1 00:00:00,000 --> 00:00:05,000 So a quick recap of the OSI model. 2 00:00:05,000 --> 00:00:10,000 At layer 1 we have the physical layer, physicals specification like Rj-45 3 00:00:10,000 --> 00:00:16,000 are defined at this layer, option such as cables specifications, voltage 4 00:00:16,000 --> 00:00:19,000 and other physical parametera are defined at the physical layer. 5 00:00:19,000 --> 00:00:26,000 At layer 2 we have the data link layer and this is where Mac address is reside. 6 00:00:26,000 --> 00:00:29,000 I’m going to explain Mac addresses in more detail in a moment. 7 00:00:29,000 --> 00:00:34,000 At layer 3 we have IP addresses and this is where routing takes place 8 00:00:34,000 --> 00:00:42,000 routers reside at this layer and addresses such as IPv4 or IPv6 exist at layer 3. 9 00:00:42,000 --> 00:00:46,000 At layer 4 we have protocol such as TCP and UDP 10 00:00:46,000 --> 00:00:50,000 now some of this information was covered in the OSI model videos 11 00:00:50,000 --> 00:00:53,000 but here I'm gonna look at some of the layers in more detail. 12 00:00:53,000 --> 00:00:56,000 So Ethernet was born in the 1970's which for a lot of us 13 00:00:56,000 --> 00:00:59,000 seems like a really long time ago. 14 00:00:59,000 --> 00:01:04,000 Robert Metcalfe was one of the people involved in Ethernet development. 15 00:01:04,000 --> 00:01:10,000 he also started the company called 3Com in 1979 which is subsequently 16 00:01:10,000 --> 00:01:16,000 been purchase by Hewlett-Packard, what's important to understand is that Ethernet 17 00:01:16,000 --> 00:01:21,000 and networking is very young when compared to telephony environments. 18 00:01:21,000 --> 00:01:26,000 Alexander Graham Bell invented the telephone system many, many years ago 19 00:01:26,000 --> 00:01:28,000 long before the advent of Ethernet. 20 00:01:28,000 --> 00:01:32,000 I’m gonna give you a brief history lesson but it's worthwhile knowing 21 00:01:32,000 --> 00:01:35,000 some of the history of the Ethernet because it explaines 22 00:01:35,000 --> 00:01:37,000 how we've got to where we are today, it also explains some of the concepts 23 00:01:37,000 --> 00:01:42,000 which are still relevant in today’s networks. 24 00:01:42,000 --> 00:01:45,000 Now in the original Ethernet implementation. 25 00:01:45,000 --> 00:01:48,000 The network architecture that was use was a bus topology. 26 00:01:48,000 --> 00:01:53,000 In a bus topology, each devices connected to a single cable 27 00:01:53,000 --> 00:01:59,000 and the clients therefore share a communication line or bus. 28 00:01:59,000 --> 00:02:02,000 This is similar in concept to what we use to have in telephony environment 29 00:02:02,000 --> 00:02:07,000 which was called a telephone party line, where a single cable 30 00:02:07,000 --> 00:02:12,000 was used to provide telephone services to remote areas. 31 00:02:12,000 --> 00:02:16,000 In that example you would have a single cable and multiple telephones 32 00:02:16,000 --> 00:02:20,000 would hang off the so called party line, now before you made a call 33 00:02:20,000 --> 00:02:24,000 in those days you have to listen to hear if anyone else was speaking. 34 00:02:24,000 --> 00:02:28,000 So before you made a call you would pick up your handset and listen 35 00:02:28,000 --> 00:02:32,000 and make sure that no one else was using the line. 36 00:02:32,000 --> 00:02:35,000 When someone made a call to that telephone line 37 00:02:35,000 --> 00:02:40,000 all telephones connected to the party line would ring. 38 00:02:40,000 --> 00:02:44,000 And the same thing happens in an Ethernet environment when using a bus topology. 39 00:02:44,000 --> 00:02:51,000 When traffic is sent on that cable it is received by all devices connected to the bus. 40 00:02:51,000 --> 00:02:58,000 This is the shared infrastructure and it means that when any device on that network 41 00:02:58,000 --> 00:03:04,000 sends traffic all other devices connected to the same cable will receive the traffic. 42 00:03:04,000 --> 00:03:10,000 When a device wants to speak or communicate it needs to ensure that no other device 43 00:03:10,000 --> 00:03:12,000 speaking, otherwise collision can occur. 44 00:03:12,000 --> 00:03:17,000 In some of the original implementations of Ethernet, we had what was called 45 00:03:17,000 --> 00:03:23,000 10 base5 also called thicknet and it had a maximum segment size of 500 meters. 46 00:03:23,000 --> 00:03:28,000 There was also another physical implementation known as 10base2 47 00:03:28,000 --> 00:03:32,000 also called thinnet this had a maximum distance of 185 meters. 48 00:03:32,000 --> 00:03:38,000 This early implementations of Ethernet use a bus topology which means 49 00:03:38,000 --> 00:03:43,000 that when a device on the cable sends a signal all devices 50 00:03:43,000 --> 00:03:46,000 connected to that cable will receive the signal. 51 00:03:46,000 --> 00:03:52,000 So now let’s discuss one of the implementations 10base2 which will hopefully help you 52 00:03:52,000 --> 00:03:47,000 understand the reasons why we do things in Ethernet today. 53 00:03:47,000 --> 00:04:01,000 Basically because of historical reasons, certain things are done in the certain way. 54 00:04:01,000 --> 00:04:08,000 So 10base2 use coaxial cable or coax cable it had a maximum speed of 10mbps 55 00:04:08,000 --> 00:04:18,000 the 10 in 10base2 indicates a speed of 10mbps, 2 indicates a maximum segment length 56 00:04:18,000 --> 00:04:24,000 of 185 meters, and the word base indicates baseband rather than broadband. 57 00:04:24,000 --> 00:04:28,000 Now what is the difference between baseband and broadband 58 00:04:28,000 --> 00:04:36,000 now baseband only allows for a single signal to traverse the wire at any given time. 59 00:04:36,000 --> 00:04:39,000 The signal uses all the frequencies. 60 00:04:39,000 --> 00:04:42,000 Broadband on the other hand which in some cases 61 00:04:42,000 --> 00:04:46,000 is used for cable television also uses coaxial cable. 62 00:04:46,000 --> 00:04:52,000 Broadband allows for multiple signals to be sent across the wire at any given time. 63 00:04:52,000 --> 00:04:57,000 If you had a cable television service which only allowed you to receive 64 00:04:57,000 --> 00:05:00,000 a single television station that wouldn’t be a very good service. 65 00:05:00,000 --> 00:05:05,000 Broadband on the other hands allows for multiple signals 66 00:05:05,000 --> 00:05:09,000 to be sent across a single coaxial cable at any given time. 67 00:05:09,000 --> 00:05:16,000 So once again broadband television uses coaxial cable which is similar to the 68 00:05:16,000 --> 00:05:21,000 coaxial cable that was use in the early Ethernet implementations.