Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 36 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 319 Tuesday May 20 2003 2 53 PM 10-Mbps and 100-Mbps Ethernet 319 Manchester encoding relies on the direction of the edge transition in the middle of the timing window to determine the binary value for that bit period. In the encoding example shown in Figure 6-3 one timing window is highlighted vertically through all four waveform examples. The top waveform has a falling edge in the center of the timing window so it is interpreted as a binary 0. The result is that in the center of the timing window for the second waveform there is a rising edge which is interpreted as a binary 1. Instead of a repeating sequence of the same binary value in the third waveform example there is an alternating binary sequence. In the first two examples the signal must transition back between each bit period so that it can make the same-direction transition each time in the center of the timing window. With alternating binary data there is no need to return to the previous voltage level in preparation for the next edge in the center of the timing window. Thus any time there is a long separation between one edge and the next you can be certain that both edges represent the middle of a timing window. The fourth waveform example is random data that enables you verify that whenever there is a wide separation between two transitions both edges are in the center of a timing window and represent the binary value for that timing window. Legacy 10-Mbps Ethernet has some common architectural features. All of these legacy versions are referred to as shared Ethernet because they share a common collision domain. It is not only allowed but it is expected that an Ethernet network could contain multiple types of media for example 10BASE5 10BASE2 10BASE-T and so on . The standard goes out of its way to ensure that interoperability is maintained. However when implementing a mixed-media network it is important to pay particular attention to the overall architecture design. It becomes easier
