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The OSI Networking Reference Model Lesson overview. In this lesson, we will cover: ● ● ● A brief history. Networking reference models. OSI and TCP/IP comparison. A brief history. The Open Systems Interconnection (OSI) reference model is a conceptual model with two major components. The first main component of the OSI networking reference model is an abstract model of networking—a seven-layer model. The second is a set of specific protocols, which allow differing computing systems to communicate with one another despite their different architectures. Why a networking model was required. A networking model was required because early networks communicated using proprietary languages. Because of their proprietary languages, early networks could only communicate with like systems, so an IBM network could only communicate with another IBM network. In addition to that, the U.S. government desired a robust computer communications system that could survive disasters. Highlights: ● ● Early networks could only communicate with like systems. The U.S. government desired a robust computer communications system that could survive disasters. TCP/IP reference model. The first networking reference model that was developed was the TCP/IP (Transmission Control Protocol/Internet Protocol) reference model. The TCP/IP reference model was published as the standard for the U.S. Department of Defense (DoD) in 1982. All of the major system manufacturers adopted the TCP/IP reference model beginning in 1984. AT&T moved the UNIX implementation of TCP/IP to open source in 1989, further cementing TCP/IP's place in networking. Highlights: ● ● ● The TCP/IP model was published as the U.S. DoD standard in 1982. Adopted by the big players beginning in 1984. AT&T moved the UNIX implementation to open source in 1989. OSI reference model. The OSI reference model was developed a year later than the TCP/IP reference model. The OSI model was published in 1983 and it defines the relationships between differing protocols and hardware. Highlights: ● ● The OSI model was published in 1983. The OSI model defines the relationships between differing protocols and the relationship between protocols and hardware. Networking reference models. The OSI networking reference model. While all networks rely upon the TCP/IP reference model, the OSI reference model is more commonly used for troubleshooting and for describing how networks operate. The reason for this is that the seven layers of the OSI model make it easier to break down the function and operation of both the protocols and hardware that make up networks. A brief description of each of the seven layers follows. Layer 1, the physical layer. Layer 1, the physical layer, standardizes the electrical signals that networks use. It also defines cable standards and how the bits of data are placed on the physical media. Network cables and hubs are part of Layer 1 of the OSI model. Highlights: ● ● The physical layer standardizes the electrical signal that networks use; it also defines cable standards and how bits are placed on the physical media. Network cables and hubs are part of Layer 1. Layer 2, the data link layer. Layer 2, the data link layer, is responsible for identifying the individual nodes—both the sending node and the receiving node. It also introduces an error correction method known as the frame check sequence (FCS). Layer 2 is composed of two sublayers. The first of those is the logical link control (LLC) layer, which is mainly responsible for flow control and error correction. The second is the media access control (MAC) layer, which is mainly responsible for node addressing. Switches and bridges are Layer 2 devices. Highlights: ● ● ● The data link layer is responsible for identifying the individual nodes; it also introduces an error correction method known as the FCS. Layer 2 is composed of two sub-layers: ○ The LLC layer is mainly responsible for flow control and error correction. ○ The MAC layer is mainly responsible for node addressing. Switches and bridges are Layer 2 devices. Layer 3, the network layer. Layer 3, the network layer, is responsible for routing functions between networks. It also identifies networks and nodes on the network. Routers are Layer 3 devices. Highlights: ● ● The network layer is responsible for routing functions between networks and it defines node and network addresses. Routers are Layer 3 devices. Layer 4, the transport layer. Layer 4, the transport layer, is responsible for breaking the data into smaller pieces for the lower layers and for the actual data transport protocols—two of which are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). The transport layer may be required to confirm the actual delivery of the data stream and it may be required to offer error correction. If so, it does this through the use of TCP. Highlights: ● ● ● The transport layer is responsible for breaking the data into smaller pieces for the lower layers and for the actual data transport protocols. The two most common Layer 4 protocols are TCP and UDP. TCP offers both packet delivery confirmation and error control. Layer 5, the session layer. Layer 5, the session layer, is the layer that is responsible for establishing the initial parameters between two systems. It sets up and tears down the communication channel. Highlights: ● ● The session layer is responsible for establishing the initial parameters between two systems. Layer 5 sets up and tears down the communication channels. Layer 6, the presentation layer. Layer 6, the presentation layer, is responsible for taking data and converting it from a machinedependent language to a machine-independent language. This is also the layer that has the main responsibility for encryption between networks. Highlights: ● ● The presentation layer is responsible for taking data and converting it from a machine dependent language to a machine independent language. Layer 6 is also the layer with the main responsibility for encryption between networks. Layer 7, the application layer. Layer 7, the application layer, is the layer that is responsible for the protocols that request services or functions from other systems. These protocols may not be the actual application. For instance, Internet Explorer is an application that uses HTTP at Layer 7 to request Web pages. Highlights: ● ● The application layer is responsible for the protocols that request services or functions from other systems. The protocols may not be the actual application. TCP/IP reference model. The TCP/IP reference model is a four-layer reference model that was designed to meet the requirements of the U.S. DoD standards for a robust network communication system. All networks that communicate with other networks rely upon the TCP/IP reference model. The four layers that comprise the model are discussed below. Network interface layer (a.k.a. Link layer). The lowest layer is the Network interface layer, which is also known as the Link layer. It handles electrical signaling, flow control, error detection, and node addressing. This layer can be mapped to Layer 1 and Layer 2 of the OSI reference model. Highlights: ● ● The Network interface layer handles electrical signaling, flow control, error detection, and node addressing. This layer maps to Layer 1 and Layer 2 of the OSI reference model. Internet layer. The Internet layer handles routing functions and identifies network systems and nodes on those networks. This layer can be mapped to Layer 3 of the OSI reference model. Highlights: ● ● The Internet layer handles routing functions and identifies network systems and nodes. This layer maps to Layer 3 of the OSI reference model. Transport layer. The Transport layer handles breaking the data into more manageable pieces for the lower layers. It is also the layer that is responsible for the delivery method—which can be either reliable or unreliable. When reliable delivery is used, the Transport layer also handles error correction. This layer maps to Layer 4 of the OSI reference model. Highlights: ● ● ● The Transport layer handles breaking the data into more manageable chunks for lower layers. It is the layer responsible for the delivery method, either reliable or unreliable, and error correction for reliable delivery. This layer maps to Layer 4 of the OSI reference model. Application layer. The Application layer handles requests for services from applications. It also handles translation to machine-independent languages and encryption. Additionally, the Application layer sets up and tears down communication sessions between systems. This layer maps to the top three layers of the OSI model (Layer 5, Layer 6, and Layer 7). Highlights: ● ● ● The Application layer handles requests for services from applications, translation to machine independent languages, and encryption. It also sets up and tears down sessions. This layer maps to Layer 5, Layer 6, and Layer 7 of the OSI reference model. OSI and TCP/IP comparison. While TCP/IP is the dominant model, most technicians communicate issues using the OSI reference model because it allows them to be more specific. When problems occur—and they will—it is easier to resolve them with a more highly defined set of specifications (e.g., Layer 5 of the OSI model vs. the Application layer of the TCP/IP model). Both the OSI and TCP/IP models are references only. It is not mandatory that they be followed. Each developer and manufacturer determines its own method of implementing the reference model. While, in theory, there will never be a problem in communication between devices and systems, it is only a theory. What was covered. A brief history. The OSI and TCP/IP networking models were created to allow disparate systems and networks to communicate easily between each other. Networking reference models. The OSI reference model has seven layers: physical, data link, network, transport, session, presentation, and application. The TCP/IP reference model has four layers: Network interface, Internet, Transport, and Application. OSI and TCP/IP comparison. TCP/IP is the dominant model in use; however, the OSI model is more commonly used when discussing problems. Both models are references only; developers and manufacturers determine their own methods of implementing network models.