Lecture 1: Networking Fundamentals and the OSI Model and part of Lecture 2: Networking Layer Interaction and More!
Right before we jump into the glorious world of networking, we need to ask ourselves what is networking.
Why are we doing all this stuff with Switches, Routers, Hubs and who knows what else!!
Well yes that is 2 questions, but you get the point…. Networking was once very very simple. We had a few end users with terminals and a printer… and well that was about it!
People were so fascinated of the idea that 10 different people could send data to that one printer from the other side of the room and it would work, they just found it fascinating!!
But of course humans being humans, we got bored of that after a while and though well what else can we do? And that’s where it started to get complicated. Companies went from wanting to transfer files from person to person, to being able to save data to one central location to wanting to hold voice conferencing… we turned the fantasies people had when the first computer networks came about, services that we can use today without a second thought into reality. Networks have grown and grown and grown in order to handle these services and make them available to our end users in an efficient manner.
And then of course we wanted to secure our networks which made things get a little bit more difficult and can make the way a network operates more complicated….
In short we have a collection of devices that can be put together to make up a network like the ones shown below and it’s our job to get them to work together!!
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If you’re new to networking you probably haven’t heard of a lot of these devices and it can be very daunting, I remember my first job in the industry walking into a server room and all the flashing your lights and just thinking what the hell is all this, it is very intimidating!!
And the key to success in the field is it’s all about the fundamentals, success in networking whether it be in the exams or in a server room is all about knowing and applying the fundamentals. They are probably the most important things to know throughout the whole CCNA!!!
I’m mentioning this now for one reason and trust me because I thought the same thing, one day if you decided to take a path in cisco you will be studying and sat there thinking Do I REALLY need to know the fundamentals are, do I really need to know the OSI model….. YES YOU DO!!!!
You’ll see why as we dive into the material more and more so let’s get too it!!!
The OSI Networking Model
The OSI model isn’t just something to remember for the exam and then forget. Its something youll find useful for real-world troubleshooting and for breaking networking down into pieces that are easier to learn, I just it on pretty much every job I go on!!
We’ll first take an introductory look at the OSI mode, getting familiar with what’s going on at each level, and then we will use it to create a path for CCENT and CCNA exam success depending on which route you go for!!
Here is the OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
As network administrators our primary concern is with the bottom three layers. But as CCENT and CCNA candidate, we are concerned with all seven layers as they will be in the exam so we will start at the top and work our way down
The Application Layer
The Application layer is where out end users interact with the network.
It performs important behind the scenes tasks such as:
Makes sure the remote communications partner is available
(Remember it takes two to network)
Ensures that both ends agree on a myriad of rules including data integrity, privacy and error recovery
The Presentation Layer
This layer answers one basic question…. How should this date be presented?
Ever opened a PDF file with a non-PDF-Friendly word processing app? You end up with hundreds of lines of unreadable rubbish that you don’t understand, that’s a presentation layer issue
Encryption takes place at this layer, you’re going to hear a lot about encryption later!
The Session Layer
This layer is the manager of the overall data and transfer process. It handles the creation, maintenance, and teardown of the communication channel between two parties
The Transport Layer
The main purpose of the Transport layer is to establish a logical end-to-end connection between two systems.
That’s not the only this going on here! Most oft eh additional transport layer functions involve either the Transmission Control Protocol (UDP) or the User Datagram Protocol (UDP), thos two protocols are so important that they have their own sections on the exam!! (A section can be worth 100+ points on the exam so I’ll explain in a lot of detail!!)
Now we come to the OSI layers you will mainly work in as Network Admins/Consultants in the working world.
The Network Layer
For the router, the network later processes answers tow basic questions:
What valid path exists from here to point B?
Of which of those paths is the best to get there
And that’s about it!!
Of course it’s not that easy, we will dive into the details later at the moment, it’s enough to know that ip addresses (172.12.123.1 for example) are used at the routing layer.
There’s another important address we use in our networks and that is in the next layer
The Data Link Layer
We’ll be spending A LOT of time with switches, and our switches run at this layer, as do these protocols:
Ethernet
HDLC (High Data Link Control)
PPP (Point-to-Point Protocol)
Frame Relay
This address running at this layer is known as the MAC address, but it had four other names you will need to know as they don’t always call it the MAC in the exam, they are:
Layer 2 address
Hardware address
Burned-in address (BIA)
Physical address
You may be wondering the same as I did, why does it need all these names???.... Well the first name makes perfect sense since we are at Layer 2, but what about the others?
This address is actually burned into the hardware so you see where the “hardware and Burning in” names come from. Be careful with the last name though. We sometimes call the MAC address the physical address because it physically exists on the hardware NOT because it runs at the physical layer of the OSI model because it doesn’t!!
Right now I want to introduce you to a set of terms that sound like they do the same thing, but we need to be very clear on the difference:
Error detection Error Correction
Remember: Detecting something doesn’t mean you’re correcting it.
Here why in bringing it up!!
The data link layer performs error detection via the frame check sequence (FCS). The actual operation of the FCS goes beyond the scope of the CCENT exam, but as network admins you really should know the FCS fundamentals:
- The sender runs a mathematical formula (an algorithm) against the contents of the frame.
- The sender places the result of that value in the FCS file of the frame, and then sends the frame
- The receiver of the frame runs the same algorithm against the contents of the frame, if the result value matches the contained FCS field, the frame is fine. If that resulting value doesn’t match the frame is considered corrupt and deleted.
So why no error recovery? It’s the recipient of the frame that detects the error, not the sender and the recipient can’t retransmit the frame to itself. All the recipient can do is let the sender know there was a problem
The Physical Layer
All the work we do at the upper layers of the OSI model is all about sending data across the physical layer in the form of ones and zeroes.
The data our end users create is going to be eventually translated into 1s and 0s. Anything having to do with a physical cable or the standards in use – the pins, connectors and the actually electric current – is running at the physical layer
With our end users entering data / sending photos /watching videos or whatever, it sound like we have a lot of work to turn all of that into ones and zeroes
It’s almost like we need a plan… well here it is!!