044 - What is Network OSI Model

The Open Systems Interconnection (OSI) model was created to help standardize communication between computer systems. It divides communications into seven different layers. Each layer performs a specific tasks and builds upon the preceding layer until the communications are complete.

Open System Interconnection (OSI) reference model has become an International standard and serves as a guide for networking. This model is the best known and most widely used guide to describe networking environments. Vendors design network products based on the specifications of the OSI model. It provides a description of how network hardware and software work together in a layered fashion to make communications possible. It also helps with trouble shooting by providing a frame of reference that describes how components are supposed to function.

The OSI Model

The Seven Layer Of OSI Model

Layer 7 Application • Message Passing - This is the layer at which communication partners are identified, network capacity is assessed, and that creates a thing to send or opens the thing received. L7A examples include: Telnet, WWW, DSN, HTTP, HTTPS, FTP, NFS, POP, SNMP, SMTP, SSH.

Layer 6 Presentation • Encoding - This layer is usually part of an operating system (OS) 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. L6P examples include: encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

Layer 5 Session • Authentication and Encryption - 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 Datagram Protocol (UDP) provide these services for most applications. L5S examples include: NFS, NetBios names, RPC, SQL.

Layer 4 Transport • Streams and Segments - This layer manages packetization of data, then the delivery of the packets, including checking for errors in the data once it arrives. On the Internet, TCP and UDP provide these services for most applications as well. L4T examples include: SPX, TCP, UDP.

Layer 3 Network • Datagrams - This layer handles the addressing and routing of the data. Sending in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level. IP is the network layer for the Internet. L3N examples include: AppleTalk DDP, IP, IPX.

Layer 2 Data link • Frames and Packets - 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 in use. L2D examples include: PPP, FDDI, ATM, IEEE 802.5/802.2, IEEE 802.3/802.2, HDLC, Frame Relay.

Layer 1 Physical • Signaling and Wiring - This layer conveys the bit stream through the network at the electrical, optical or radio level. It provides the hardware means of sending and receiving data on a carrier network. L1P examples include: Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.

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