In Chapter 1, “Shooting Trouble,” you learned that the protocol data unit (PDU) for Layer 2 is frames. Frames refer to the entire message from Layer 2 to Layer 7. Control bits mark the beginning and end of frames just as picture frames mark the edges of a picture. Because you are already familiar with how Layer 2 allows different devices to take turns on the media (media access control), and how the network works (logical topologies), now I want you to look at how Ethernet data is actually packaged at Layer 2. In particular, compare DIX Ethernet to IEEE Ethernet, including the various Layer 2 headers in Figure 5-3.