‫OCI open systems interconnection.
‫Is a reference model for how applications can communicate over a network, a reference model is a conceptual
‫framework for understanding relationships.
‫The purpose of the U.S. reference model is to guide vendors and developers so that the digital communication
‫products and software programs they create will interoperate and to facilitate clear comparisons among
‫communications tools.
‫Most vendors involved in the telecommunications make an attempt to describe their products and services
‫in relation to the Osai model.
‫And although useful for guiding discussion and evaluation, Osai is rarely actually implemented.
‫As few network products or standard tools keep all related functions together in well-defined layers
‫as related to the model developed by representatives of major computer and telecommunication companies.
‫Beginning in 1983, Osai was originally intended to be a detailed specification of actual interfaces.
‫Instead, the committee decided to establish a common reference model for which others could then develop
‫detailed interfaces, which in turn could become standards.
‫OSA was officially adopted as the international standard by the International Organization of Standards
‫ISO.
‫So what about the Oh as I layers?
‫The main concept of Osai is that the process of communication between two end points in a telecommunication
‫network can be divided into seven distinct groups of related functions or layers.
‫Each communicating user, a program is at a computer that can provide those seven layers a function.
‫So in a given message between users, there will be a flow of data down through the layers in the source
‫computer across the network and then up through the layers in the receiving computer.
‫The seven layers of function are provided by a combination of applications, operating systems, network
‫card device drivers and networking hardware that enable a system to put a signal on a network cable
‫or out over Wi-Fi or any other wireless protocol.
‫The seven open systems interconnection layers are.
‫Layer seven, the application layer.
‫This is a layer at which communication partners are identified.
‫Is there someone to talk to?
‫Network capacity is assessed.
‫Will the network let me talk to them right now and.
‫That creates a thing to send or opens the thing received.
‫Please note that this layer is not the application itself.
‫It is this set of services and applications should be able to make use of directly, although some applications
‫may perform application layer functions.
‫Layer six, the presentation layer, this layer is usually part of an operating system and converts
‫incoming and outgoing data from one presentation format to another.
‫For example, from clear text to encrypted text at one end and back to clear text at the other.
‫Layer five, the session layer, this layer sets up coordinates and terminates conversations.
‫Services include authentication and reconnection after an interruption on the internet transmission
‫control protocol, TCP and user data Graham Protocol UDP provide these services for most applications.
‫Layer four, the transport layer, this layer manages packet ization of data, then the delivery of
‫the packet, including checking for errors and the data once it arrives.
‫On the Internet, TCP and UDP provide these services for most applications as well.
‫Layer three, the network layer, this layer handles the addressing and routing of the data, sending
‫it in the right direction to the right destination on outgoing transmissions and receiving incoming
‫transmissions at the packet level.
‫IB is a network layer for the internet.
‫Layer two, the data link layer.
‫This layer sets up links across the physical network, putting packets into network frames.
‫This layer has two sub layers the logical link control layer and the media access control layer.
‫Ethernet is the main data link, layer and use.
‫Layer one, the physical layer, this layer conveys the bitstream through the network at the electrical,
‫optical or radio level.
‫It provides the hardware means of sending and receiving data on a carrier network.
‫Let's see some of the most famous protocols of the layers.
‫Or ease of use?
‫It's better to talk about physical layer and data link layer together.
‫Here we have some protocols and data link layer and some physical media and connection methodologies
‫in physical layer Ethernet and 8.2.1 one wireless LAN are the most known protocols for the data link
‫layer.
‫Ethernet is the name of the most commonly used network protocol that controls how data is transmitted
‫over land, which is a local area network.
‫You need to have network interface cards in the devices that you want to connect to the network.
‫A wireless local area network, LAN is a wireless computer network protocol that links two or more devices
‫using wireless communication within a limited area such as a home school, computer, laboratory or
‫office building.
‫This gives users the ability to move around with a local coverage area and yet still be connected to
‫the network.
‫Most modern LANs are based on AI Tripoli 8.2.1 one standard and are marketed under the Wi-Fi brand name.
‫IP Internet Protocol is responsible for addressing host encapsulating data into transferred packet and
‫routing packets from a source host to a destination host across one or more IP networks.
‫The best known transport protocol is the transmission control protocol, or TCP.
‫It's used for connection oriented transmissions, whereas the connection was user data grand protocol
‫UDP is used for simpler messaging transmissions.
‫We're going to talk more in-depth about these protocols in the next lecture.
‫Again, for ease of use, the last three layers session, layer, presentation, layer and application
‫layer are thought of together as the application layer.
‫Let me put another parenthesis here.
‫We're talking about the OSAI reference model here.
‫In addition, there is another reference model called TCP IP reference model and instead of OSAI models
‫last three layers.
‫There is only a single application layer in the TCP IP reference model.
‫Just keep that in the back of your mind.
‫And back to our subject.
‫The application layer protocols are classified according to the protocol they are using in the transport
‫layer and these protocols in Iraq with the end user via applications.
‫Therefore, they are the most known protocols by just about everybody.
‫Some of the most well-known TCP based application layer protocols are.
‫HTTP Hypertext Transfer Protocol.
‫Simply a communications protocol used to send and receive web pages and files on the internet.
‫Telnet is one of the simplest ways to exchange data between two computers.
‫It allows two computers anywhere on a computer network, including the worldwide internet, to exchange
‫text and, well, other data in real time.
‫FTP file transfer protocol is a communication protocol for the rapid, simple transmission of files
‫across a network.
‫SMTP.
‫Simple Mail Transfer Protocol is used to send and relay an email message between e-mail servers.
‫Note that it is not used to retrieve e-mail messages from a server.
‫Instead, either IMAP or Pop is used to retrieve email messages.
‫DNS, the domain name system is a system used to convert a computer's hostname into an IP address on
‫the internet.
‫For example, if a computer needs to communicate with a web server, NHS Dot UK, your computer needs
‫the IP address of the web server.
‫NHS Dot UK It is the job of the DNS to convert the hostname to the IP address of the web server.
‫The DNS uses both the UDP and TCP.
‫SNMP Simple Network Management Protocol is used in network management systems to monitor status of devices
‫and also spot problems.
‫So let's see what happens to a packet inside the network traffic.
‫Please note, the data unit transferred between two end points has different names in each layer and
‫layers five, six and seven application layers.
‫It's called data in layer four transport layer.
‫It's called segment where TCP and data gram for UDP in layer three the network layer.
‫It's called packet in layer two.
‫The data link layer.
‫The data unit is called frame.
‫Now I usually use packet for the transferred data unit in each layer to make it less complicated for
‫you.
‫Makes sense before looking at the road trip of a DNS query in detail.
‫Let's look at the relationship between the osai layers and computer systems.
‫The packets and layer one and layer two are managed by network interfaces of your device, for example,
‫by your Ethernet card, and the packets from three to seven are managed by the processing unit in software
‫detail.
‫The layer three and layer four packets are managed by the operating system of your devices, and the
‫packets of layer five to seven are managed by the related application or service, for example, a web
‫browser.
‫When you write a URL in the address bar of your browser and hit Enter.
‫The first thing it sends is a DNS query, a DNS query is the process of a computer networking device,
‫making an enquiry to get an IP address for a DNS name such as Mail Dot Yahoo.com.
‫The client computer will send a DNS query to one of their internet service providers DNS servers.
‫The DNS server looks in its DNS database to tell whether it can answer the query authoritatively.
‫If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process
‫is complete.
‫So let's see the road trip of a desk query from your computer to a DNS server when data is transmitted
‫by the source toward a specific destination.
‫It passes through the application presentation and the session layers, and the protocol data unit arrives
‫at the transport layer.
‫Layer four layers five, six and seven are displayed as a single layer to simplify the presentation.
‫At this layer, a 20 byte data header is placed in front of the data.
‫A DNS query can also use TCP.
‫But let's assume that it uses UDP at this time the data and the layer for header.
‫Here it's UDP header, which together form a segment or data.
‫Graham is passed down to layer three.
‫The network layer.
‫The network layer places it's layer three header.
‫Here are the IP header in front of the received segment, and this group becomes a packet.
‫The layer three header contains important fields, such as the logical address, the IP address of both
‫the source and the destination device.
‫The newly formed packet is then passed down to layer two, the data link layer creates a new data unit
‫called a frame by adding the layer to frame header, which is the Ethernet header here.
‫Like layer three and addressing structure is also applied in the layer to header.
‫That is the Mac address.
‫The frame is then passed down to the physical layer.
‫Which converts the information into one bits that are sent over the physical media using electrical
‫signals on a copper link, for instance.
‫Finally, the data is sent over the wire using a wide variety of methods such as Ethernet or token ring.
‫The headers are a specific form of control information that allows the data to go through the network
‫properly.
‫That's the data at each layer is encapsulated in the information appropriate for this specific layer,
‫including addressing and error checking.
‫The overall size of the information increases as the data travels through the lower layers from layer
‫four to layer one, the destination device, which is the DNS server here, receives the data and this
‫additional information is analyzed and then is removed as the data passes through the higher layers
‫up to the application layer where the data is de capsule8 it.