1 00:00:00,200 --> 00:00:03,200 Aussie Open Systems interconnection. 2 00:00:03,810 --> 00:00:08,370 Is a reference model for how applications can communicate over a network. 3 00:00:08,400 --> 00:00:13,440 A reference model is a conceptual framework for understanding relationships. 4 00:00:14,010 --> 00:00:20,430 The purpose of the OSI reference model is to guide vendors and developers so that the digital communication 5 00:00:20,430 --> 00:00:27,510 products and software programs they create will interoperate and to facilitate clear comparisons among 6 00:00:27,510 --> 00:00:29,010 communications tools. 7 00:00:29,040 --> 00:00:35,400 Most vendors involved in the telecommunications make an attempt to describe their products and services 8 00:00:35,400 --> 00:00:42,210 in relation to the OSI model and although useful for guiding discussion and evaluation. 9 00:00:42,240 --> 00:00:50,730 OSI is rarely actually implemented as few network products or standard tools keep all related functions 10 00:00:50,730 --> 00:00:58,020 together in well defined layers as related to the model developed by representatives of major computer 11 00:00:58,020 --> 00:01:01,770 and telecommunication companies beginning in 1983. 12 00:01:02,010 --> 00:01:07,740 OSI was originally intended to be a detailed specification of actual interfaces. 13 00:01:07,770 --> 00:01:14,770 Instead, the committee decided to establish a common reference model for which others could then develop 14 00:01:14,770 --> 00:01:18,880 detailed interfaces, which in turn could become standards. 15 00:01:19,390 --> 00:01:25,300 OSI was officially adopted as the international standard by the International Organization of Standards 16 00:01:25,330 --> 00:01:26,290 ISO. 17 00:01:26,830 --> 00:01:30,220 So what about the ozone layers? 18 00:01:30,860 --> 00:01:38,180 The main concept of ozone is that the process of communication between two end points in a telecommunication 19 00:01:38,180 --> 00:01:44,420 network can be divided into seven distinct groups of related functions or layers. 20 00:01:45,010 --> 00:01:52,330 Each communicating user or program is at a computer that can provide those seven layers of function. 21 00:01:52,330 --> 00:01:58,750 So in a given message between users, there will be a flow of data down through the layers in the source 22 00:01:58,750 --> 00:02:04,240 computer across the network and then up through the layers in the receiving computer. 23 00:02:04,270 --> 00:02:10,690 The seven layers of function are provided by a combination of applications operating systems, network 24 00:02:10,690 --> 00:02:17,650 card device drivers and networking hardware that enable a system to put a signal on a network cable 25 00:02:17,650 --> 00:02:21,610 or out over a Wi-Fi or any other wireless protocol. 26 00:02:21,850 --> 00:02:25,660 The seven Open Systems interconnection layers are. 27 00:02:26,950 --> 00:02:29,200 Layer seven, the application layer. 28 00:02:29,440 --> 00:02:33,280 This is a layer at which communication partners are identified. 29 00:02:33,310 --> 00:02:35,140 Is there someone to talk to? 30 00:02:36,260 --> 00:02:38,480 Network capacity is assessed. 31 00:02:38,510 --> 00:02:42,740 Will the network let me talk to them right now and. 32 00:02:43,400 --> 00:02:47,660 They create a thing to send or opens the thing received. 33 00:02:48,140 --> 00:02:51,790 Please note that this layer is not the application itself. 34 00:02:51,800 --> 00:02:57,150 It is the set of services an application should be able to make use of directly. 35 00:02:57,170 --> 00:03:01,610 Although some applications may perform application layer functions. 36 00:03:02,270 --> 00:03:04,850 Layer six, the presentation layer. 37 00:03:05,270 --> 00:03:12,800 This layer is usually part of an operating system and converts incoming and outgoing data from one presentation 38 00:03:12,800 --> 00:03:14,170 format to another. 39 00:03:14,180 --> 00:03:21,590 For example, from cleartext to encrypted text at one end and back to clear text at the other. 40 00:03:22,660 --> 00:03:24,880 Layer five, the session layer. 41 00:03:25,060 --> 00:03:29,350 This layer sets up coordinates and terminates conversations. 42 00:03:29,900 --> 00:03:32,990 Services include authentication and reconnection. 43 00:03:32,990 --> 00:03:40,130 After an interruption on the Internet Transmission control protocol, TCP and User Datagram Protocol 44 00:03:40,160 --> 00:03:43,760 UDP provide these services for most applications. 45 00:03:44,760 --> 00:03:47,040 Layer four, the transport layer. 46 00:03:47,070 --> 00:03:50,310 This layer manages packetization of data. 47 00:03:50,340 --> 00:03:56,190 Then the delivery of the packets, including checking for errors in the data once it arrives. 48 00:03:56,920 --> 00:04:01,810 On the Internet, TCP and UDP provide these services for most applications as well. 49 00:04:02,850 --> 00:04:04,920 Layer three, the network layer. 50 00:04:05,250 --> 00:04:10,530 This layer handles the addressing and routing of the data, sending it in the right direction to the 51 00:04:10,530 --> 00:04:17,519 right destination on outgoing transmissions and receiving incoming transmissions at the packet level. 52 00:04:18,060 --> 00:04:21,029 IP is a network layer for the internet. 53 00:04:21,860 --> 00:04:24,230 Layer two, The data link layer. 54 00:04:24,780 --> 00:04:30,840 This layer sets up links across the physical network putting packets into network frames. 55 00:04:31,110 --> 00:04:33,510 This layer has two sublayers. 56 00:04:33,540 --> 00:04:37,830 The logical link control layer and the media access control layer. 57 00:04:37,860 --> 00:04:40,560 Ethernet is the main data link layer in use. 58 00:04:41,270 --> 00:04:43,700 Layer one, the physical layer. 59 00:04:44,030 --> 00:04:50,000 This layer conveys the bitstream through the network at the electrical, optical or radio level. 60 00:04:50,270 --> 00:04:55,250 It provides the hardware means of sending and receiving data on a carrier network. 61 00:04:55,580 --> 00:04:58,790 Let's see some of the most famous protocols of the layers. 62 00:04:59,670 --> 00:05:00,390 For ease of use. 63 00:05:00,420 --> 00:05:04,500 It's better to talk about physical layer and data link layer together. 64 00:05:05,130 --> 00:05:10,710 Here we have some protocols and data link layer and some physical media and connection methodologies 65 00:05:10,710 --> 00:05:12,030 in physical layer. 66 00:05:12,060 --> 00:05:18,570 Ethernet and 802.11 Wireless LAN are the most known protocols of the data link layer. 67 00:05:19,280 --> 00:05:24,380 Ethernet is the name of the most commonly used network protocol that controls how data is transmitted 68 00:05:24,380 --> 00:05:27,410 over a LAN, which is a local area network. 69 00:05:28,250 --> 00:05:33,790 You need to have network interface cards in the devices that you want to connect to the network. 70 00:05:34,320 --> 00:05:36,510 A wireless local area network. 71 00:05:36,510 --> 00:05:43,560 W Wlan is a wireless computer network protocol that links two or more devices using wireless communication 72 00:05:43,560 --> 00:05:49,590 within a limited area such as a home school, computer laboratory or office building. 73 00:05:49,620 --> 00:05:56,160 This gives users the ability to move around with a local coverage area and yet still be connected to 74 00:05:56,160 --> 00:05:57,030 the network. 75 00:05:57,060 --> 00:06:04,230 Most modern LANs are based on I triple IEEE 802.11 standards and are marketed under the Wi-Fi brand 76 00:06:04,230 --> 00:06:04,710 name. 77 00:06:05,940 --> 00:06:13,650 IP Internet protocol is responsible for addressing hosts, encapsulating data into transferred packets 78 00:06:13,650 --> 00:06:20,370 and routing packets from a source host to a destination host across one or more IP networks. 79 00:06:21,460 --> 00:06:27,130 The best known transport protocol is the transmission control protocol or TCP. 80 00:06:27,340 --> 00:06:33,520 It's used for connection oriented transmissions, whereas the Connectionless user datagram protocol. 81 00:06:33,550 --> 00:06:37,400 UDP is used for simpler messaging transmissions. 82 00:06:37,420 --> 00:06:41,140 We're going to talk more in depth about these protocols in the next lecture. 83 00:06:42,440 --> 00:06:48,230 Again, for ease of use, the last three layers session layer, presentation, layer and application 84 00:06:48,230 --> 00:06:52,250 layer are thought of together as the application layer. 85 00:06:53,910 --> 00:06:55,950 Let me put another parenthesis here. 86 00:06:56,340 --> 00:07:00,130 We're talking about the OSI reference model here. 87 00:07:00,150 --> 00:07:04,980 In addition, there is another reference model called TCP IP reference model. 88 00:07:04,980 --> 00:07:12,600 And instead of OSI models last three layers, there is only a single application layer in the TCP IP 89 00:07:12,600 --> 00:07:13,560 reference model. 90 00:07:13,590 --> 00:07:15,540 Just keep that in the back of your mind. 91 00:07:16,950 --> 00:07:18,240 And back to our subject. 92 00:07:19,040 --> 00:07:20,120 The application layer. 93 00:07:20,120 --> 00:07:27,590 Protocols are classified according to the protocol they are using in the transport layer, and these 94 00:07:27,590 --> 00:07:31,150 protocols interact with the end user via applications. 95 00:07:31,160 --> 00:07:35,750 Therefore, they are the most known protocols by just about everybody. 96 00:07:37,000 --> 00:07:41,350 Some of the most well known TCP based application layer protocols are. 97 00:07:42,420 --> 00:07:45,920 Http hypertext transfer protocol. 98 00:07:45,980 --> 00:07:51,740 Simply a communications protocol used to send and receive web pages and files on the Internet. 99 00:07:52,270 --> 00:07:56,980 Telnet is one of the simplest ways to exchange data between two computers. 100 00:07:57,310 --> 00:08:03,970 It allows two computers anywhere on a computer network, including the worldwide Internet, to exchange 101 00:08:03,970 --> 00:08:07,030 text and, well, other data in real time. 102 00:08:08,410 --> 00:08:14,950 FTP file Transfer protocol is a communication protocol for the rapid simple transmission of files across 103 00:08:14,950 --> 00:08:15,790 a network. 104 00:08:17,310 --> 00:08:24,720 Smtp simple mail transfer protocol is used to send and relay an email message between email servers. 105 00:08:24,750 --> 00:08:28,680 Note that it is not used to retrieve email messages from a server. 106 00:08:28,680 --> 00:08:33,000 Instead, either iMap or Pop is used to retrieve email messages. 107 00:08:34,440 --> 00:08:35,610 DNS. 108 00:08:35,610 --> 00:08:42,240 The domain name system is a system used to convert a computer's hostname into an IP address on the Internet. 109 00:08:42,419 --> 00:08:49,140 For example, if a computer needs to communicate with a web server nhs.uk, your computer needs the 110 00:08:49,140 --> 00:08:50,880 IP address of the web server. 111 00:08:50,890 --> 00:08:52,470 Nhs.uk. 112 00:08:52,680 --> 00:08:58,620 It is the job of the DNS to convert the hostname to the IP address of the web server. 113 00:08:58,650 --> 00:09:02,230 The DNS uses both the UDP and TCP. 114 00:09:03,390 --> 00:09:10,140 SNMp Simple Network Management Protocol is used in network management systems to monitor status of devices 115 00:09:10,140 --> 00:09:12,090 and also spot problems. 116 00:09:12,390 --> 00:09:16,500 So let's see what happens to a packet inside the network traffic. 117 00:09:17,040 --> 00:09:23,550 Please note the data unit transferred between two endpoints has different names in each layer. 118 00:09:23,760 --> 00:09:26,490 In layers five, six and seven application layers. 119 00:09:26,490 --> 00:09:30,300 It's called data in layer four transport layer. 120 00:09:30,300 --> 00:09:39,030 It's called segment for TCP and Datagram for UDP in layer three, the network layer, it's called packet 121 00:09:39,270 --> 00:09:44,220 in layer two, the data link layer, the data unit is called frame. 122 00:09:44,430 --> 00:09:51,570 Now I usually use packet for the transfer data unit in each layer to make it less complicated for you. 123 00:09:51,600 --> 00:09:52,320 Make sense? 124 00:09:52,590 --> 00:09:58,380 Before looking at the road trip of a DNS query in detail, let's look at the relationship between the 125 00:09:58,380 --> 00:10:01,380 OSI layers and computer systems. 126 00:10:02,100 --> 00:10:07,800 The packets and layer one and layer two are managed by network interfaces of your device, for example, 127 00:10:07,830 --> 00:10:09,390 by your Ethernet card. 128 00:10:09,750 --> 00:10:15,750 And the packets from 3 to 7 are managed by the processing unit in software detail. 129 00:10:16,310 --> 00:10:22,190 The layer three and layer four packets are managed by the operating system of your devices, and the 130 00:10:22,190 --> 00:10:29,120 packets of layer 5 to 7 are managed by the related application or service, for example, a web browser. 131 00:10:29,300 --> 00:10:33,170 When you write a URL in the address bar of your browser and hit enter. 132 00:10:33,890 --> 00:10:36,530 The first thing it sends is a DNS query. 133 00:10:36,830 --> 00:10:44,060 A DNS query is the process of a computer networking device making an inquiry to get an IP address for 134 00:10:44,060 --> 00:10:47,240 a DNS name such as Mail Yahoo.com. 135 00:10:48,340 --> 00:10:54,760 The client computer will send a DNS query to one of their Internet service providers DNS servers. 136 00:10:54,820 --> 00:11:02,560 The DNS server looks in its DNS database to tell whether it can answer the query authoritatively. 137 00:11:03,040 --> 00:11:10,090 If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process 138 00:11:10,120 --> 00:11:11,200 is complete. 139 00:11:11,440 --> 00:11:16,340 So let's see the road trip of a DNS query from your computer to a DNS server. 140 00:11:16,360 --> 00:11:22,930 When data is transmitted by the source toward a specific destination, it passes through the application 141 00:11:22,930 --> 00:11:28,300 presentation and session layers, and the protocol data unit arrives at the transport layer. 142 00:11:28,330 --> 00:11:29,410 Layer four. 143 00:11:29,560 --> 00:11:33,130 Layers five, six and seven are displayed as a single layer. 144 00:11:33,130 --> 00:11:39,970 To simplify the presentation at this layer, a 20 byte data header is placed in front of the data. 145 00:11:40,090 --> 00:11:46,270 A DNS query can also use TCP, but let's assume that it uses UDP at this time. 146 00:11:46,300 --> 00:11:53,530 The data and the layer four header here, it's UDP header which together form a segment or datagram 147 00:11:53,770 --> 00:11:55,690 is passed down to layer three. 148 00:11:55,720 --> 00:11:57,070 The network layer. 149 00:11:57,310 --> 00:12:04,010 The network layer places its layer three header here, the IP header in front of the received segment, 150 00:12:04,010 --> 00:12:06,500 and this group becomes a packet. 151 00:12:07,060 --> 00:12:13,810 The layer three header contains important fields such as the logical address, the IP address of both 152 00:12:13,810 --> 00:12:16,330 the source and the destination device. 153 00:12:16,880 --> 00:12:20,210 The newly formed packet is then passed down to layer two. 154 00:12:20,540 --> 00:12:27,830 The data link layer creates a new data unit called a Frame by adding the layer two frame header, which 155 00:12:27,830 --> 00:12:29,450 is the Ethernet header here. 156 00:12:30,140 --> 00:12:31,430 Like Layer three. 157 00:12:31,460 --> 00:12:35,360 An addressing structure is also applied in the layer two header. 158 00:12:35,900 --> 00:12:37,850 That is the Mac address. 159 00:12:38,090 --> 00:12:41,000 The frame is then passed down to the physical layer. 160 00:12:41,730 --> 00:12:48,120 Which converts the information into zero and one bits that are sent over the physical media using electrical 161 00:12:48,120 --> 00:12:51,090 signals on a copper link, for instance. 162 00:12:51,700 --> 00:12:58,180 Finally, the data is sent over the wire, using a wide variety of methods such as Ethernet or token 163 00:12:58,180 --> 00:12:58,690 ring. 164 00:12:58,780 --> 00:13:05,080 The headers are a specific form of control information that allows the data to go through the network 165 00:13:05,080 --> 00:13:05,980 properly. 166 00:13:05,990 --> 00:13:12,580 Thus, the data at each layer is encapsulated in the information appropriate for the specific layer, 167 00:13:12,760 --> 00:13:16,060 including addressing and error checking. 168 00:13:16,830 --> 00:13:23,130 The overall size of the information increases as the data travels through the lower layers from layer 169 00:13:23,130 --> 00:13:24,540 four to layer one. 170 00:13:24,780 --> 00:13:31,290 The destination device, which is the DNS server here, receives the data and this additional information 171 00:13:31,290 --> 00:13:40,380 is analyzed and then is removed as the data passes through the higher layers up to the application layer 172 00:13:40,380 --> 00:13:43,380 where the data is capsulated.