What are the most popular OSI Model Layers?

      What are the most popular OSI Model Layers?

        Open System Interconnection Model OSI is short for Open System Interconnection, which is an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers.

        It will help him troubleshoot problems on a network. Knowing how to identify which layer does the problem is occurring at, will help to determine what the actual cause of the problem.

        We wants to be an IT professional architect concerning with only the routers, switches and cabling. We will have greater interest in the Network Layer 4 because we are concerned about the routers. We will also have greater interest in the Data Link Layer 2 because we show concern about the switches. For cabling, he will use Physical Layer 1.
        There are seven layers in the OSI model. Layers 1-4 are the lower levels concerning about moving data around. Layer 5-7 are the upper layers that had application level data.

        The Application layer is the highest layer of the OSI model and lets you communicate with the computer. The Application Layer receives its information from the Presentation layer.
        The common application protocols are:
        • File Transfer Protocol (FTP)
        • Telnet
        • Simple Mail Transfer Protocol (SMTP)
        • Internet Message Access Protocol (IMAP),
        • Post Office Protocol (POP)
        • Hypertext Transfer Protocol (HTTP)
        • Simple Network Management Protocol (SNMP).
        • Network News Transfer Protocol (NNTP)

        The Session Layer 5 is responsible for setting up, managing and tearing down sessions between Presentation Layer. It talks to the Transport Layer and Presentation Layer.
        The Presentation Layer’s primary responsibility is to define the syntax that network hosts use to communicate. Compression and encryption fall in the functions of this layer. It is sometimes referred to as the “syntax” layer and is responsible for transforming information or data into format(s) the application layer can use.

        The functions performed at the Presentation layer of the OSI are:
        • Protocol conversion
        • Data translation.
        • Data encryption and decryption
        • Data compression
        • Character set conversion
        • Interpretation of graphics commands.

        Gateway services also function at the Presentation layer. A gateway can be defined as a connection point between networks which run different systems and applications.

        Common gateways include:
        • Gateways which cross platforms and file systems
        • Systems Network Architecture (SNA) gateways enable PCs to communicate with mainframe computers.
        • E-mail gateways enable data to be transmitted between different e-mail applications running the same protocol.

        The Session Layer 5 is responsible for setting up, managing and tearing down sessions between Presentation Layer. It talks to the Transport Layer and Presentation Layer. Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.
        It also provides dialog control between devices. It coordinates communication

        The Transport Layer 4 talks to the Session Layer and the Network Layer. It is responsible for getting the entire message. It provides end-to-end management of communication. TCP and UDP protocols are used to create data streams by segmenting and reassembling data from upper-layer applications. They provide end-to-end data transport services and can establish logical connection between the sending host and the receiving host on an internetwork.. It hides details of any network dependent information from the high layers by providing transparent data transfer. It can provide reliable and sequential packet delivery through error recovery and flow control mechanisms. If we cannot fix the errors, then it requests that the missing data be resend. It also informs the transport manager that the data arrived correctly. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

        Network Layer 3 talks to the Transport Layer and the Data Link Layer. This layer is responsible for packet forwarding, including routing through different routers. Addressing and routing are the main goals of Layer 3. It determines how data are transferred between network devices. It addresses the route packets for their destination. It reads incoming packets and only allows the relevant ones through. It fixes errors inside the packets if it can be fix. It works out the best route for the packets to use. It also provide flow and congestion control to prevent network resource depletion.

        For Data Link Layer 2, Ethernet, switches, bridges translate data frames into bits for Layer 1 processing. Data Link Layer also talks to the Network Layer and Physical Layer. The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Media Access Control (MAC) layer and the Logical Link Control (LLC) layer can be found in Layer 2. In the networking world, most switches operate at Layer 2.
        The Physical Layer is Layer 1 where wiring, fiber optic, hubs, repeaters, token ring network, and network cards are used to make two network devices communicate together. It defines physical means of sending data over network devices and interfaces between network medium and devices. Hubs connect multiple twisted pair or fiber optic devices together.
        It repeats the signal coming into one port out the others. A stackable hub is designed to be linked with other hubs in a single telecommunications closet. A standalone hub serves a workgroup of computers that are separated from the rest of the network. It is also known as a workgroup hub. A passive hub simply retransmits signals over the network. An intelligent hub has processing capabilities, can monitor network traffic, detect packet errors and collisions, poll connected devices for information and gather the data in database format It is also known as the managed hub.

        The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.

        OSI model is obsolete but it helps to frame discussions of protocols and contrast various technologies. When you understand the OSI model and its layers, then you can understand which protocols and devices that can interoperate with each other when new technologies are developed and explained.

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