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ccna 200 301 bookstore chains that closed.The data link layer reformats the message, and each piece of the edited message is called a DataFrame, which adds a custom header containing the hardware destination and source address. These added messages form a container capsule-like structure in which the original message is wrapped. This is similar to wrapping the engine, cruising deviceequipment, and other tools added to the Apollo project's circumlunar component [L2]. Different parts of this device will function in different phases of the space flight, and when their mission is complete, the individual parts will detach from the entire device and be discarded. Data transfer takes a similar form in the network.Figure 2.15 shows the Ethernet and IEEE-defined data link layers. If you look closely you will see that the IEEE 802.2 standard is combined with other IEEE standards and adds more functionality to them.Logical Link Control (LLC)Medium Access Control (MAC)Figure 2.15 Data Link LayerOne very important thing you must know is that the router works at the network layer, and it does not care at all about exactly where a particular host is located. Their only concern is where the network is located and what the best path to that network (including the remote network) is. Routers are devices that only care about the network, but after all, that's one good thing! Truly uniquely identifying each device on the local network is what the data link layer does.The data link layer uses hardware address resolution when a host in the local network sends a packet to another host, or when a packet is transmitted between routers. Each time a packet is sent between routers, the packet is encapsulated in frames at the data link layer, which includes control information. The receiving router then strips away this encapsulated information, leaving the original packet in its original form. The packet encapsulation and decapsulation process is repeated at each hop until the packet reaches the final destination host. It is important to understand that the packet itself does not change as it is forwarded along the router; it is simply encapsulated with control information that will be needed when the packet is transmitted to a different type of media.The Ethernet data link layer as defined by the IEEE is divided into the following two sub-layers.Medium Access Control (MAC) 802.3 defines how packets are placed into the medium. When all bandwidth is shared by all, competition for access to the medium is on a "first come, first served" basis. Physical addressing and logical topology are also defined here. What is logical topology? It is the signaling path through the physical topology. This sublayer can also use linediscipline [L3]. Error notification (and not correction), ordering of data frames to be sent, and optional flow control mechanisms.