Note

The Cisco FastHub 316 is a Class II repeater.

A trade-off was necessary between network diameter and collision recovery. The trade-off was between the need to reduce the impact of collision recovery and the need for the network diameter to be large enough to accommodate reasonable network sizes. The compromise was to choose a maximum network diameter (approximately 2500 m) and then to set the minimum frame length long enough to ensure detection of all worst-case collisions.

This compromise worked well for 10 Mbps, but became a problem for higher data-rate Ethernet networks. Fast Ethernet (100 Mbps) was required to provide backward compatibility with earlier Ethernet networks, including the existing IEEE 802.3 frame format and error-detection processes. Additionally, all applications and networking software running on the 10 Mbps networks needed to be supported.

The time it takes for a signal to propagate across the network medium is essentially constant for all transmission rates. The time required to transmit a frame is inversely related to the transmission rate. At 100 Mbps, a minimum-length frame can be transmitted in approximately one-tenth of the defined slot time, and the transmitting hosts would not likely detect any collisions that could occur during this transmission. Therefore, the maximum network diameter specified for 10 Mbps networks could not be used for 100 Mbps (Fast Ethernet) networks. The solution for Fast Ethernet was to reduce the maximum network diameter by approximately a factor of 10, to a little more than 200 m.

This same problem of network diameter arose during specification development for Gigabit Ethernet; however, decreasing network diameters by another factor of 10, to approximately 20 meters, for 1000 Mbps operation was not practical. The developers decided to maintain approximately the same maximum collision domain diameters as Fast Ethernet (100 Mbps) networks and to increase the minimum frame size by adding a variable-length non userdata extension field to frames that are shorter than the minimum length. (The receiver removes the extension field from the frame.)

Table 3-4 demonstrates the variables relating to network diameter for 10 Mbps Ethernet, 100 Mbps (Fast) Ethernet, and 1000 Mbps (Gigabit) Ethernet.

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