|
Lesson 3 - The Data Link LayerOne Link, One FrameAs we know, the Physical Layer is only "aware" of the transmission medium itself. The Data Link Layer is a little smarter, because it knows there is another node on the other end of a physical link. However, a Data Link Layer process can only communicate with one peer node at a time, over one physical link. Of course, many devices may be connected by the same physical link. As we have seen, all nodes connected to the same bus or hub form a single broadcast network. Each node's Physical Layer process receives every signal transmitted by every other node. However, the Data Link Layer does not keep track of all those devices. It simply follows the instructions of the Network Layer, and addresses each packet and frame to the destination the Network Layer provides. Meanwhile, only the destination node's Data Link Layer process decapsulates the frame and passes the packet up to its Network Layer. The Data Link Layer processes on other nodes recognize that the frame is not addressed to them, and they discard the frame. Each frame carries a packet of data across a single physical link. A new frame is built for the trip across each link, just as we use a different ticket to board an airplane, then a bus, then a train. However, the Network Layer packet stays intact as each frame is built and removed around it. Thus, we often say that the Data Link Layer is concerned with transmitting data to the "next node" in the network. The next node may be the data's final destination, or it could be an intermediate routing device that can forward the data to its final destination by encapsulating it within a new frame and sending it over a different physical link. As you can see in the Frames and Links Diagram, the sending computer and receiving computer are not physically connected. Thus, any data sent from the sending computer to the receiving computer must travel over two physical links, from the sending computer to the routing device, then from the routing device to the receiving computer.
Frames and linksIf data must travel between two computers not directly connected, higher layers of the OSI model coordinate the data's journey across multiple physical links. In the next lesson, we explain how the Network Layer serves as a travel agent that plans the itinerary of each data packet. ActivitiesSee the Activities and Extended Activities section in Unit 4 Lesson 3 in your textbook Introduction to Networking to test what you have learned so far.
[ Previous Section ] [ Unit Contents ] [ Next Lesson ]
 |
