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The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other. A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers .Advantages of OSI layered model: Advantages of OSI layered model It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting. It allows multiple-vendor development through standardization of network components. It allows various types of network hardware and software to communicate. It prevents changes in one layer from affecting other layers, so it does not hamper development.OSI reference model has seven layers:: OSI reference model has seven layers:The Application Layer: The Application Layer The Application layer of the OSI model marks the spot where users actually communicate to the computer. The Application layer is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist. Applications sometimes require more than only desktop resources. Often, they’ll unite communicating components from more than one network application. Prime examples are file transfers and email, as well as enabling remote access, network management activities, client/server processes, and information location.Presentation Layer: Presentation Layer It presents data to the Application layer and is responsible for data translation and code formatting. The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption, and decryption are associated with this layer. Some Presentation layer standards are involved in multimedia operations too.Session Layer: Session Layer The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex , half duplex , and full duplex . To sum up, the Session layer basically keeps different applications’ data separate from other applications’ data. Simplex Full-duplex Half-duplexTransport Layer: Transport Layer Transport layer segment and reassemble data from upper-layer applications and unite it into the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork. The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuits. The Transport layer can be connectionless (UDP) or connection-oriented (TCP).Slide 9: Flow Control The segments delivered are acknowledged back to the sender upon their reception. Any segments not acknowledged are retransmitted. Segments are sequenced back into their proper order upon arrival at their destination. A manageable data flow is maintained in order to avoid congestion, overloading, and data loss. Connection-Oriented Communication A service is considered connection-oriented if it has the following characteristics: A virtual circuit is set up (e.g., a three-way handshake like Synchronize, Negotiate connection and Acknowledge ). It uses sequencing. It uses acknowledgments. It uses flow controlSlide 11: Windowing and Buffering The quantity of data segments ( measured in bytes ) that the transmitting machine is allowed to send without receiving an acknowledgment for them is called a window . Windows are used to control the amount of outstanding, unacknowledged data segments. Acknowledgments Reliable data delivery ensures the integrity of a stream of data sent from one machine to the other through a fully functional data link. It guarantees that the data won’t be duplicated or lost . This is achieved through something called positive acknowledgment with retransmission —Network Layer: Network Layer The Network layer (also called layer 3) manages device addressing, tracks the location of devices on the network, and determines the best way to move data, which means that the Network layer must transport traffic between devices that aren’t locally attached. Routers (layer 3 devices) are specified at the Network layer and provide the routing services within an internetwork. Two types of packets are used at the Network layer: data and route updates: Data packets Used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols ; examples of routed protocols are IP and IPv6.Slide 13: Route update packets Used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route update packets are called routing protocols; examples of some common ones are RIP, RIPv2, EIGRP, and OSPF . Route update packets are used to help build and maintain routing tables on each router. Logical addressing . It also define the route .Data Link Layer: Data Link Layer The Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit. The Data Link layer formats the message into pieces, each called a data frame , and adds a customized header containing the hardware destination and source address.Slide 15: The IEEE Ethernet Data Link layer has two sublayers: Media Access Control (MAC) 802.3 Defines how packets are placed on the media. Contention media access is “first come/first served” Physical addressing is defined here, Line discipline, error notification (not correction), ordered delivery of frames, and optional flow control can also be used at this sublayer. Logical Link Control (LLC) 802.2 Responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the Data Link layer what to do with a packet once a frame is received.Physical Layer: Physical Layer At the bottom, we find that the Physical layer does two things: It sends bits and receives bits. Bits come only in values of 1 or 0 — a Morse code with numerical values. The Physical layer communicates directly with the various types of actual communication media. The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems.Slide 17: Each layer depends on the service function of the ISO/OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it can add whatever headers and trailers the layer will use to perform its function. As networks perform services for users, the flow and packaging of the information changes. In this example of internetworking, five conversion steps occur:Slide 18: What do the 7 layers really do? You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.