Would you like to inspect the original subtitles? These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:03,000 align:middle line:84% MAC addresses are once again, 48 bits in length 2 00:00:03,000 --> 00:00:07,000 align:middle line:84% but rather than showing MAC addresses as 48 bit values 3 00:00:07,000 --> 00:00:15,000 align:middle line:84% in this demonstrations I’m gonna represent MAC addresses 4 00:00:15,000 --> 00:00:17,000 align:middle line:84% by letter such as A, B, C and D and I’m doing that just for simplicity sake. 5 00:00:17,000 --> 00:00:20,000 align:middle line:84% So what is a hub do with received traffic. 6 00:00:20,000 --> 00:00:24,000 align:middle line:84% So in this example, let's assume that A is sending traffic to C. 7 00:00:24,000 --> 00:00:31,000 align:middle line:84% So the source address of the frame is A and the destination address of the frame is C. 8 00:00:31,000 --> 00:00:35,000 align:middle line:84% A sends that frame to the hub what will a hub do with the frame? 9 00:00:35,000 --> 00:00:42,000 align:middle line:84% now because a hub is a multi port repeater in other words it's simply a repeater 10 00:00:42,000 --> 00:00:47,000 align:middle line:84% with multiple ports and it has no understanding of the traffic it receives 11 00:00:47,000 --> 00:00:54,000 align:middle line:84% it will simply amplify the signal and send the traffic or frames out of all ports. 12 00:00:54,000 --> 00:00:58,000 align:middle line:84% So it literally receives a frame, amplifies it 13 00:00:58,000 --> 00:01:01,000 align:middle line:84% and sends it out of all other ports except the port on which it was received. 14 00:01:01,000 --> 00:01:07,000 align:middle line:84% so every device in this topology will receive the frame sent from A to C. 15 00:01:07,000 --> 00:01:11,000 align:middle line:84% so once again A is sending a frame to C 16 00:01:11,000 --> 00:01:16,000 align:middle line:84% but all devices except A have received the frame. 17 00:01:16,000 --> 00:01:20,000 align:middle line:84% The network interface cards or NICs of B and D will receive the frame 18 00:01:20,000 --> 00:01:20,000 align:middle line:84% and read the destination MAC address, they will see in this example 19 00:01:20,000 --> 00:01:26,000 align:middle line:84% that the destination MAC address is C and therefore the frame is not destined 20 00:01:26,000 --> 00:01:33,000 align:middle line:84% to themselves and the Network Interface Cards will therefore drop the frame. 21 00:01:33,000 --> 00:01:37,000 align:middle line:84% So the frames sent to D and B will be dropped 22 00:01:37,000 --> 00:01:41,000 align:middle line:84% by the Network Interface Cards or NICs of those PCs 23 00:01:41,000 --> 00:01:45,000 align:middle line:84% Host c however will accept the frame because the frame is destined to it. 24 00:01:45,000 --> 00:01:51,000 align:middle line:84% So the Network Interface Card or NIC on PC C will read the destination MAC address 25 00:01:51,000 --> 00:01:55,000 align:middle line:84% and we'll see that the destination MAC address of the frame is at self 26 00:01:55,000 --> 00:01:59,000 align:middle line:84% and it will therefore received the frame, strip the Layer 2 headers 27 00:01:59,000 --> 00:02:04,000 align:middle line:84% and pass the packet to the higher layer protocols on the machine 28 00:02:04,000 --> 00:02:07,000 align:middle line:84% in other words if this is an IPv4 packet it will send 29 00:02:07,000 --> 00:02:13,000 align:middle line:84% the packet to the IPv4 process running on the machine for further processing 30 00:02:13,000 --> 00:02:18,000 align:middle line:84% Now let's assume that A ping C, so it requires return traffic 31 00:02:18,000 --> 00:02:23,000 align:middle line:84% so C replies with the frame with source Mac address being C 32 00:02:23,000 --> 00:02:26,000 align:middle line:84% and the destination MAC address being A. 33 00:02:26,000 --> 00:02:30,000 align:middle line:84% C sends that frame to the hub and what does the hub do with the frame? 34 00:02:30,000 --> 00:02:34,000 align:middle line:84% Now once again a hub is simply a multi port repeater 35 00:02:34,000 --> 00:02:41,000 align:middle line:90% and it will therefore just amplify the signal 36 2:37 -0> 2:40 37 00:02:41,000 --> 00:02:44,000 align:middle line:84% without understanding of the data in the frames. 38 00:02:44,000 --> 00:02:47,000 align:middle line:84% So the frame is sent to both D and B 39 00:02:47,000 --> 00:02:51,000 align:middle line:84% which drop a frame because the destination MAC address is not themselves 40 00:02:51,000 --> 00:02:56,000 align:middle line:84% A will accept the frame because it destined to it, it will then strip the layer 2 41 00:02:56,000 --> 00:03:01,000 align:middle line:84% headers and send the data to higher layer protocols for further processing. 42 00:03:01,000 --> 00:03:06,000 align:middle line:84% So A and C are communicating with one another but it’s important to realize 43 00:03:06,000 --> 00:03:12,000 align:middle line:84% that the hub is a physical layer device that is simply a multi port repeater 44 00:03:12,000 --> 00:03:15,000 align:middle line:84% and will therefore amplify frames out of all interfaces. 45 00:03:15,000 --> 00:03:21,000 align:middle line:84% So B and D will see all the frames sent between A and C. 46 00:03:21,000 --> 00:03:27,000 align:middle line:84% Physically this topology is a star topology but logically it doesn’t work that way. 47 00:03:27,000 --> 00:03:33,000 align:middle line:84% The physical topology of a hub is a star but logically it's a bus. 48 00:03:33,000 --> 00:03:36,000 align:middle line:84% It’s very important to realize that there’s a difference 49 00:03:36,000 --> 00:03:41,000 align:middle line:84% between a physical and logical topology in networks. 50 00:03:41,000 --> 00:03:44,000 align:middle line:84% The way the network is physically cabled 51 00:03:44,000 --> 00:03:47,000 align:middle line:84% isn’t necessarily the way the network is going to operate. 52 00:03:47,000 --> 00:03:52,000 align:middle line:84% It is important to remember that when a device sends traffic in a hub environment 53 00:03:52,000 --> 00:03:59,000 align:middle line:84% all devices receive a frame, that's exactly the way it works in 10base2 or 10base5. 54 00:03:59,000 --> 00:04:06,000 align:middle line:84% A hub operates in the same way is 10base2 because when A sends a frame 55 00:04:06,000 --> 00:04:13,000 align:middle line:84% unto the network all devices receive the frame in the same way as 10base2. 56 00:04:13,000 --> 00:04:17,000 align:middle line:84% Just like in 10base2 environment when there's a collision on the network 57 00:04:17,000 --> 00:04:20,000 align:middle line:84% it will affect all devices in the network. 58 00:04:20,000 --> 00:04:22,000 align:middle line:84% This is a single collision domain. 59 00:04:22,000 --> 00:04:28,000 align:middle line:84% A collision anywhere will cause devices to back off, send a jamming signal 60 00:04:28,000 --> 00:04:30,000 align:middle line:84% and then attempt to transmit again. 61 00:04:30,000 --> 00:04:34,000 align:middle line:84% As you increase the number of devices in a hub environment 62 00:04:34,000 --> 00:04:39,000 align:middle line:84% the number of collisions increases and your network throughput goes down. 63 00:04:39,000 --> 00:04:45,000 align:middle line:84% In addition broadcast are received by everyone as this is a single broadcast domain. 64 00:04:45,000 --> 00:04:48,000 align:middle line:84% A broadcast sent by B is received by everyone. 65 00:04:48,000 --> 00:04:53,000 align:middle line:84% It’s a single broadcast domain because all devices need to process broadcast 66 00:04:53,000 --> 00:04:56,000 align:middle line:84% sent by every other device in the network. 67 00:04:56,000 --> 00:04:59,000 align:middle line:84% Broadcast traffic will flood through the entire network 68 00:04:59,000 --> 00:05:04,000 align:middle line:84% and interrupt the CPU of every device which is obviously not ideal. 69 00:05:04,000 --> 00:05:08,000 align:middle line:84% From a bandwidth point of view this maybe 10baseT 70 00:05:08,000 --> 00:05:16,000 align:middle line:84% where 10 means 10 Mbps but its 10 Mbps shared between all devices. 71 00:05:16,000 --> 00:05:20,000 align:middle line:84% So assuming that we have 10 Mbps like we do in this example. 72 00:05:20,000 --> 00:05:26,000 align:middle line:84% And they are four devices in the network with a maximum utilization of 30% 73 00:05:26,000 --> 00:05:31,000 align:middle line:84% that means that each device only gets 0.75 Mbps throughput 74 00:05:31,000 --> 00:05:40,000 align:middle line:84% its not 10 Mbps dedicated its 10 Mbps shared between all the devices. 75 00:05:40,000 --> 00:05:44,000 align:middle line:84% Once again because it's shared you need to divide the bandwidth 76 00:05:44,000 --> 00:05:48,000 align:middle line:84% by the number of devices in a shared Ethernet environment. 77 00:05:48,000 --> 00:05:52,000 align:middle line:84% And because you’re not generally getting more than 30 to 40% utilization 78 00:05:52,000 --> 00:05:56,000 align:middle line:84% because of collisions on the network you need to multiply that 79 00:05:56,000 --> 00:05:59,000 align:middle line:84% by 30%, 30% being a conservative value. 80 00:05:59,000 --> 00:06:08,000 align:middle line:84% So your bandwidth is 10 divided by 4*30% which equates to 0.75 Mbps 81 00:06:08,000 --> 00:06:13,000 align:middle line:84% which is obviously not very good. 10200