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Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 30 is the Cisco approved textbook to use alongside version 3.1 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. | 1102.book Page 259 Tuesday May 20 2003 2 53 PM History and Evolution of Ethernet 259 source device sends data out on a network the data carries the MAC address of its intended destination. As this data propagates along the network medium the NIC in each device on the network checks to see if its MAC address matches the physical destination address carried by the data frame. If there is no match the NIC discards the data frame. If there is a match the NIC verifies the destination address in the frame header to determine whether the packet is properly addressed. When the data passes its destination station the NIC for that station makes a copy takes the data out of the envelope and gives the data to the computer to be processed by upper-layer protocols such as IP and TCP. Framing in General Encoded bit streams on physical media represent a tremendous technological accomplishment but they alone are not enough to make communication happen. Framing helps obtain essential information that could not otherwise be obtained with coded bit streams alone. Examples of such information are listed here Which computers are communicating with one another When communication between individual computers begins and when it terminates Recognition of errors that occur during the communication Whose turn it is to talk in a computer conversation Where the data is located within the frame When you have a way to identify computers you can move on to framing. Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit. Figure 5-5 illustrates the ideas of bits and frames. When you work with bits the most accurate diagram you can use to visualize them is a graph showing voltage versus time. However because you usually deal with larger units of data and addressing and control information this type of graph could become ridiculously large and confusing. Another type of diagram you could use is the frame format diagram which is based on voltage versus time graphs. You
