14.6 100-Mbps Configuration Guidelines: Model 2

Transmission System Model 2 for 100BASE-T segments provides a set of calculations for verifying the round trip signal delay of more complex Fast Ethernet LANs. The physical size and the number of links and repeaters in a 100BASE-T system are primarily limited by the round-trip signal timing required to ensure that the collision detect mechanism will work correctly. The Model 2 configuration calculations provide the information you need to verify the timing budget of a set of standard 100BASE-T components, to make sure that their combined signal delays fit within the timing budget required by the standard.

You may notice that these calculations appear to have a different round trip timing budget than the ones used for the 10-Mbps media system. This is because media segments in the Fast Ethernet system are based on different signalling systems than 10-Mbps Ethernet, and because the conversion of signals between the Ethernet MAC and the media segments consumes a number of bit times. You may also notice that there is no calculation for interframe gap shrinkage, unlike the 10-Mbps Model 2 rules. That's because the maximum number of repeaters allowed in a Fast Ethernet system does not result in enough gap shrinkage to worry about.

- Finding the Worst-Case Path
- Calculating Round Trip Delay Time
- Segment Delay Value
- Calculating Your Own Segment Delay Values
- Typical Propagation Values for Cables

Quick Reference Guide to 100-Mbps Multi-Segment Configuration - 09 SEP 95

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