1-net-2-models (1).pdf

IT – 312; Networking – II 1 Network Models OSI MODEL Serial data communications rely...

IT – 312; Networking – II 1 Network Models OSI MODEL Serial data communications rely upon networks to transmit data. The two most important network models are the Open Systems Interconnection Reference (OSI) model and the Internet model: The Open Systems Interconnection Reference model was developed by the Open System Interconnection Subcommittee in 1984. The OSI model consists of seven layers: the physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer. Though it isn’t widely used today, it still has value as a foundational understanding of networking. TCP/IP MODEL (Internet) The Internet model, though actually older than OSI, is the network model that has arisen to be the dominant model for all current hardware and software. Also referred to as the Transfer Control Protocol/Internet Protocol (TCP/IP) model, it combines the top three OSI layers into a single layer, making it a five-layer model consisting of a physical layer, data link layer, network layer, transport layer and application layer. The Internet model allows different independent networks to connect one another and then together, create what we know as the global Internet. | Dr. EDDIE I. PERU – Subject Professor ISU-Cauayan IT – 312; Networking – II 2 | Dr. EDDIE I. PERU – Subject Professor ISU-Cauayan IT – 312; Networking – II 3 | Dr. EDDIE I. PERU – Subject Professor ISU-Cauayan IT – 312; Networking – II 4 | Dr. EDDIE I. PERU – Subject Professor ISU-Cauayan IT – 312; Networking – II 5 The OSI model. The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO) and others in the late 1970s. It was published in its first form in 1984 as ISO 7498, with the current version being ISO/IEC 7498-1:1994. Physical layer The physical layer refers to the physical communication medium and the technologies to transmit data across that medium. At its core, data communication is the transfer of digital and electronic signals through various physical channels like fiber-optic cables, copper cabling, and air. The physical layer includes standards for technologies and metrics closely related with the channels, such as Bluetooth, NFC, and data transmission speeds. Data link layer The data link layer refers to the technologies used to connect two machines across a network where the physical layer already exists. It manages data frames, which are digital signals encapsulated into data packets. Flow control and error control of data are often key focuses of the data link layer. Ethernet is an example of a standard at this level. The data link layer is often split into two sub-layers: the Media Access Control (MAC) layer and Logical Link Control (LLC) layer. Network layer The network layer is concerned with concepts such as routing, forwarding, and addressing across a dispersed network or multiple connected networks of nodes or machines. The network layer may also manage flow control. Across the internet, the Internet Protocol v4 (IPv4) and IPv6 are used as the main network layer protocols. Transport layer The primary focus of the transport layer is to ensure that data packets arrive in the right order, without losses or errors, or can be seamlessly recovered if required. Flow control, along with error control, is often a focus at the transport layer. At this layer, commonly used protocols include the Transmission Control Protocol (TCP), a near-lossless connection-based protocol, and the User Datagram Protocol (UDP), a lossy connectionless protocol. TCP is commonly used where all data | Dr. EDDIE I. PERU – Subject Professor ISU-Cauayan IT – 312; Networking – II 6 must be intact (e.g. file share), whereas UDP is used when retaining all packets is less critical (e.g. video streaming). Session layer The session layer is responsible for network coordination between two separate applications in a session. A session manages the beginning and ending of a one-to-one application connection and synchronization conflicts. Network File System (NFS) and Server Message Block (SMB) are commonly used protocols at the session layer. Presentation layer The presentation layer is primarily concerned with the syntax of the data itself for applications to send and consume. For example, Hypertext Markup Language (HTML), JavaScipt Object Notation (JSON), and Comma Separated Values (CSV) are all modeling languages to describe the structure of data at the presentation layer. Application layer The application layer is concerned with the specific type of application itself and its standardized communication methods. For example, browsers can communicate using HyperText Transfer Protocol Secure (HTTPS), and HTTP and email clients can communicate using POP3 (Post Office Protocol version 3) and SMTP (Simple Mail Transfer Protocol). How does communication happen in the OSI model? The layers in the Open Systems Interconnection (OSI) model are designed so that an application can communicate over a network with another application on a different device, no matter the complexity of the application and underlying systems. To do this, various standards and protocols are used to communicate with the layer above or below. Each of the layers is independent and only aware of the interfaces to communicate with the layer above and below it. By chaining together all these layers and protocols, complex data communications can be sent from one high-level application to another. The process works as follows: 1. The sender’s application layer passes data communication down to the next lower layer. 2. Each layer adds its own headers and addressing to the data before passing it on. 3. Data communication moves down the layers until it is eventually transmitted through the physical medium. 4. At the other end of the medium, each layer processes the data according to the relevant headers at that level. 5. At the receiver end, data moves up the layer and is gradually unpacked until the application at the other end receives it. What are alternatives to the OSI model? Various networking models were used in the past, such as Sequenced Packet Exchange/Internet Packet Exchange (SPX/IPX) and Network Basic Input Output System (NetBIOS). Today, the main alternative to the Open Systems Interconnection (OSI) model is the TCP/IP model. The TCP/IP model is comprised of five different layers: While layers like the physical layer, network layer, and application layer appear to map directly to the OSI model, this isn’t quite the case. Instead, the TCP/IP model most accurately maps to the structure and protocols of the internet. The OSI model remains a popular networking model to describe how networking operates from a holistic perspective for educational purposes. However, the TCP/IP model is now more commonly used in practice. | Dr. EDDIE I. PERU – Subject Professor ISU-Cauayan