Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 31 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 269 Tuesday May 20 2003 2 53 PM Ethernet Operation 269 Figure 5-10 CSMA CD Carrier Sense Multiple Access Collision Collision Detection Backoff Algorithm Networking devices are capable of detecting when a collision has occurred because the amplitude of the signal on the networking media increases CD collision detect . When a collision occurs each device that is transmitting continues to transmit data for a short time to ensure that all devices see the collision. When all devices on the network have seen that a collision has occurred each transmitting device invokes an algorithm known as a backoff algorithm. When all transmitting devices on the network have backed off for a certain period of time random and therefore different for each device any device can attempt to gain access to the networking media once again. When data transmission resumes on the network the devices that were involved in the collision do not have priority to transmit data. Figure 5-11 summarizes the CSMA CD process. Page 270 Tuesday May 20 2003 2 53 PM 270 Chapter 5 Ethernet Fundamentals Figure 5-11 CSMA CD Process Ethernet is a broadcast transmission technology. This means that all devices on a network can see all frames that pass along the networking medium. However not all the devices on the network will process the data. Only the device whose MAC address matches the destination MAC address carried by the frame copies the frame into its buffer. Ethernet is not concerned with Layer 3 network addresses such as IP or IPX. If the MAC addresses match the frame is copied and passed up to Layer 3 to check the destination IP or IPX address for a match. Page 271 Tuesday May 20 2003 2 53 PM Ethernet Operation 271 After a device has verified the destination MAC and IP addresses carried by the data it checks the data packet for errors. If the device detects errors the data packet is discarded. The destination device does not notify the source device regardless of .
