Working With Bit Time Values

Some vendors note that their repeater delay values are smaller than the value listed in table 14.2, which will also make it easier to meet the 512 bit time maximum. While theoretically these extra bit times could be used to provide a longer than 5 meter inter-repeater segment length, for example, this approach could lead to problems.

While providing a longer inter-repeater link might seem to be an advantage, you should also consider what would happen if that vendor's repeater failed and had to be replaced with another vendor's repeater whose delay time was larger. If that happened, then the worst-case path in your network might end up with excessive delay due to the bit times consumed by the longer inter-repeater segment you had implemented. You can avoid this problem by designing your network conservatively and not pushing things to the edge of the timing budget.

Note that it is possible to use more than one Class I or two Class II repeaters in a given collision domain if the segment lengths are kept short enough to provide the extra bit time budget required by the repeaters. However, the majority of network installations are based on building cabling systems with 100 meter segment lengths (typically implemented as 90 meters "in the walls" and 10 meters for patch cables, etc.). A network design with so many repeaters that the network requires very short segments to meet the timing specifications is not going to be useful in most situations.

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

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