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1.8 Signal Topology and Media System Timing

Round Trip Timing

In order for the media access control system to work properly, all Ethernet interfaces must be capable of responding to one another's signals within a specified amount of time. The signal timing is based on the amount of time it takes for a signal to get from one end of the complete media system and back, which is known as the "round trip time." The maximum round trip time of signals on the shared Ethernet channel is strictly limited to ensure that every interface can hear all network signals within the specified amount of time provided in the Ethernet medium access control system.

The longer a given network segment is, the more time it takes for a signal to travel over it. The intent of the configuration guidelines is to make sure that the round trip timing limits are met, no matter what combination of media segments are used in the system. The configuration guidelines provide rules for combining segments with repeaters so that the correct signal timing is maintained for the entire LAN. If the specifications for individual media segment lengths and the configuration rules for combining segments are not followed, then computers may not hear one another's signals within the required time limit, and could end up interfering with one another.

The correct operation of an Ethernet LAN depends upon media segments that are built according to the rules published for each media type. More complex LANs built with multiple media types must be designed according to the multi-segment configuration guidelines provided in the Ethernet standard. These rules include limits on the total number of segments and repeaters that may be in a given system, to ensure that the correct round trip timing is maintained.

Quick Reference Guide to the Ethernet System - 04 SEP 95
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