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