以前总是搞不清楚1000BASE-PX20中的PX20是怎么回事，今天看到了在IEEE802.3ah中的解释
原来20是距离，而不是刚开始以为的管脚数
1.4.xxx 1000BASE-PX10: IEEE 802.3 Physical Layer specification for a 1000 Mb/s point to multi-point
link over one single mode optical fiber, with a reach of up to 10 km. (See IEEE 802.3 Clauses 60, 65, and
66.)
1.4.xxx 1000BASE-PX20: IEEE 802.3 Physical Layer specification for a 1000 Mb/s point to multi-point
link over one single mode optical fiber, with a reach of up to 20 km. (See IEEE 802.3 Clauses 60, 65, and
66.)