TA2 OSI MODEL PDF

Introduction to Data Communication 1.10.2 Why Should We Learn the OSI model? Learning the OSI Model helps us to understand what functions occur where and when The OSI Model helps us to understand how a Web browser works The OSI Model helps us to understand what Internet Protocol does and how...

Introduction to Data Communication 1.10.2 Why Should We Learn the OSI model? Learning the OSI Model helps us to understand what functions occur where and when The OSI Model helps us to understand how a Web browser works The OSI Model helps us to understand what Internet Protocol does and how it works The OSI Model helps us to understand why we need ARP The OSI Model helps us to understand what is MAC address Learning the OSI Model makes it easier to learn. Learning the OSI Model makes it easier to perform troubleshooting. Learning the OSI Model makes it easier to troubleshoot any problem, including computer problems. Learning the OSI Model makes it easier to communicate with other technical people and discuss technical issues. 1.10.3 OSI model Fig 1.17 OSI model Network architecture based on OSI model is shown in fig 1.17 (above) & its functional description is given below. The Open Systems Interconnection (OSI) model defines a networking framework to implement protocols in layers, with control passed from one layer to the next. It is primarily used today as a teaching tool. It conceptually divides computer network architecture into 7 layers in a logical progression. The lower layers deal with electrical signals, chunks of binary data, and routing of these data across networks. Higher levels cover network requests and responses, representation of data, and network protocols as seen from a user's point of view. Follow fig 1.18. The OSI model was originally conceived as a standard architecture for building network systems and indeed, many popular network technologies today reflect the layered design of OSI. IRISET 15 TA2 – Data Communication & Networking Introduction to Data Communication Fig.1.18 OSI layers Physical Layer At Layer 1, the Physical layer of the OSI model is responsible for ultimate transmission of digital data bits from the Physical layer of the sending (source) device over network communications media to the Physical layer of the receiving (destination) device. Examples of Layer 1 technologies include Ethernet cables and Token Ring networks. Additionally, hubs and other repeaters are standard network devices that function at the Physical layer, as are cable connectors. Follow fig 1.19. Fig. 1.19 Physical layer At the Physical layer, data are transmitted using the type of signaling supported by the physical medium: electric voltages, radio frequencies, or pulses of infrared or ordinary light. Data Link Layer When obtaining data from the Physical layer, the Data Link layer checks for physical transmission errors and packages bits into data "frames". The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of any various network devices to the physical medium. Because the Data Link layer is the single most complex layer in the OSI model, it is often divided into two parts, the "Media Access Control" sub layer and the "Logical Link Control" sublayer.Follow fig 1.20. Fig 1.20 Data Link layer IRISET 16 TA2 – Data Communication & Networking Introduction to Data Communication Network Layer The Network layer adds the concept of routing above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination, Fig 1.21 Network Layer This Layer 3 formats the data into packets delivered up to the Transport layer. Otherwise, the Network layer updates the destination address and pushes the frame back down to the lower layers Follow fig 1.21. To support routing, the Network layer maintains logical addresses such as IP addresses for devices on the network. The Network layer also manages the mapping between these logical addresses and physical addresses. In IP networking, this mapping is accomplished through the Address Resolution Protocol (ARP). Transport Layer The Transport Layer delivers data across network connections. TCP is the most common example of a Transport Layer 4 network protocol. Different transport protocols may support a range of optional capabilities including error recovery, flow control, and support for re- transmission follow Fig 1.22. Fig 1.22 Transport layer IRISET 17 TA2 – Data Communication & Networking Introduction to Data Communication Session Layer The Session Layer manages the sequence and flow of events that initiate and tear down network connections. At Layer 5, it is built to support multiple types of connections that can be created dynamically and run over individual networks. Follow fig 1.23. Fig 1.23 Session Layer Presentation Layer The Presentation layer is the simplest in function of any piece of the OSI model. At Layer 6, it handles syntax processing of message data such as format conversions and encryption / decryption needed to support the Application layer above it.Follow fig 1.24.. Fig 1.24, Presentation Layer Application Layer The Application layer supplies network services to end-user applications. Network services are typically protocols that work with user's data. For example, in a Web browser application, the Application layer protocol HTTP packages the data needed to send and receive Web page content. This Layer 7 provides data to (and obtains data from) the Presentation layer. Follow fig 1.25. Fig 1.25 Application Layer IRISET 18 TA2 – Data Communication & Networking Introduction to Data Communication 1.10.4 An Operational Example of the OSI Model This example assumes that we are on a local area network and that we are using an Ethernet card to communicate with the network. Follow table 1.2 LAYER EXAMPLE FUNCTION/ACTIVITY A Web browser such as Internet Explorer or Netscape provides the means for computer to contact a Web server and download several files that go together to produce a single Web page. We can request a Web page by typing in a Web address or by clicking a link in an open Web page. The Web APPLICATION Web browser is an APPLICATION. The Web browser Browser application gives the means to select a Web server, contact the server and request a Web page. The Web browser handles the process of finding the Web server, requesting the desired file and displaying all the files contained in the Web page. Web browser supports various image file formats, audio files and HTML. The Web browser handles PRESENTATION of the Web page to the user by converting the files stored at the Web server into formats used to display them on computer. Conversion of data from one format to another is the job of the PRESENTATION PRESENTATION layer. A Web browser can convert these file formats into the local formats used on the local computer for displaying images, playing sounds and displaying text; if it cannot, it often can launch an application which does understand the format. Much of the PRESENTATION layer HTTP conversions are handled in the program we are running. SESSION When we request a Web page, the Web browser opens a TCP connection to the Web server. The Web server sends back the Web page and closes the connection. Web browser then opens the Web page. Within the Web page instructions are written in HTML tags which tell the browser where to find additional files to be displayed within the Web page such as style sheets, sound files, images, movies, Flash files and applets. Web browser automatically opens additional TCP connections to the Web server. Each TCP connection is a SESSION. To communicate with a Web server computer must open a TCP connection to the Web server and request a Web page. The TCP connection breaks up the Web page into TRANSPORT TCP manageable chunks, labels them with numbers so they can be reassembled in the correct order and TRANSPORTS the pieces across the correct SESSION. Internet Protocol (IP) is a NETWORK layer protocol that uses unique addresses for the Web server and for computer. IP provides the means for computer to determine whether the Web server is a local computer or a computer located somewhere on the Internet. To reach a Web server on the IP NETWORK Internet, IP protocol also allows computer to figure out how ARP to reach the Internet Web server via default gateway. Computer creates a message addressed to the Web server with computer's return IP address. Computer uses ARP to figure out the physical MAC address of the default gateway and then passes the data to the NETWORK layer. IRISET 19 TA2 – Data Communication & Networking Introduction to Data Communication LLC Once the request from Web browser has been created it is sent to the network card. Once it reaches network card it E must be converted into a message that is sent from DATA LINK T computer to the default gateway which will forward the H MAC message to the Internet. At the DATA LINK layer, the Web E request is inserted inside a network request to the default R N gateway. E T CSMA The physical layer provides the means to transmit the Web PHYSICAL /CD page request to the default gateway. Table 1.2 Operational examples of OSI Layers 1.10.5 SUMMARY OF OSI LAYERS Fig 1.26 Summary of OSI model Video links for understanding of OSI layers https://www.youtube.com/watch?v=vv4y_uOneC0 https://www.youtube.com/watch?v=KUoJKwgnGlQ IRISET 20 TA2 – Data Communication & Networking