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I don't want to spend too much time on theoretical information or to understand the packets sent to

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the destination systems and the reply packets as well.

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We should talk about the TCP/IP Internet Protocol suite. OSI (open systems interconnection) is a reference

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model for how applications can communicate over a network. A reference model is a conceptual framework

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for understanding relationships.

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The purpose of the OSI reference model is to guide vendors and developers so that the digital communication

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products and software programs they create will operate and to facilitate clear comparisons among communications

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tools.

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Most vendors involved in the telecommunications make an attempt to describe their products and services

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in relation to the OSI model and although useful for guiding discussion and evaluation.

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OSI is rarely actually implemented. As few network products or standard tools

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Keep all related functions together in well-defined layers as related to the model. Developed by representatives

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of major computer and telecommunication companies beginning in 1983,

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OSI was originally intended to be a detailed specification of actual interfaces.

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Instead, the committee decided to establish a common reference model for which others could then develop

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detailed interfaces, Which in turn could become standards.

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OSI was officially adopted as the international standard by the International Organization of standards.

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ISO

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So, what about the, OSI Layers

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The main concept of OSI is that the process of communication between two endpoints in

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a telecommunication network can be divided into seven distinct groups of related functions or layers.

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Each communicating user or program is at a computer that can provide those seven layers of function.

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So in a given message between users there will be a flow of data down through the layers in the source

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computer across the network and then up through the layers in the receiving computer.

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The seven layers of function are provided by a combination of applications, operating systems, network

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card, device drivers and networking hardware that enable a system to put a signal on a network cable

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or out over Wi-Fi or any other wireless protocol.

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The seven Open Systems Interconnection layers are: Layer 7.

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The application layer.

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This is a layer at which communication partners are identified.

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Is there someone to talk to?

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Network capacity is assessed (Will the network

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let me talk to them right now?), and that creates a thing to send or opens the thing received.

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Please note that this layer is not the application itself.

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It is this set of services an application should be able to make use of directly. Although some applications

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may perform application layer functions. Layer 6 - The presentation layer.

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This layer is usually part of an operating system and converts incoming and outgoing data from one presentation

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format to another.

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For example from clear text to encrypted text at one end and back to clear text the other. Layer 5 - The

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session layer. This layer sets up coordinates and terminate conversations. Services include authentication

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and reconnection after an interruption. On the Internet Transmission Control Protocol.

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TCP and User Datagram Protocol UDP provide these services for most applications.

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Layer 4. The transport layer.

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This layer manages packetization of data then the delivery of the packets. Including checking for errors

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in the data once it arrives. On the internet.

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TCP and UDP provide these services for most applications as well.

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Layer 3. The network layer. This layer handles the addressing and routing of the data. Sending it in the

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right direction to the right destination on outgoing transmissions and receiving incoming transmissions

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at the packet level.

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IP is a network layer for the Internet. Layer 2. The data link layer. This layer sets up links across

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the physical network,

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putting packets into network frames. This layer has two sub-layers.

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The Logical Link Control Layer and the Media Access Control Layer. Ethernet is the main data link layer in use.

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Layer 1. The physical layer. This layer conveys the bitstream through the network

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at electrical, optical or radio level.

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It provides a hardware means of sending and receiving data on a carrier network.

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Let's see some of the most famous protocols of the layers.

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For ease of use, it's better to talk about physical layer and data link layer together where we have some protocols

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and data link layer and some physical media and connection methodologies in physical layer. Ethernet

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and 802.11 wireless LAN are the most known protocols of the data link layer either net is the

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name of the most commonly used network protocol that controls how data is transmitted over a LAN, which

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is a local area network. You need to have Network Interface Cards in the devices that you want to connect

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to the network. A wireless local area network (WLAN) is a wireless computer network protocol that links

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two or more devices using wireless communication within a limited area such as a home, school, computer

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laboratory or office building.

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This gives users the ability to move around with a local coverage area and yet still be connected to the

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network.

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Most modern WLANs are based on

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IEEE 802.11 standard and are marketed under the Wi-Fi brand name. IP (internet protocol)

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is responsible for addressing host, encapsulating data into transferred packets and routing packets from

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a source host to a destination host across one or more IP networks.

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The best known transport protocol is the Transmission Control Protocol or TCP.

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It is used for connection oriented transmissions whereas the connectionless User Datagram Protocol (UDP) is

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used for simpler messaging transmissions.

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We're going to talk more in-depth about these protocols in the next lecture.

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Again, for ease of use the last three layers session layer presentation layer and application layer are

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thought of together as the application layer. Let me put another parenthesis here,

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We're talking about the OSI reference model here.

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In addition there is another reference model called TCP/IP reference model and instead of OSI model's

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last 3 layers. There is only a single application layer in the TCP/IP reference model.

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Just keep that in the back of your mind. And back to our subject. The application layer protocols are classified

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according to the protocol they are using in the transport layer and these protocols interact with the

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end user via applications.

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Therefore they are the most known protocols by just about everybody.

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Some of the most well known TCP based application layer protocols are HTTP (Hypertext Transfer Protocol)

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simply a communications protocol used to send and receive web pages and files on the internet.

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Telnet is one of the simplest ways to exchange data between two computers.

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It allows two computers anywhere on a computer network including the worldwide internet to exchange

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text and that well other data in real time. FTP (file transfer protocol) is a communication protocol for

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the rapid simple transmission of files across a network.

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SMTP (simple mail transfer protocol) is used to send and relay an e-mail message between email servers.

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Note that it is not used to retrieve email messages from a server instead either IMAP or POP is used

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to retrieve email messages.

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DNS (The domain name system) is the system used to convert a computer's hostname into an IP address on the

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Internet.

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For example if a computer needs to communicate with a web server nhs.uk. Your computer needs

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the IP address of the web server

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nhs.uk

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.

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It is the job of the DNS to convert the hostname to the IP address of the web server. The DNS uses both

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the UDP and TCP. SNMP (simple network management protocol) is used in network management systems to

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monitor status of devices and also spot problems.