1 00:00:00,420 --> 00:00:02,680 Now let's look at a MAC address in more detail. 2 00:00:02,690 --> 00:00:05,140 It's once again six bytes in length. 3 00:00:05,370 --> 00:00:09,200 And if you remember a byte is eight bits in length. 4 00:00:09,270 --> 00:00:19,710 So six times eight gives you 48 bits 3 bytes or 24 bits is the UI portion of the Address 3 bytes or 5 00:00:19,710 --> 00:00:20,960 24 bits. 6 00:00:21,000 --> 00:00:27,670 Is a network interface card specific and is the unique identifier of that network interface card. 7 00:00:27,780 --> 00:00:33,720 Now in the UI portion in the first octet will most significant octet. 8 00:00:33,720 --> 00:00:37,870 In other words the first bite in the UI the least is significant. 9 00:00:37,870 --> 00:00:45,570 But in other words the last bit of the first octet or first byte is either set to 0 which indicates 10 00:00:45,570 --> 00:00:53,850 he unicast or it's set to 1 which indicates multicast unicast traffic if you remember is a conversation 11 00:00:53,850 --> 00:00:55,710 between two devices. 12 00:00:55,710 --> 00:01:00,210 One device is sending the traffic and the other devices receiving the traffic. 13 00:01:00,210 --> 00:01:08,190 So device say is talking to device B multicast is a way one device is sending traffic to multiple devices 14 00:01:08,190 --> 00:01:11,580 that have subscribed to the multicast. 15 00:01:11,580 --> 00:01:16,690 Now this makes it very efficient for Ethan it switches to know whether they should flood the frame out 16 00:01:16,690 --> 00:01:23,310 of all ports when multicast traffic is received by layer to switch that traffic is flooded out of all 17 00:01:23,310 --> 00:01:27,640 ports whereas unicast traffic is typically not flooded. 18 00:01:27,990 --> 00:01:35,160 So by reading the but in the frame the layer to switch knows how to process traffic. 19 00:01:35,160 --> 00:01:38,420 The second least significant bit in the first octet. 20 00:01:38,430 --> 00:01:44,460 So in other words we still looking at the first octet But second the least significant but he's either 21 00:01:44,460 --> 00:01:52,050 set to zero which means that it's a globally unique MAC address or it's set to one which means that 22 00:01:52,050 --> 00:01:54,920 an administrator has changed the MAC address. 23 00:01:55,020 --> 00:02:01,800 So that would be for the example that I did previously where I changed the MAC address on my PC to 0 24 00:02:01,830 --> 00:02:10,920 means it's a unique MAC address designated by the manufacturer where as a one means that a administrator 25 00:02:11,040 --> 00:02:14,980 locally changed the MAC address of the interface. 26 00:02:15,060 --> 00:02:22,680 Now in Ethernet when a bus topology is used to Vice's use what's called carious sains multiple access 27 00:02:22,890 --> 00:02:27,420 slash collision detection for CSM a slash CD. 28 00:02:27,510 --> 00:02:28,880 This operates as follows. 29 00:02:29,010 --> 00:02:35,950 When a device wants to send traffic it should first check to hear if any other devices speaking. 30 00:02:35,970 --> 00:02:43,320 So the device will not communicate onto the network if it hears another device that's called Carry a 31 00:02:43,320 --> 00:02:50,430 sense carious sense is essentially sensing the network to hear if another device is speaking. 32 00:02:50,430 --> 00:02:57,870 Multiple Access means that any device can communicate across that segment as long as no other devices 33 00:02:57,870 --> 00:02:59,480 are communicating. 34 00:02:59,490 --> 00:03:07,710 Now this is different to the old mainframe days where a central device would pull terminals to allow 35 00:03:07,710 --> 00:03:15,240 them to communicate in Ethernet wi using a distributed environment where each device can independently 36 00:03:15,240 --> 00:03:19,820 communicate across the network without permission from other devices. 37 00:03:19,830 --> 00:03:25,050 However a device should only same traffic if no other devices speaking. 38 00:03:25,080 --> 00:03:29,820 And that's because we want to avoid collisions in an Ethernet environment. 39 00:03:30,030 --> 00:03:38,500 As another analogy with traditional telephones are connected to a PBX the PBX is in charge of the communications. 40 00:03:38,520 --> 00:03:41,360 That's not true in an Ethernet environment. 41 00:03:41,400 --> 00:03:45,150 Every device is independent of other devices. 42 00:03:45,150 --> 00:03:51,960 However if collisions do take place there's an option in the net to detect collisions when a device 43 00:03:51,960 --> 00:03:54,950 detects that a collision has taken place. 44 00:03:54,990 --> 00:04:01,340 It may send a backoff or jamming signal to indicate that a collision has taken place. 45 00:04:01,530 --> 00:04:08,250 Once again in this environment terminate is used at the end of the cable to ensure that signals don't 46 00:04:08,370 --> 00:04:11,880 bounce back causing additional collisions. 47 00:04:11,880 --> 00:04:20,190 Now in a given scenario it may happen that two devices one to communicate at exactly the same time but 48 00:04:20,190 --> 00:04:24,090 at that point in time no devices are speaking. 49 00:04:24,090 --> 00:04:31,950 So let's say that in this example a wants to communicate with C so he wants to send traffic onto the 50 00:04:31,950 --> 00:04:35,990 network with a source address Ave and destination address of C.. 51 00:04:36,120 --> 00:04:40,500 But at that exact point time D also wants to communicate. 52 00:04:40,650 --> 00:04:46,350 In this case D wants to communicate with B said wants to send a frame onto the network with a source 53 00:04:46,350 --> 00:04:50,260 address of D and destination address of B. 54 00:04:50,310 --> 00:04:58,110 Now in line with CSM a slash CD both A&E fusee check to see if anyone is speaking. 55 00:04:58,200 --> 00:05:03,140 So they use carious saints or C-s to check the wire. 56 00:05:03,210 --> 00:05:07,070 At this point in time no device is communicating on the network. 57 00:05:07,070 --> 00:05:14,280 However because of multiple access any device can access the cable without permission from any other 58 00:05:14,280 --> 00:05:15,140 device. 59 00:05:15,180 --> 00:05:18,630 So both A and D send traffic onto the network. 60 00:05:18,780 --> 00:05:20,910 But because this is 10 days too. 61 00:05:21,000 --> 00:05:28,770 Or in other words baseband only one signal is allowed across the wire at any given time so in this example 62 00:05:28,980 --> 00:05:30,890 a collision takes place. 63 00:05:30,960 --> 00:05:38,490 Now if a transmitting data station or PC detects another signal on the wire while transmitting its frame 64 00:05:38,850 --> 00:05:45,930 it will stop transmitting that frame and then send a jamming signal as well as waiting a random period 65 00:05:45,930 --> 00:05:51,890 of time known as a backoff delay before trying to send the signal again. 66 00:05:51,900 --> 00:05:58,370 This will prevent machines or PCs from repeatedly attempting to transmit at the same time. 67 00:05:58,440 --> 00:06:06,630 However the probability of collisions becomes greater as the cable length increases and as more devices 68 00:06:06,630 --> 00:06:08,400 are added onto the network. 69 00:06:08,400 --> 00:06:15,540 In other words it's more likely that collisions will take place with a longer cable lengths and more 70 00:06:15,540 --> 00:06:16,620 devices. 71 00:06:16,680 --> 00:06:24,180 So as you add more and more devices to this network and extend the cable length the probability of collisions 72 00:06:24,180 --> 00:06:25,610 increases dramatically. 73 00:06:27,350 --> 00:06:29,640 Now there were other issues with the 10 base too. 74 00:06:29,810 --> 00:06:32,540 The first issue is cable length. 75 00:06:32,630 --> 00:06:36,560 The longer the cable the greater the signal degradation was. 76 00:06:36,680 --> 00:06:43,100 In other words as your cable increased in length the more likely it was that one host's signal wouldn't 77 00:06:43,130 --> 00:06:48,530 be received by another host the host on one side of the cable might send a signal. 78 00:06:48,530 --> 00:06:54,680 But because of segregation a host at the other end of the cable may not be able to receive or interpret 79 00:06:54,800 --> 00:06:55,760 the signal. 80 00:06:55,760 --> 00:07:02,960 Another problem is cable breaks a cable break at any point would cause the entire network to fail. 81 00:07:03,110 --> 00:07:07,940 So if someone accidentally broke the cable at this point the whole network would fail. 82 00:07:07,970 --> 00:07:11,800 Homestay cannot communicate with other devices in the network. 83 00:07:11,810 --> 00:07:19,250 Those devices cannot communicate with state however because of those cable break there's no terminator 84 00:07:19,340 --> 00:07:22,300 on the cable the cable is also damaged. 85 00:07:22,310 --> 00:07:29,810 So what happens is signals get reflected deam might send a signal to see but it's going to continue 86 00:07:29,810 --> 00:07:35,480 across the cable and then it's going to be reflected back causing collisions in the network. 87 00:07:35,480 --> 00:07:42,230 So this network wasn't very robust in that cable breaks could bring down the entire network. 88 00:07:42,230 --> 00:07:48,590 Now it gets worse 10 base 2 implies 10 megabits per second Ethernet. 89 00:07:48,620 --> 00:07:53,510 However this is not 10 megabits per second for each device. 90 00:07:53,510 --> 00:07:58,950 It's 10 megabits per second shared between all devices on that segment. 91 00:07:58,970 --> 00:08:04,830 In addition because of collisions you can only use between 30 and 40 percent. 92 00:08:05,030 --> 00:08:12,430 See only getting 30 to 40 percent utilization collisions increased dramatically above that utilization. 93 00:08:12,440 --> 00:08:16,200 So a conservative figure would be 30 percent utilization. 94 00:08:16,250 --> 00:08:22,180 That means 10 megabits per second would be shared between all devices on that segment. 95 00:08:22,310 --> 00:08:24,470 So in this case we have four devices. 96 00:08:24,470 --> 00:08:31,640 So that means that 10 megabits per second divided by four devices times 30 percent only gives you zero 97 00:08:31,640 --> 00:08:37,590 point seven five megabits per second and not 10 megabits per second per device. 98 00:08:37,610 --> 00:08:43,390 This is not ideal because the bandwidth available to your PC is very low. 99 00:08:43,430 --> 00:08:49,520 He's spacially in a large network so it is more devices are added to the network the bandwidth available 100 00:08:49,520 --> 00:08:51,740 to each device is decreased. 101 00:08:51,740 --> 00:08:55,600 This is also known as a single collision domain. 102 00:08:55,610 --> 00:09:01,570 In other words if a collision takes place at any point in the network all devices in this network are 103 00:09:01,570 --> 00:09:04,490 affected by that collision and would need to back off.