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MII接口也有一些不足之处，主要是其接口信号线很多，发送和接收和指示接口有14根数据线(不包括MDIO接口的信号线，因为其被所有MII接口?所共享)，当交换芯片的端口数据较多时，会造成芯片的管脚数目很多的问题，这给芯片的设计和单板的设计都带来了一定的问题。为了解决这些问题，人们设计了?两种新的MII接口，它们是RMII接口(Reduced?MII接口)和SMII接口(StreamMII接口)。?
这两种接口都减少了MII接口的数据线，不过它们一般只用在以太网交换机的交换MAC芯片和多口物理层芯片中，而很少用于单口的MAC层芯片和物理?层芯片中。RMII接口和SMII接口都可以用于10M以太网和100M以太网，但不可能用于1000M以太网，因为此时时钟频率太高，不可能实现。?
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【转自】http://dpinglee.blog.163.com/blog/static/144097753201041131115262/?
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关于PHYAD:?
The?PHYAD?field?for?an?MDIO?frame?is?a?5‐bit?binary?value?capable?of?addressing?32?unique?addresses.?However,?every?MDIO?slave?must?respond?to?physical?address?0.?This?requirement?dictates?that?the?physical?address?for?any?particular?PHY?must?not?be?set?to?0?to?avoid?MDIO?contention.?Physical?Addresses?1?through?to?31?can?be?used?to?connect?up?to?31?PHY?devices?onto?a?single?MDIO?bus.Physical?Address?0?can?be?used?to?write?a?single?command?that?is?obeyed?by?all?attached?PHYs,?such?as?a?reset?or?power‐down?command.??