Original subtitles

So a quick recap of the OSI model.

At layer 1 we have the physical layer, physicals specification like Rj-45

are defined at this layer, option such as cables specifications, voltage

and other physical parametera are defined at the physical layer.

At layer 2 we have the data link layer and this is where Mac address is reside.

I�m going to explain Mac addresses in more detail in a moment.

At layer 3 we have IP addresses and this is where routing takes place

routers reside at this layer and addresses such as IPv4 or IPv6 exist at layer 3.

At layer 4 we have protocol such as TCP and UDP 10

now some of this information was covered in the OSI model videos 11

but here I'm gonna look at some of the layers in more detail. 12

So Ethernet was born in the 1970's which for a lot of us 13

seems like a really long time ago. 14

Robert Metcalfe was one of the people involved in Ethernet development. 15

he also started the company called 3Com in 1979 which is subsequently 16

been purchase by Hewlett-Packard, what's important to understand is that Ethernet 17

and networking is very young when compared to telephony environments. 18

Alexander Graham Bell invented the telephone system many, many years ago 19

long before the advent of Ethernet. 20

I�m gonna give you a brief history lesson but it's worthwhile knowing 21

some of the history of the Ethernet because it explaines 22

how we've got to where we are today, it also explains some of the concepts 23

which are still relevant in today�s networks. 24

Now in the original Ethernet implementation. 25

The network architecture that was use was a bus topology. 26

In a bus topology, each devices connected to a single cable 27

and the clients therefore share a communication line or bus. 28

This is similar in concept to what we use to have in telephony environment 29

which was called a telephone party line, where a single cable 30

was used to provide telephone services to remote areas. 31

In that example you would have a single cable and multiple telephones 32

would hang off the so called party line, now before you made a call 33

in those days you have to listen to hear if anyone else was speaking. 34

So before you made a call you would pick up your handset and listen 35

and make sure that no one else was using the line. 36

When someone made a call to that telephone line 37

all telephones connected to the party line would ring. 38

And the same thing happens in an Ethernet environment when using a bus topology. 39

When traffic is sent on that cable it is received by all devices connected to the bus. 40

This is the shared infrastructure and it means that when any device on that network 41

sends traffic all other devices connected to the same cable will receive the traffic. 42

When a device wants to speak or communicate it needs to ensure that no other device 43

speaking, otherwise collision can occur. 44

In some of the original implementations of Ethernet, we had what was called 45

10 base5 also called thicknet and it had a maximum segment size of 500 meters. 46

There was also another physical implementation known as 10base2 47

also called thinnet this had a maximum distance of 185 meters. 48

This early implementations of Ethernet use a bus topology which means 49

that when a device on the cable sends a signal all devices 50

connected to that cable will receive the signal. 51

So now let�s discuss one of the implementations 10base2 which will hopefully help you 52

understand the reasons why we do things in Ethernet today. 53

Basically because of historical reasons, certain things are done in the certain way. 54

So 10base2 use coaxial cable or coax cable it had a maximum speed of 10mbps 55

the 10 in 10base2 indicates a speed of 10mbps, 2 indicates a maximum segment length 56

of 185 meters, and the word base indicates baseband rather than broadband. 57

Now what is the difference between baseband and broadband 58

now baseband only allows for a single signal to traverse the wire at any given time. 59

The signal uses all the frequencies. 60

Broadband on the other hand which in some cases 61

is used for cable television also uses coaxial cable. 62

Broadband allows for multiple signals to be sent across the wire at any given time. 63

If you had a cable television service which only allowed you to receive 64

a single television station that wouldn�t be a very good service. 65

Broadband on the other hands allows for multiple signals 66

to be sent across a single coaxial cable at any given time. 67

So once again broadband television uses coaxial cable which is similar to the 68

coaxial cable that was use in the early Ethernet implementations.