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Quick Reference Guide to 10-Mbps Multi-Segment Configuration

7.4 10-Mbps Configuration Guidelines: Model 1

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The first configuration model provides a set of multi-segment configuration rules for combining various 10-Mbps Ethernet segments. The bold text is taken directly from IEEE Std 802.3j-1993 (p.26). [Italic text indicates my comments and is not from the standard.]

1. Repeater sets are required for all segment interconnection. [The repeaters used must comply with all IEEE specifications in section 9 of the 802.3 standard, and do signal retiming and reshaping, preamble regeneration, etc.]

2. MAUs that are part of repeater sets count toward the maximum number of MAUs on a segment. [Thick Ethernet repeaters typically use an outboard MAU to connect to the thick Ethernet coax. Thin coax and twisted-pair repeater hubs use internal MAUs located inside each repeater port.]

3. The transmission path permitted between any two DTEs may consist of up to five segments, four repeater sets (including optional AUIs), two MAUs, and two AUIs. [The repeater sets are assumed to have their own MAUs, which are not counted in this rule.]

4. AUI cables for 10BASE-FP and 10BASE-FL shall not exceed 25 m. (Since two MAUs per segment are required, 25 m per MAU results in a total AUI cable length of 50 m per segment).

5. When a transmission path consists of four repeaters and five segments, up to three of the segments may be mixing and the remainder must be link segments. When five segments are present, each fiber optic link segment (FOIRL, 10BASE-FB, or 10BASE-FL) shall not exceed 500 m, and each 10BASE-FP segment shall not exceed 300 m.

6. When a transmission path consists of three repeater sets and four segments, the following restrictions apply:
   • The maximum allowable length of any inter-repeater fiber segment shall not exceed 1000 m for FOIRL, 10BASE-FB, and 10BASE-FL segments and shall not exceed 700 m for 10BASE-FP segments.

   • The maximum allowable length of any repeater to DTE fiber segment shall not exceed 400 m for 10BASE-FL segments and shall not exceed 300 m for 10BASE-FP segments and 400 m for segments terminated in a 10BASE-FL MAU.

   • There is no restriction on the number of mixing segments in this case. [In other words, when using three repeater sets and four segments, all segments may be mixing segments if desired.]

FIGURE 7.3 One possible maximum 10-Mbps configuration

The figure shows an example of a maximum Ethernet configuration that meets the canned configuration rules. The maximum packet transmission path in this system is between DTE 1 and DTE 2, since there are four repeaters and five media segments in that particular path. Two of the segments in the path are mixing segments, and the other three are link segments. You can find more examples in Appendix A, "10-Mbps Configuration Examples."

While the canned configuration rules are based on conservative timing calculations, you shouldn't let that lead you to believe that you can bend these rules and always get away with it. There isn't a lot of engineering margin left in maximum-sized Ethernets, despite the allowances made in the standards for manufacturing tolerances and equipment variances. If you want maximum performance and reliability, then you need to stick to the published guidelines.

Also, while the configuration guidelines emphasize the maximum limits of the system, you should beware of stretching things as far as they can go. Ethernets, like many other systems, work best when they are not being pushed to their limits.

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Quick Reference Guide to 10-Mbps Multi-Segment Configuration - 09 SEP 95

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