ccna qos

ccna qos.The data link layer reconstructs the bit value into the original data link frame (Ethernet) At the MAC layer, the NIC checks the FCS to verify that the frame is valid, and it also looks at the target MAC address to verify that the Ethernet frame was sent to it. If the destination MAC address is different from its own MAC address and is not a multicast or broadcast address, the NIC discards the frame; otherwise the NIC processes the frame. In this example, the NIC finds that the encapsulated packet is a TCP/IP packet, so it strips (unwraps) the Ethernet frame information and hands the packet off to the TCP/IP stack at the network layer. If the encapsulated packet is an IPX packet, the NIC hands it off to the network layer's IPX stack.Next, the network layer looks at the logical destination address in the packet header. If the logical destination address is different from its address and is not a multicast or broadcast address, the network layer will discard the packet. If the logical addresses are the same, the target device looks at the protocol information in the packet header to determine which protocol the packet should be given to. In this example, the logical address is the same and the specified protocol is TCP, so the network layer strips the packet information and gives the encapsulated data segment to the TCP protocol at the transport layer.After receiving the data segment, the transport layer protocol may perform many functions, depending on whether the connection is reliable or unreliable. The focus here is on the multiplexing function of the transport layer. In this example, the transport layer looks at the destination port number in the header of the data segment. In this example, the user is using Telnet on PC.A to transmit information to PC-B, so the destination port number is 23. The transport layer sees this port number and knows that it needs to give the encapsulated data to the Telnet application. If PC-B does not support Telnet, the transport layer will discard the data segment; otherwise, the transport layer will strip the data segment information and give the encapsulated data to the Telnet application. If this is a new connection request, the operating system will start the Telnet process.Note that logical communication occurs between the corresponding layers of the two devices; for example, logical communication occurs between the transport layer of PC-A and PC.B, as well as between the network layer of PC-A and PC-B and between the data link layers.Exam Points Be sure to familiarize yourself with the process of passing data down (encapsulation) and up (unencapsulation) along the protocol stack when transferring data between devices.2.3.3 Layering and CommunicationFrom the encapsulation and unencapsulation processes described earlier, it is clear that in order to send and receive information, both the source and target devices need to perform a significant amount of processing. The situation is further complicated if the source and target devices are located on different network segments and there are network devices such as hubs, switches and routers between these segments. Figure 2.6 provides an example of such a situation.In this example, PC-A wants to send data to PC-B. Note that each device needs to process information at a specific layer; for example, after PC-A puts information on the cable, the switch connected to PC-A needs to process that information. As stated earlier in this chapter, the switch runs at Layer 2 of the OSI reference model. The router selects a path based on the Layer 3 destination address, while the switch selects a path based on the Layer 2 destination MAC address in the frame. Therefore, the physical layer of the switch must convert the physical layer signals into bits, pass them up to the data link layer, and reorganize them into frames here. The switch looks at the destination MAC address, makes switching decisions, and determines which port the frame needs to be forwarded out of. Next, it passes the frame down to the physical layer, which converts the frame's bits into physical layer signals. Switching is discussed in Chapter 4.At the physical layer, the next device encountered is the router. As stated earlier in this chapter, the router runs at Layer 3 of the OSI reference model. This router first converts physical layer signals into bits at the physical layer; these bits are passed up to the data link layer, where they are reconfigured into frames. Next, the router looks at the destination MAC address in the frame, and if that MAC address is different from its own, the router discards the frame.If the same, the router will strip the frame information and pass the packet up to the network layer.