1 00:00:00,210 --> 00:00:03,030 ‫OCI open systems interconnection. 2 00:00:04,000 --> 00:00:10,840 ‫Is a reference model for how applications can communicate over a network, a reference model is a conceptual 3 00:00:10,840 --> 00:00:13,240 ‫framework for understanding relationships. 4 00:00:14,210 --> 00:00:20,480 ‫The purpose of the U.S. reference model is to guide vendors and developers so that the digital communication 5 00:00:20,480 --> 00:00:27,560 ‫products and software programs they create will interoperate and to facilitate clear comparisons among 6 00:00:27,560 --> 00:00:28,760 ‫communications tools. 7 00:00:29,330 --> 00:00:35,480 ‫Most vendors involved in the telecommunications make an attempt to describe their products and services 8 00:00:35,480 --> 00:00:37,820 ‫in relation to the Osai model. 9 00:00:38,420 --> 00:00:45,440 ‫And although useful for guiding discussion and evaluation, Osai is rarely actually implemented. 10 00:00:46,280 --> 00:00:52,970 ‫As few network products or standard tools keep all related functions together in well-defined layers 11 00:00:53,000 --> 00:00:59,780 ‫as related to the model developed by representatives of major computer and telecommunication companies. 12 00:00:59,780 --> 00:01:07,670 ‫Beginning in 1983, Osai was originally intended to be a detailed specification of actual interfaces. 13 00:01:07,970 --> 00:01:14,690 ‫Instead, the committee decided to establish a common reference model for which others could then develop 14 00:01:14,690 --> 00:01:18,560 ‫detailed interfaces, which in turn could become standards. 15 00:01:19,340 --> 00:01:24,920 ‫OSA was officially adopted as the international standard by the International Organization of Standards 16 00:01:25,340 --> 00:01:26,060 ‫ISO. 17 00:01:27,090 --> 00:01:30,030 ‫So what about the Oh as I layers? 18 00:01:31,030 --> 00:01:38,260 ‫The main concept of Osai is that the process of communication between two end points in a telecommunication 19 00:01:38,260 --> 00:01:44,200 ‫network can be divided into seven distinct groups of related functions or layers. 20 00:01:45,200 --> 00:01:52,190 ‫Each communicating user, a program is at a computer that can provide those seven layers a function. 21 00:01:52,580 --> 00:01:58,820 ‫So in a given message between users, there will be a flow of data down through the layers in the source 22 00:01:58,820 --> 00:02:04,100 ‫computer across the network and then up through the layers in the receiving computer. 23 00:02:04,550 --> 00:02:10,760 ‫The seven layers of function are provided by a combination of applications, operating systems, network 24 00:02:10,760 --> 00:02:17,690 ‫card device drivers and networking hardware that enable a system to put a signal on a network cable 25 00:02:17,690 --> 00:02:21,410 ‫or out over Wi-Fi or any other wireless protocol. 26 00:02:22,160 --> 00:02:25,520 ‫The seven open systems interconnection layers are. 27 00:02:27,080 --> 00:02:29,000 ‫Layer seven, the application layer. 28 00:02:29,750 --> 00:02:33,140 ‫This is a layer at which communication partners are identified. 29 00:02:33,590 --> 00:02:35,000 ‫Is there someone to talk to? 30 00:02:36,420 --> 00:02:38,310 ‫Network capacity is assessed. 31 00:02:38,790 --> 00:02:42,540 ‫Will the network let me talk to them right now and. 32 00:02:43,520 --> 00:02:47,540 ‫That creates a thing to send or opens the thing received. 33 00:02:48,330 --> 00:02:51,660 ‫Please note that this layer is not the application itself. 34 00:02:52,020 --> 00:02:59,190 ‫It is this set of services and applications should be able to make use of directly, although some applications 35 00:02:59,190 --> 00:03:01,410 ‫may perform application layer functions. 36 00:03:02,490 --> 00:03:09,510 ‫Layer six, the presentation layer, this layer is usually part of an operating system and converts 37 00:03:09,510 --> 00:03:13,920 ‫incoming and outgoing data from one presentation format to another. 38 00:03:14,490 --> 00:03:21,450 ‫For example, from clear text to encrypted text at one end and back to clear text at the other. 39 00:03:22,730 --> 00:03:29,150 ‫Layer five, the session layer, this layer sets up coordinates and terminates conversations. 40 00:03:30,080 --> 00:03:35,750 ‫Services include authentication and reconnection after an interruption on the internet transmission 41 00:03:35,750 --> 00:03:43,550 ‫control protocol, TCP and user data Graham Protocol UDP provide these services for most applications. 42 00:03:44,940 --> 00:03:51,690 ‫Layer four, the transport layer, this layer manages packet ization of data, then the delivery of 43 00:03:51,690 --> 00:03:56,010 ‫the packet, including checking for errors and the data once it arrives. 44 00:03:57,080 --> 00:04:01,640 ‫On the Internet, TCP and UDP provide these services for most applications as well. 45 00:04:02,890 --> 00:04:09,100 ‫Layer three, the network layer, this layer handles the addressing and routing of the data, sending 46 00:04:09,100 --> 00:04:15,250 ‫it in the right direction to the right destination on outgoing transmissions and receiving incoming 47 00:04:15,250 --> 00:04:17,320 ‫transmissions at the packet level. 48 00:04:18,160 --> 00:04:20,860 ‫IB is a network layer for the internet. 49 00:04:22,090 --> 00:04:24,070 ‫Layer two, the data link layer. 50 00:04:24,990 --> 00:04:30,600 ‫This layer sets up links across the physical network, putting packets into network frames. 51 00:04:31,440 --> 00:04:37,890 ‫This layer has two sub layers the logical link control layer and the media access control layer. 52 00:04:37,950 --> 00:04:40,380 ‫Ethernet is the main data link, layer and use. 53 00:04:41,420 --> 00:04:48,140 ‫Layer one, the physical layer, this layer conveys the bitstream through the network at the electrical, 54 00:04:48,140 --> 00:04:49,790 ‫optical or radio level. 55 00:04:50,600 --> 00:04:55,040 ‫It provides the hardware means of sending and receiving data on a carrier network. 56 00:04:55,910 --> 00:04:58,610 ‫Let's see some of the most famous protocols of the layers. 57 00:04:59,730 --> 00:05:00,460 ‫Or ease of use? 58 00:05:00,480 --> 00:05:04,320 ‫It's better to talk about physical layer and data link layer together. 59 00:05:05,310 --> 00:05:10,710 ‫Here we have some protocols and data link layer and some physical media and connection methodologies 60 00:05:10,860 --> 00:05:18,120 ‫in physical layer Ethernet and 8.2.1 one wireless LAN are the most known protocols for the data link 61 00:05:18,120 --> 00:05:18,450 ‫layer. 62 00:05:19,420 --> 00:05:24,460 ‫Ethernet is the name of the most commonly used network protocol that controls how data is transmitted 63 00:05:24,460 --> 00:05:27,250 ‫over land, which is a local area network. 64 00:05:28,400 --> 00:05:33,560 ‫You need to have network interface cards in the devices that you want to connect to the network. 65 00:05:34,460 --> 00:05:42,080 ‫A wireless local area network, LAN is a wireless computer network protocol that links two or more devices 66 00:05:42,080 --> 00:05:48,620 ‫using wireless communication within a limited area such as a home school, computer, laboratory or 67 00:05:48,620 --> 00:05:49,430 ‫office building. 68 00:05:50,000 --> 00:05:56,210 ‫This gives users the ability to move around with a local coverage area and yet still be connected to 69 00:05:56,210 --> 00:05:56,840 ‫the network. 70 00:05:57,350 --> 00:06:04,520 ‫Most modern LANs are based on AI Tripoli 8.2.1 one standard and are marketed under the Wi-Fi brand name. 71 00:06:06,010 --> 00:06:14,170 ‫IP Internet Protocol is responsible for addressing host encapsulating data into transferred packet and 72 00:06:14,170 --> 00:06:20,170 ‫routing packets from a source host to a destination host across one or more IP networks. 73 00:06:21,600 --> 00:06:27,300 ‫The best known transport protocol is the transmission control protocol, or TCP. 74 00:06:27,570 --> 00:06:33,570 ‫It's used for connection oriented transmissions, whereas the connection was user data grand protocol 75 00:06:33,570 --> 00:06:37,230 ‫UDP is used for simpler messaging transmissions. 76 00:06:37,650 --> 00:06:41,040 ‫We're going to talk more in-depth about these protocols in the next lecture. 77 00:06:42,520 --> 00:06:48,310 ‫Again, for ease of use, the last three layers session, layer, presentation, layer and application 78 00:06:48,310 --> 00:06:52,090 ‫layer are thought of together as the application layer. 79 00:06:54,150 --> 00:06:55,710 ‫Let me put another parenthesis here. 80 00:06:56,610 --> 00:07:00,030 ‫We're talking about the OSAI reference model here. 81 00:07:00,330 --> 00:07:07,530 ‫In addition, there is another reference model called TCP IP reference model and instead of OSAI models 82 00:07:07,530 --> 00:07:08,550 ‫last three layers. 83 00:07:08,910 --> 00:07:13,560 ‫There is only a single application layer in the TCP IP reference model. 84 00:07:13,740 --> 00:07:15,390 ‫Just keep that in the back of your mind. 85 00:07:17,060 --> 00:07:17,960 ‫And back to our subject. 86 00:07:19,140 --> 00:07:26,100 ‫The application layer protocols are classified according to the protocol they are using in the transport 87 00:07:26,100 --> 00:07:31,170 ‫layer and these protocols in Iraq with the end user via applications. 88 00:07:31,290 --> 00:07:35,640 ‫Therefore, they are the most known protocols by just about everybody. 89 00:07:37,200 --> 00:07:41,190 ‫Some of the most well-known TCP based application layer protocols are. 90 00:07:42,720 --> 00:07:45,720 ‫HTTP Hypertext Transfer Protocol. 91 00:07:46,230 --> 00:07:51,510 ‫Simply a communications protocol used to send and receive web pages and files on the internet. 92 00:07:52,420 --> 00:07:56,770 ‫Telnet is one of the simplest ways to exchange data between two computers. 93 00:07:57,610 --> 00:08:04,000 ‫It allows two computers anywhere on a computer network, including the worldwide internet, to exchange 94 00:08:04,000 --> 00:08:06,850 ‫text and, well, other data in real time. 95 00:08:08,480 --> 00:08:14,690 ‫FTP file transfer protocol is a communication protocol for the rapid, simple transmission of files 96 00:08:14,690 --> 00:08:15,650 ‫across a network. 97 00:08:17,350 --> 00:08:18,220 ‫SMTP. 98 00:08:18,280 --> 00:08:24,550 ‫Simple Mail Transfer Protocol is used to send and relay an email message between e-mail servers. 99 00:08:25,030 --> 00:08:28,720 ‫Note that it is not used to retrieve e-mail messages from a server. 100 00:08:28,720 --> 00:08:32,830 ‫Instead, either IMAP or Pop is used to retrieve email messages. 101 00:08:34,540 --> 00:08:41,500 ‫DNS, the domain name system is a system used to convert a computer's hostname into an IP address on 102 00:08:41,500 --> 00:08:41,980 ‫the internet. 103 00:08:42,700 --> 00:08:49,030 ‫For example, if a computer needs to communicate with a web server, NHS Dot UK, your computer needs 104 00:08:49,030 --> 00:08:50,980 ‫the IP address of the web server. 105 00:08:51,010 --> 00:08:58,660 ‫NHS Dot UK It is the job of the DNS to convert the hostname to the IP address of the web server. 106 00:08:58,810 --> 00:09:02,380 ‫The DNS uses both the UDP and TCP. 107 00:09:03,430 --> 00:09:10,240 ‫SNMP Simple Network Management Protocol is used in network management systems to monitor status of devices 108 00:09:10,240 --> 00:09:11,860 ‫and also spot problems. 109 00:09:12,700 --> 00:09:16,360 ‫So let's see what happens to a packet inside the network traffic. 110 00:09:17,230 --> 00:09:24,130 ‫Please note, the data unit transferred between two end points has different names in each layer and 111 00:09:24,130 --> 00:09:26,550 ‫layers five, six and seven application layers. 112 00:09:26,560 --> 00:09:30,390 ‫It's called data in layer four transport layer. 113 00:09:30,400 --> 00:09:37,210 ‫It's called segment where TCP and data gram for UDP in layer three the network layer. 114 00:09:37,480 --> 00:09:40,650 ‫It's called packet in layer two. 115 00:09:40,720 --> 00:09:41,650 ‫The data link layer. 116 00:09:41,890 --> 00:09:43,990 ‫The data unit is called frame. 117 00:09:44,710 --> 00:09:51,340 ‫Now I usually use packet for the transferred data unit in each layer to make it less complicated for 118 00:09:51,340 --> 00:09:51,550 ‫you. 119 00:09:51,700 --> 00:09:56,140 ‫Makes sense before looking at the road trip of a DNS query in detail. 120 00:09:56,380 --> 00:10:01,210 ‫Let's look at the relationship between the osai layers and computer systems. 121 00:10:02,250 --> 00:10:07,860 ‫The packets and layer one and layer two are managed by network interfaces of your device, for example, 122 00:10:07,860 --> 00:10:14,970 ‫by your Ethernet card, and the packets from three to seven are managed by the processing unit in software 123 00:10:14,970 --> 00:10:15,570 ‫detail. 124 00:10:16,530 --> 00:10:22,260 ‫The layer three and layer four packets are managed by the operating system of your devices, and the 125 00:10:22,260 --> 00:10:28,470 ‫packets of layer five to seven are managed by the related application or service, for example, a web 126 00:10:28,470 --> 00:10:28,980 ‫browser. 127 00:10:29,580 --> 00:10:33,000 ‫When you write a URL in the address bar of your browser and hit Enter. 128 00:10:34,040 --> 00:10:40,610 ‫The first thing it sends is a DNS query, a DNS query is the process of a computer networking device, 129 00:10:40,820 --> 00:10:47,120 ‫making an enquiry to get an IP address for a DNS name such as Mail Dot Yahoo.com. 130 00:10:48,540 --> 00:10:54,510 ‫The client computer will send a DNS query to one of their internet service providers DNS servers. 131 00:10:55,140 --> 00:11:02,370 ‫The DNS server looks in its DNS database to tell whether it can answer the query authoritatively. 132 00:11:03,250 --> 00:11:10,120 ‫If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process 133 00:11:10,240 --> 00:11:10,990 ‫is complete. 134 00:11:11,740 --> 00:11:17,770 ‫So let's see the road trip of a desk query from your computer to a DNS server when data is transmitted 135 00:11:17,770 --> 00:11:20,320 ‫by the source toward a specific destination. 136 00:11:21,100 --> 00:11:27,070 ‫It passes through the application presentation and the session layers, and the protocol data unit arrives 137 00:11:27,070 --> 00:11:28,240 ‫at the transport layer. 138 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. 139 00:11:35,350 --> 00:11:39,790 ‫At this layer, a 20 byte data header is placed in front of the data. 140 00:11:40,270 --> 00:11:42,630 ‫A DNS query can also use TCP. 141 00:11:42,970 --> 00:11:48,250 ‫But let's assume that it uses UDP at this time the data and the layer for header. 142 00:11:48,820 --> 00:11:52,900 ‫Here it's UDP header, which together form a segment or data. 143 00:11:52,900 --> 00:11:55,540 ‫Graham is passed down to layer three. 144 00:11:55,930 --> 00:11:56,890 ‫The network layer. 145 00:11:57,610 --> 00:12:00,070 ‫The network layer places it's layer three header. 146 00:12:00,100 --> 00:12:06,160 ‫Here are the IP header in front of the received segment, and this group becomes a packet. 147 00:12:07,200 --> 00:12:13,800 ‫The layer three header contains important fields, such as the logical address, the IP address of both 148 00:12:13,890 --> 00:12:16,080 ‫the source and the destination device. 149 00:12:17,050 --> 00:12:23,800 ‫The newly formed packet is then passed down to layer two, the data link layer creates a new data unit 150 00:12:23,800 --> 00:12:29,260 ‫called a frame by adding the layer to frame header, which is the Ethernet header here. 151 00:12:30,400 --> 00:12:35,140 ‫Like layer three and addressing structure is also applied in the layer to header. 152 00:12:36,220 --> 00:12:37,660 ‫That is the Mac address. 153 00:12:38,410 --> 00:12:40,840 ‫The frame is then passed down to the physical layer. 154 00:12:41,900 --> 00:12:48,170 ‫Which converts the information into one bits that are sent over the physical media using electrical 155 00:12:48,170 --> 00:12:50,900 ‫signals on a copper link, for instance. 156 00:12:51,850 --> 00:12:58,510 ‫Finally, the data is sent over the wire using a wide variety of methods such as Ethernet or token ring. 157 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 158 00:13:05,140 --> 00:13:05,860 ‫properly. 159 00:13:06,250 --> 00:13:12,370 ‫That's the data at each layer is encapsulated in the information appropriate for this specific layer, 160 00:13:13,060 --> 00:13:15,880 ‫including addressing and error checking. 161 00:13:16,930 --> 00:13:23,170 ‫The overall size of the information increases as the data travels through the lower layers from layer 162 00:13:23,170 --> 00:13:30,130 ‫four to layer one, the destination device, which is the DNS server here, receives the data and this 163 00:13:30,130 --> 00:13:38,110 ‫additional information is analyzed and then is removed as the data passes through the higher layers 164 00:13:38,560 --> 00:13:43,180 ‫up to the application layer where the data is de capsule8 it.