Internetworking allows these relatively unrelated

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Unformatted text preview: yers 7 and 6. The presentation layer software performs text compression, code conversion, security encryption, and so on, on the received message, and after adding a header (H 6) to it, it passes the resulting message on to the session layer (5). Depending on the type of dialog, the session layer software establishes a dialog between the sender and the receiver processes. It also regulates the direction of message flow. A header (//$) is added to the message at this layer, and the resulting message is passed on to the transport layer (4). The transport layer software now splits the message into smaller units (Mi and M2) called packets and adds a header (H4) to each packet. These headers contain the sequence numbers of the message packets. The packets are then passed on to the network layer (3). The network layer software makes routing decisions for the received packets and sets up a logical path between the sending and receiving nodes for transmission of the packets. It then adds a header (Hi) to each packet and passes them on to the data-link layer (2). The data-Jink layer software adds a header (H2) and a trailer (T2) to each of these packets. The trailers contain the checksum of the data in the corresponding packets. The resulting message units are called frames, which are passed on to the physical layer (1). The physical layer software simply transmits the raw bits from the sender's machine to the receiver's machine using the physical connection between the two machines. On the receiver's machine, the message data traverses up from the physical layer to the application layer. As the message data traverses to higher-level layers, each layer performs the functions assigned to it and strips off the headers or trailers added by its peer layer at the sending node. For example, the data-link layer at the receiving machine performs error detection by recalculating the checksum for each frame and comparing it with the checksum in the trailer of the frame. It strips off the header (Hi) a...
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