1 00:00:00,240 --> 00:00:03,060 As I open systems interconnection. 2 00:00:04,000 --> 00:00:10,870 Is a reference model for how applications can communicate over a network, a reference model is a conceptual 3 00:00:10,870 --> 00:00:13,260 framework for understanding relationships. 4 00:00:14,180 --> 00:00:20,510 The purpose of the U.S. reference model is to guide vendors and developers so that the digital communication 5 00:00:20,510 --> 00:00:27,590 products and software programs they create will interoperate and to facilitate clear comparisons among 6 00:00:27,590 --> 00:00:28,770 communications tools. 7 00:00:29,360 --> 00:00:35,480 Most vendors involved in the telecommunications make an attempt to describe their products and services 8 00:00:35,480 --> 00:00:37,840 in relation to the OWI model. 9 00:00:38,390 --> 00:00:46,700 And although useful for guiding discussion and evaluation, OAC is rarely actually implemented as few 10 00:00:46,700 --> 00:00:54,080 network products or standard tools keep all related functions together in well-defined layers as related 11 00:00:54,080 --> 00:00:59,800 to the model developed by representatives of major computer and telecommunication companies. 12 00:00:59,810 --> 00:01:07,650 Beginning in 1983, OAC was originally intended to be a detailed specification of actual interfaces. 13 00:01:07,940 --> 00:01:14,720 Instead, the committee decided to establish a common reference model for which others could then develop 14 00:01:14,720 --> 00:01:18,590 detailed interfaces, which in turn could become standards. 15 00:01:19,580 --> 00:01:24,920 As I was officially adopted as the international standard by the International Organization of Standards 16 00:01:25,310 --> 00:01:26,060 ISO. 17 00:01:27,060 --> 00:01:30,060 So what about the OK as I layard's? 18 00:01:30,970 --> 00:01:38,290 The main concept of Ossi is that the process of communication between two end points in a telecommunication 19 00:01:38,290 --> 00:01:44,200 network can be divided into seven distinct groups of related functions or layers. 20 00:01:45,170 --> 00:01:52,200 Each communicating user program is at a computer that can provide those seven layers a function. 21 00:01:52,580 --> 00:01:58,850 So in a given message between users, there will be a flow of data down through the layers in the source 22 00:01:58,850 --> 00:02:04,120 computer across the network and then up through the layers in the receiving computer. 23 00:02:04,550 --> 00:02:10,790 The seven layers of function are provided by a combination of applications operating systems, network 24 00:02:10,790 --> 00:02:17,720 card device drivers and networking hardware that enable a system to put a signal on a network cable 25 00:02:17,720 --> 00:02:21,430 or out over Wi-Fi or any other wireless protocol. 26 00:02:22,130 --> 00:02:25,520 The seven open systems interconnection layers are. 27 00:02:27,080 --> 00:02:33,140 Layer seven, the application layer, this is a layer at which communication partners are identified. 28 00:02:33,560 --> 00:02:35,000 Is there someone to talk to? 29 00:02:36,420 --> 00:02:42,540 Network capacity is assessed, will the network let me talk to them right now and. 30 00:02:43,520 --> 00:02:47,540 That creates a thing to send or opens the thing received. 31 00:02:48,360 --> 00:02:54,780 Please note that this layer is not the application itself, it is the set of services and applications 32 00:02:54,780 --> 00:03:01,440 should be able to make use of directly, although some applications may perform application layer functions. 33 00:03:02,490 --> 00:03:09,540 Layer six, the presentation layer, this layer is usually part of an operating system and converts 34 00:03:09,540 --> 00:03:16,530 incoming and outgoing data from one presentation format to another, for example, from clear text to 35 00:03:16,530 --> 00:03:21,480 encrypted text at one end and back to clear text to the other. 36 00:03:22,730 --> 00:03:29,180 Layer five, the session layer, this layer sets up coordinates and terminates conversations. 37 00:03:30,080 --> 00:03:35,780 Services include authentication and reconnection after an interruption on the Internet Transmission 38 00:03:35,780 --> 00:03:43,610 Control Protocol TCP and User Data Graem Protocol UDP provide these services for most applications. 39 00:03:44,940 --> 00:03:52,350 Layer for the transport layer, this layer manages packetized of data, then the delivery of the packet, 40 00:03:52,590 --> 00:03:56,040 including checking for errors and the data once it arrives. 41 00:03:57,080 --> 00:04:01,670 On the Internet, TCP and UDP provide these services for most applications as well. 42 00:04:02,890 --> 00:04:09,130 Layer three, the network layer, this layer handles the addressing and routing of the data, sending 43 00:04:09,130 --> 00:04:15,280 it in the right direction to the right destination on outgoing transmissions and receiving incoming 44 00:04:15,280 --> 00:04:17,350 transmissions at the packet level. 45 00:04:18,160 --> 00:04:20,890 IP is a network layer for the Internet. 46 00:04:22,090 --> 00:04:24,070 Layer to that datalink layer. 47 00:04:24,990 --> 00:04:32,070 This layer sets up links across the physical network, putting packets into network frames, this layer 48 00:04:32,070 --> 00:04:37,920 has two sub layers the logical link control layer and the media access control layer. 49 00:04:37,950 --> 00:04:40,380 Ethernet is the main data link layer and use. 50 00:04:41,420 --> 00:04:48,170 Layer one, the physical layer, this layer conveys the bitstream through the network at the electrical, 51 00:04:48,170 --> 00:04:55,040 optical or radio level, it provides the hardware means of sending and receiving data on a carrier network. 52 00:04:55,900 --> 00:04:58,640 Let's see some of the most famous protocols of the layers. 53 00:04:59,730 --> 00:05:04,320 Reserve use, it's better to talk about physical layer and data link layer together. 54 00:05:05,280 --> 00:05:10,890 Here we have some protocols and datalink layer and some physical media and connection methodologies 55 00:05:10,890 --> 00:05:18,120 in physical layer Ethernet and Eddo two one one wireless lan are the most known protocols of the datalink 56 00:05:18,120 --> 00:05:18,450 layer. 57 00:05:19,390 --> 00:05:24,490 Ethernet is the name of the most commonly used network protocol that controls how data is transmitted 58 00:05:24,490 --> 00:05:27,280 over a land which is a local area network. 59 00:05:28,400 --> 00:05:33,590 You need to have network interface card in the devices that you want to connect to the network. 60 00:05:34,430 --> 00:05:42,080 A wireless local area network LAN is a wireless computer network protocol that links two or more devices 61 00:05:42,080 --> 00:05:48,620 using wireless communication within a limited area such as a home, school, computer, laboratory or 62 00:05:48,620 --> 00:05:49,430 office building. 63 00:05:49,970 --> 00:05:56,240 This gives users the ability to move around with a local coverage area and yet still be connected to 64 00:05:56,240 --> 00:05:56,850 the network. 65 00:05:57,380 --> 00:06:03,950 Most modern plans are based on, I believe, two point one one standard and are marketed under the Wi-Fi 66 00:06:03,950 --> 00:06:04,550 brand name. 67 00:06:05,950 --> 00:06:13,570 IP Internet Protocol is responsible for addressing host, encapsulating data into transferred packets 68 00:06:13,870 --> 00:06:20,200 and routing packets from a source host to a destination host across one or more IP networks. 69 00:06:21,570 --> 00:06:28,650 The best known transport protocol is the Transmission Control Protocol, or TCP, it's used for connection 70 00:06:28,650 --> 00:06:36,000 oriented transmissions, whereas the connection less user data Graham Protocol UDP is used for simpler 71 00:06:36,000 --> 00:06:37,290 messaging transmissions. 72 00:06:37,650 --> 00:06:41,040 We're going to talk more in depth about these protocols in the next lecture. 73 00:06:42,490 --> 00:06:48,340 Again, for ease of use, the last three layers session, layer, presentation layer and application 74 00:06:48,340 --> 00:06:52,090 layer are thought of together as the application layer. 75 00:06:54,150 --> 00:07:00,010 Let me put another parenthesis here, we're talking about the ISI reference model here. 76 00:07:00,330 --> 00:07:04,860 In addition, there is another reference model called TCP IP reference model. 77 00:07:05,220 --> 00:07:12,720 And instead of OSFI models last three layers, there is only a single application layer in the TCP IP 78 00:07:12,720 --> 00:07:13,560 reference model. 79 00:07:13,740 --> 00:07:15,390 Just keep that in the back of your mind. 80 00:07:17,030 --> 00:07:18,050 And back to our subject. 81 00:07:19,140 --> 00:07:26,100 The application layer protocols are classified according to the protocol they are using in the transport 82 00:07:26,100 --> 00:07:31,250 layer, and these protocols interact with the end user via applications. 83 00:07:31,260 --> 00:07:35,640 Therefore, they are the most known protocols by just about everybody. 84 00:07:37,170 --> 00:07:41,190 Some of the most well-known TCP based application layer protocols are. 85 00:07:42,450 --> 00:07:50,190 HTP Hypertext Transfer Protocol, simply a communications protocol used to send and receive Web pages 86 00:07:50,190 --> 00:07:51,530 and files on the Internet. 87 00:07:52,450 --> 00:07:58,900 Telnet is one of the simplest ways to exchange data between two computers, it allows two computers 88 00:07:58,900 --> 00:08:05,740 anywhere on a computer network, including the worldwide Internet, to exchange text and, well, other 89 00:08:05,740 --> 00:08:06,880 data in real time. 90 00:08:08,300 --> 00:08:14,690 FTP File Transfer Protocol is a communication protocol for the rapid, simple transmission of files 91 00:08:14,690 --> 00:08:15,650 across a network. 92 00:08:17,170 --> 00:08:24,570 SMTP, simple mail transfer protocol is used to send in relay an email message between email servers, 93 00:08:25,030 --> 00:08:28,720 note that it is not used to retrieve email messages from a server. 94 00:08:28,730 --> 00:08:32,860 Instead, either IMAP or Pop is used to retrieve email messages. 95 00:08:34,540 --> 00:08:41,410 DNS, the domain name system, is a system used to convert a computer's hostname into an IP address 96 00:08:41,410 --> 00:08:41,980 on the Internet. 97 00:08:42,730 --> 00:08:48,760 For example, if a computer needs to communicate with a Web server, NHS, dot, UK, your computer 98 00:08:48,760 --> 00:08:50,960 needs the IP address of the Web server. 99 00:08:51,020 --> 00:08:58,630 NHS, UK, it is the job of the DNS to convert the hostname to the IP address of the web server. 100 00:08:58,780 --> 00:09:02,370 The DNS uses both the UDP and TCP. 101 00:09:03,250 --> 00:09:10,270 SMP simple network management protocol is used in network management systems to monitor status of devices 102 00:09:10,270 --> 00:09:11,900 and also spot problems. 103 00:09:12,700 --> 00:09:16,360 So let's see what happens to a packet inside the network traffic. 104 00:09:17,260 --> 00:09:24,160 Please note the data unit transferred between two end points, has different names in each layer and 105 00:09:24,160 --> 00:09:26,570 layers five, six and seven application layers. 106 00:09:26,590 --> 00:09:30,390 It's called data in layer for transport layer. 107 00:09:30,430 --> 00:09:34,540 It's called segment for TCP and data gram for UDP. 108 00:09:34,990 --> 00:09:37,200 And Layer three, the network layer. 109 00:09:37,480 --> 00:09:41,680 It's called packet and layer two, the data link layer. 110 00:09:41,890 --> 00:09:44,010 The data unit is called frame. 111 00:09:44,730 --> 00:09:51,550 Now I usually use packet for the transfer data unit in each layer to make it less complicated for you. 112 00:09:51,670 --> 00:09:52,150 Makes sense. 113 00:09:52,840 --> 00:09:58,330 Before looking at the road trip of a DNS query in detail, let's look at the relationship between the 114 00:09:58,330 --> 00:10:01,210 oscillators, layers and computer systems. 115 00:10:02,280 --> 00:10:07,890 The packets in layer one and layer two are managed by network interfaces of your device, for example, 116 00:10:07,890 --> 00:10:14,970 by your Ethernet card, and the packets from three to seven are managed by the processing unit in software 117 00:10:14,970 --> 00:10:15,570 detail. 118 00:10:16,560 --> 00:10:22,650 The layer three and layer four packets are managed by the operating system of your devices and the packets 119 00:10:22,650 --> 00:10:28,980 of layer five to seven are managed by the related application or service, for example, a Web browser, 120 00:10:29,580 --> 00:10:33,000 when you write a new URL in the address bar of your browser and hit enter. 121 00:10:34,010 --> 00:10:40,640 The first thing it sends is a DNS query, a DNS query is the process of a computer networking device 122 00:10:40,820 --> 00:10:47,150 making an inquiry to get an IP address for a DNS name such as Mail Dot, Yahoo Dotcom. 123 00:10:48,510 --> 00:10:54,540 The client computer will send a DNS query to one of their Internet service providers, DNS servers, 124 00:10:55,170 --> 00:11:02,370 the DNS server looks in its DNS database to tell whether it can answer the query authoritatively. 125 00:11:03,220 --> 00:11:10,150 If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process 126 00:11:10,240 --> 00:11:11,010 is complete. 127 00:11:11,710 --> 00:11:16,210 So let's see the road trip of a DNS query from your computer to a DNS server. 128 00:11:16,600 --> 00:11:22,900 When data is transmitted by the source toward a specific destination, it passes through the application 129 00:11:23,110 --> 00:11:28,240 presentation and Sesson layers and the protocol data unit arrives at the transport layer. 130 00:11:28,510 --> 00:11:34,810 Layer four layers five, six and seven are displayed as a single layer to simplify the presentation. 131 00:11:35,320 --> 00:11:39,820 At this layer, a 20 byte data header is placed in front of the data. 132 00:11:40,210 --> 00:11:46,120 A DNS query can also use TCP IP, but let's assume that it uses UDP at this time. 133 00:11:46,600 --> 00:11:53,320 The data and the layer for header here, its UDP header, which together form a segment or data gram 134 00:11:54,070 --> 00:11:55,570 is passed down to layer three. 135 00:11:55,930 --> 00:11:58,930 The network layer, the network layer places. 136 00:11:58,930 --> 00:12:05,650 It's layer three header here, the IP header in front of the received segment and this group becomes 137 00:12:05,650 --> 00:12:06,190 a packet. 138 00:12:07,170 --> 00:12:13,890 The Layer three header contains important fields such as the logical address, the IP address of both 139 00:12:13,890 --> 00:12:16,110 the source and the destination device. 140 00:12:17,050 --> 00:12:23,830 The newly formed packet is then passed down the layer to the datalink layer creates a new data unit 141 00:12:23,830 --> 00:12:29,290 called a frame by adding the layer to frame header, which is the Ethernet header here. 142 00:12:30,400 --> 00:12:35,170 Like layer three, an addressing structure is also applied in the layer to header. 143 00:12:36,190 --> 00:12:40,840 That is the Mac address, the frame is then passed down to the physical layer. 144 00:12:41,930 --> 00:12:48,200 Which converts the information into zero and one bits that are sent over the physical media using electrical 145 00:12:48,200 --> 00:12:50,930 signals on a copper link, for instance. 146 00:12:51,850 --> 00:12:58,540 Finally, the data is sent over the wire using a wide variety of methods such as Ethernet or token ring, 147 00:12:59,080 --> 00:13:05,140 the headers are a specific form of control information that allows the data to go through the network 148 00:13:05,140 --> 00:13:05,860 properly. 149 00:13:06,220 --> 00:13:12,370 That's the data at each layer is encapsulated in the information appropriate for this specific layer, 150 00:13:13,060 --> 00:13:15,910 including a dressing and error checking. 151 00:13:16,900 --> 00:13:23,200 The overall size of the information increases as the data travels through the lower layers from layer 152 00:13:23,200 --> 00:13:30,130 four to layer one, the destination device, which is the DNS server here, receives the data and this 153 00:13:30,130 --> 00:13:38,140 additional information is analyzed and then is removed as the data passes through the higher layers 154 00:13:38,530 --> 00:13:43,210 up to the application layer where the data is the capsule lated. 155 00:13:44,200 --> 00:13:50,380 The physical address, commonly the Mac address, which is located in a special field in the data link 156 00:13:50,380 --> 00:13:57,490 layer header, changes as the packet passes from one device to another, for example, from the source 157 00:13:57,490 --> 00:14:03,310 PC to a switch to a router to another switch and finally to the destination PC. 158 00:14:04,740 --> 00:14:10,650 However, the original IP source and destination addresses do not change when transiting the network 159 00:14:11,280 --> 00:14:15,540 because the packet is stripped of its layer three header only when it goes beyond a router. 160 00:14:16,850 --> 00:14:23,060 When it stays within the same land, it only passes through switches which related at the layer to heter 161 00:14:23,060 --> 00:14:24,380 containing the Mac address. 162 00:14:25,620 --> 00:14:32,190 As a result, the header changes as the packet is rean capitulated, as does the Mac address Fields'.