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  • Cisco Certified Network Associate - Day 2. This is Imran Rafai, your trainer for this

  • entire series. Welcome back everybody for the second day in our training series, as

  • you already know this is a FREE training on YouTube for all the people who are interested

  • to start a career in networking. In our last video we learnt about Network Fundamentals.

  • Today we would be dealing with the OSI Model and the TCP/IP Model. When I say TCP/IP Models

  • or OSI Model, it is nothing but a set of rules... its a set of standards. Now you might ask

  • why we need a set of rules or set of standards in the computer industry! To understand that

  • we need to learn a little about the history of the computer industry. Not long ago, there

  • was a fierce battle between IBM and Digital Equipment Corporation (DEC) for being the

  • leading computer manufacturer. But there was a problem. Both these manufacturers manufactured

  • devices which were not compatible with each other. So if you bought a computer from IBM

  • you had to buy a monitor, printer and every thing from IBM. Similarly if you bought a

  • device from DEC then you had to buy all the accessories and other devices from DEC so

  • that you could actually use it. There were many companies which bought equipments from

  • both equipment manufacturers such that the accounts department had IBM computers and

  • devices and the marketing department had Digital Equipment Corporation computer. But the problem

  • is they could not communicate with each other or they could not even share information with

  • each other. Now that was not the right way forward. That was about the time when international

  • organization for standards or more commonly known as ISO thought that there was a need

  • for a standard. ISO developed OSI which was an acronym for Open System Interconnect which

  • is more commonly known as OSI reference model. At about the same time, a competing standard

  • known as the TCP/IP model which was promoted by the department of defence came into being.

  • The TCP/IP model is more like a stripped down version of the OSI Model and because it was

  • more relevant it went onto become the industry standard. To understand little bit more about

  • the models, we need to understand about layers approaches. To understand that let us talk

  • about this office that is on the screen. Every office has different employees and employee

  • levels. So you might have a CEO, you might have a Senior office staff, you might have

  • payroll manager, an account manager, maintenance manager, you have maintenance staff, you have

  • junior office staff, and you have different staffs. The reason every companies have different

  • employees with different designations, is because all those people have different roles

  • and responsibilities. So when you know that something is not done, or some task is not

  • accomplished, you know who is responsible. For instance if salary is not credited on

  • time, you know the responsibility has to be with the payroll manager, whether the bank

  • is involved or not... that is secondary, but the payroll is your first point of contact.

  • You will need to go and speak to the payroll manager. If you come to the office one fine

  • day and you see that your office is really dirty, it is not cleaned, you know the janitor

  • is responsible or the maintenance department is responsible. So you know where the responsibility

  • lies. Similarly we learn about the OSI Model and the TCP/IP model because in learning about

  • the computer network or tomorrow when we are trouble shooting computer networks, we need

  • to know what works at what layer. So if something isn't working, instead of troubleshooting

  • all the eqipments, we know exactly where the problem is! That is why we have a layer approach.

  • Alright, knowing what we know now, lets get into the Models! Lets understand how models

  • work, what are the layers involved and lets compare TCP/IP and the OSI Model. What you

  • see on the left is the OSI Model. What you see on the right is the TCP/IP model. Let

  • me take a highlighter so that I can highlight as I talk! Alright, for the moment, dont worry

  • about the colors. I have color coded it to compare between the OSI Model and the TCP/IP

  • Model. So lets start discussing from the left, from OSI Model. Before we go any further,

  • I need to stress on this point that as a student of networking, all of you should know the

  • layers by-heart! So when I say that, you need to know exactly the sequences of the layers.

  • You need to know the layer 7 is the Application Layer, Layer 6 is Presentation Layer, Layer

  • 5 is Sessions layer. You need to know where the layers are! One way of remembering the

  • sequences of the layers is by having MNEMONICS. One of the most popular mnemonics to remember

  • the OSI model is the one given here. So if you remember, "Please Do Not Throw Sausage

  • Pizza Away", you can remember the first letter of the layers from bottom up! So "Please Do

  • Not Throw Sausage Pizza Away" is PDNTSPA! Similarly some people like to remember it

  • from top to bottom, which is perfectly fine. So if you want to remember it from top to

  • bottom, then you have another MNEMONICS, which says "All People Seem To Need Data Processing".

  • Well it is upto you. You can do it the way you like it, as long as you remember that

  • physical layer is layer 1 and application layer is layer 7. Some people get confused

  • when they remember it from top to bottom, that Application is layer 1.... NO.... Application

  • is layer 7 and Physical is layer 1! Alright and if you wan to remember TCPIP model, then

  • you could remember by just remembering this MNEMONICS, which is "TCP/IP comes in a TIN".

  • Well it does not mean anything, but you can just remember - ATIN .... that is Application

  • Transport Internet and Network Interface! Like I said, if you have a better way of remembering,

  • please use it.... as long as you remember the sequence of these layers, it should be

  • perfectly fine! Alrght, lets start with the Application layer. Application layer is the

  • point of contact for all Network Aware applications. Let me repeat that, it is a point of contact

  • for all Network Aware Applications. A lot of authors, I have heard them say/write that

  • all applications on your computer resides on this application layer, which is not true.

  • Application layer ..... firstly deals with only application that is network aware and

  • secondly all the application on your computer has nothing do with the application layer,

  • if they do not communicate on the network! To understand that a little bit more, let

  • us assume that you have uninstalled all the drivers from your PC, your wireless interface

  • card, your ethernet interface card and all network interface cards, you uninstall. Uninstall

  • the driver or physically remove if you can. Now if you fire up microsoft word for instance,

  • microsoft word will not communicate with the application layer or for that matter it will

  • not communicate with any of these layers. That is because microsoft cannot access your

  • network because because your Operating system does not even have a network interface card!

  • Similarly, on a normal system, if you fire up your web browser, maybe chrome, internet

  • explorer, safari, any of the webbrowsers and if you type www.cnn.com, that web browser

  • creates an http request and it communicates with your application layer. The application

  • layer gives that data to the presentation layer. The presentation layer does what it

  • has to do, gives it to the sessions layer. Sessions layer does the same thing and it

  • gives to transport and it keeps passing it to the next layer until it reaches the physical

  • layer! So this is how it works! So if an application communicates with the networks, that is the

  • only time the application layer is invoked! Some of the protocols that work at this layer

  • are FTP, TFTP, SNMP, DNS, HTTP and there are quite a few. So you can just do a GOOGLE...

  • you will get the entire list of all the protocols that work at this layer! Next we will deal

  • with the presentation layer. The presentation layer is the layer which GENERIFIES data.

  • When I say GENERIFIES data, it is the layer which takes the data coming out of sessions

  • layer on the way back, converts it into presentable form and gives it to the application layer.

  • So if you have raw data coming from sessions layer, to the presentation layer, this is

  • where data is converted. So if it is an image, it creates an image. If its a word document,

  • it creates a word document. So presentation layer is where all data conversion happens.

  • Also all encryption services like TLS, SSL all that are taken care of at this layer!

  • Next we have the sessions layer. Sessions layer is the layer which creates and maintains

  • session. When I say that, lets assume your computer has two applications. You have yout

  • telnet program and you have your web browsing application, maybe internet explorer! Now

  • both of these applications are accessing the network. So this layer is the layer that create

  • 2 different sessions and maintains them! So when your web browser session is terminated,

  • your telnet session will also not get termnated because they are separate sessions! So it

  • maintains different sessions! Now I have color coded the top three layers in blue, and the

  • reason I have done that is because as a network engineer, and especially as a Cisco Network

  • Engineer, there is very little that we need to deal with layer 5,6,7. That is because

  • most of the cisco devices will not look beyond layer 4. So layer 1, 2, 3 &4 are very critical

  • for Cisco Engineer. But 5,6,7 are not so critical. You just need to know how it works and you

  • know as a rough idea, you know what it does, that should be perfectly fine because layer

  • 5,6,7 are taken care of by the operating system. But layer 4,3,2,1 is where as a network engineer,

  • you have to know exactly how they work! Next we learn about the transport layer. Transport

  • layer is also one of the critical layers for a network engineer. When the information coming

  • from the upper layers comes to the transport layers, transport layer breaks them into manageable

  • segments. Each segment, the transport layer adds its own header to create encapsulation.

  • Two critical decisions the transport layer takes is:1) Whether to use Reliable Communication

  • or Unreliable Communication. 2) Create Port Numbers. When applications need a reliable

  • communication, they use TCP which is nothing but an acronym for Transmission Control Protocol.

  • If it wants an unreliable communication, it uses UDP, which is an acronym for User Datagram

  • Protocol. When I say 'Reliable', it doesnt mean it is better than 'Unreliable' in that,

  • the only difference is that the reliable communication has to get an acknowledgement for every packet

  • that is sent! In unreliable, there is not acknowledgement that is received for every

  • segment transmission. So unreliable is actually faster because of the less overhead. So if

  • it is realtime applications, they will use UDP, because it is faster and it is realtime.

  • So most of the time, if you have tried watching an online transmission or a live feed, there

  • are times when we see those green pixels on the screen. This is nothing but the information

  • for that pixel has not been received and the receiving device has no way of telling the

  • sending device that it did not receive that pixel. Similarly another function that is

  • taken care of by the transport layer is to create port numbers. Port number is a number

  • that is attached with the IP Address, to identify which application this information is coming

  • from. The transport layer, 1)Creates a random port source port number and attaches the port

  • number for the destination. So if your traffic is going to a webserver, the webserver has

  • a standard port number of 80! So if you are going to an IP Address of 10.10.10.10 for

  • instance as an example, it takes 10.10.10.10 and adds a port number for 80. So it creates

  • a socket.... socket is nothing but an ip address and port number. It creates a socket and send

  • down to the network layer. It also adds a source port number. Why does it require source

  • port number? The reason it creates a source port number is because if there are 2 applications

  • running on your computer and it is going to the same IP address, when the reply comes

  • ack the transport layer needs to know which data goes to which application. And it will

  • identify that by actually having our source port number. Next we will learn about the

  • Network Layer. When the network layer gets a segment from the transport layer, it adds

  • a network layer header. When a network layer header is added, that information is called

  • as a Packet! So one of the critcal functions of the Network Layer is of IP Address or also

  • known as Logical Addressing. We will learn about IP addressing in the next video, DAY3.

  • But just know that this is where the IPAddress is added. It also finds the best path. So

  • when I say best path, it compares the IPaddresses and it checks if the destination ip address

  • is in the local subnet. If it is not on the local subnet, it will find the best path to

  • the destination. Next we have the DataLink Layer. When the packet comes from the Network

  • layer to the data link layer,the data link layer will add data link header to it. And

  • that information is called a FRAME. The data link layer is responsible for MAC ADDRESSING.

  • MAC is nothing but an acronym for Media Access Control, and MAC address is a hardware address.

  • That means every network interface card of your computer, has its own MAC address. MAC

  • Address is also known as Burn-In-Address and its a address that cannot be changed. Yes

  • there are ways of spoofing the address, but it cannot be changed physically. This is a

  • layer where Error Checking happens. So when information coming from the physical layer

  • to the data link layer, the data link layer has the ability to check if there were error

  • that were induced during transmission. So it could use Cyclic Redundancy Check (CRC)

  • or it could use Parity Check or any other error checking mechanism that Data Link Layer

  • uses to check for errors. Next is the physical layer and physical layer is where the actually

  • data transfer happen. This is the layer that deals with wires, cables and hardware ports/connectors

  • and all similar things that happen at this layer. As a computer engineer or as a network

  • engineer, when somebody tells you that the internet doesnt work, as an engineer you will

  • start working or troubleshooting from Physical Layer. First you will check the cables to

  • see if it works. If the cables are fine, next you will check for Layer 2 problem. Then you

  • will go to Layer 3, Layer 4.... so you will trouble shoot by layers and that is why we

  • have to learn about the models. Also any change in one of the layers will not affect the adjacent

  • layers as long as the standard is followed. So if a physical layer, instead of the ethernet

  • cable I choose to use Wireless. It should not matter because the change is happening

  • on the physical layer only. So datalink layer, as long as it gets the information the way

  • it is expecting to get, it does not matter how physical layer gets those information.

  • Similarly the Network Layer - We are in the midst of transition phase of going from IPv4

  • to IPv6, so that change is only to layer 3. Layer 4 and Layer 2 will not be affected as

  • long as the standard is followed and it is receiving the data as it was receiving earlier.

  • So that is the beauty of the layered approach. Now if you compare the left and the right

  • we can see that the layer 5,6,7 is comparable to Layer 4 in the TCP/IP model. Transport

  • Layer and Transport Layer in TCP/IP, they have comparable functions. Similarly the Network

  • Layer and the Internet Layer. The layer 1 and layer 2 of OSI Model is comparable to

  • Layer 1 of the TCP/IP model. Next we will just see the OSI Model working. Let us assume

  • that the guy on the left wants to send some to the guy on the right. So he creates a data.

  • Like we mentioned the top 3 layers are generally known as the upper layers and upper layer

  • we really do not deal much. So lets assume the data goes to the transport layer. What

  • the transport layer does is, it takes the data, it segments it and adds its own header.

  • So if you see, you have a transport header here. Next lets assume we are dealing with

  • UDP here, so it does not wait for acknowledgements. It will send segment 1 to the network layer.

  • The network layer as we know will add a network header to the segment and it becomes a packet

  • now. Network layer will give the segment 1 or the packet to the datalink layer. The data

  • link layer takes that and adds a Data Link header. And if you can see the segment 2 also

  • goes through the same process. Next the segment 1 goes to the physical link layer and the

  • physical link layer as we know will convert it into 0s and 1s. So our segment 1 got converted.

  • Next segment 2 also will get converted. By that time the physical layer of the destination

  • device would have taken those physical bits and converted it into segment with the appropriate

  • headers. What the receiving layers will do is, it will strip the headers of the corresponding

  • layers from the source. So the data link layer will strip the datalink header and give that

  • information to the network layer. So if you see there was a datalink header to segment

  • 1 which the data link layer stripped, and gave that information to network layer. Similarly

  • it will happen to Segment 2 as well. So for segment 1 when it goes to transport layer,

  • this network header will get removed. So if you see there, the network header got removed

  • and it reached the transport layer. Next the transport layer will wait until it gets all

  • the segment. That is what the transport layer will do. So transport layer waits until segment

  • 2 comes there. Now transport layer will strip all the transport layer headers and combine

  • that information and give it to the upper layers. So the transport layer combines it

  • and the receiving guy receives the same data. And this is how a very basic representation

  • of how a data network works. I think thats about all the information that we had to learn

  • in this day 2 video. Please go through this entire cloud map and I am sure you understand

  • all the topics here. But if you do not, please feel free to write into me at 'imran.rafai@nwking.org'.

  • Please dont forget to share our videos, like our videos, and please subscribe to our YouTube

  • channel. You love and your respect is what motivates us to create free contents. Thank

  • you so much.

Cisco Certified Network Associate - Day 2. This is Imran Rafai, your trainer for this

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    Cen Yo Lin   に公開 2019 年 10 月 15 日
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