to capture some data on your network

and view that data inside of an application such as Wireshark.

I have a Wireshark protocol decode on my screen

where I've captured data on my network,

and this Wireshark has three separate windows

that is displaying different views of the same data.

The top window is a frame-by-frame breakdown.

For example, I have highlighted frame 88 on this top window.

The middle window describes more detail

of what different information is contained

within that single frame.

And on the bottom window, we have a hexadecimal and ASCII

breakdown of the data itself.

Let's focus our view on this middle window,

which is a summary of this highlighted frame 88.

You can see that there are 1, 2, 3, 4, 5 different lines of data

inside of this single frame where the protocol decode has

separated out the individual protocols

within the single ethernet frame.

Let's start with the one at the very top,

which describes frame 88, which has 2,005 bytes on the wire,

and 2,005 bytes were captured by Wireshark.

If you wanted to associate that line with the OSI model,

that would be best associated with layer 1

or the physical model.

Next down is Ethernet II, which has a source MAC address

and a destination MAC address.

And since we are describing MAC addresses,

everything within that line of the Wireshark decode

is associated with layer 2 of the OSI model,

or the data link layer.

The next line down describes the Internet Protocol, or IP.

And as we described before, IP fits squarely

in layer 3 of the OSI model, or the network layer.

And in fact, there are individual IP addresses

listed here for the source IP address and the destination IP

address.

And you can see there is a DNS resolution here that shows that

the IP address of 72.14.247.19 is also associated with

googlemail.l.google.com.

As we move down this list, we're moving up

the layers of the OSI model.

And the next one is the Transmission Control Protocol,

which means we're communicating via TCP for this Google Mail

communication.

Since TCP is in this list, we must

be at OSI layer 4, or the transport layer.

And indeed, you can see TCP port numbers for the source

and the destination of this traffic.

Since the destination port is port 443,

Wireshark has already filled in that we're

communicating over an encrypted channel using HTTPS.

And if we go one layer down into the secure socket layer,

this effectively encapsulates the top three layers

of the OSI model.

So secure socket layer and everything underneath it

are referring to layer 5, the session layer, layer 6,

presentation layer, and layer 7, the application layer.

Here's a summary of that protocol decode broken out

into the different layers of the OSI model.

We had electrical signals at the physical layer.

Layer 2 at the data link layer had ethernet frame information.

Our IP addresses were at layer 3, or the network layer.

Our TCP encapsulation with our TCP port numbers

is at the transport layer.

Our session layer linked the presentation information

to the transport.

Presentation layer was obviously performing that SSL or TLS

encryption and decryption.

And layer 7, or the application layer,

is referring to that Google Mail front end

and being able to send and receive emails

using mail.google.com.

Hopefully you're now able to see how this very broad description

of an OSI model can be applied to practically

any communication that's occurring over the network.

And this allows us to talk with other it professionals

and communicate in a way where everyone understands

exactly what we're referring to when we talk about a layer 1

physical problem or a layer 7 application problem.