802.11n兼容a/b/g问题（Legacy mode，Mixed mode，Greenfield mode）

中文的没看到有什么人去解释，大致翻下查到的英语的。

For interoperability, 802.11n supports three modes, explains Hung:
对于兼容性，802.11n支持三种模式：

* Legacy mode.
延伸模式

Addresses 11n APs and an 11a/g/b client environment. In this case, the client decodes the first three fields of a six-field header, which is specific to 11a and 11g. If the 5-GHz option isn’t supported in the 11n AP, there will be no backward compatibility with 802.11a.

用来解决802.11n的AP与a/g/b的兼容模式。在这种模式下，客户端首先解码首先得三部分在第六层（six-field header，我觉得是第六层也就是mac的意思）的报头，对于802.11a和g市特定的部分。如果AP是不支持5G的频带，那么这时候AP就不能向下兼容802.11a.

 

* Mixed mode.
混合模式

Addresses 11n APs and a mixed 11n/a/g/b environment. As in legacy mode, 11a and 11g/b clients will read the first three fields of the AP’s 11n header, while 802.11n will also parse the additional three “high throughput” fields of the full 11n header. Again, if the 5-GHz option isn’t supported in the 11n AP, there will be no backward compatibility with 802.11a.

用来解决802.11n的AP与一个n/a/g/b混合的模式。如同延伸模式一样，11a与11g/b的客户端将会读首先mac帧的三个字段，而802.11n还会解析增加的三个“高吞吐量”字段在整个802.11n的帧报头中。如同延伸模式一样，如果AP不支持5G频段，那么不能够兼容802.11a.

 

* Greenfield mode.
格林菲爾德模式?这个不知道怎么翻，greenfield模式解释如下

PS:The Greenfield mode is an optional high-throughput mode in the 802.11n standard, which is not backward compatible with legacy

(802.11a/b/g) protocols and is expected to provide maximum performance benefits of 802.11n.
这个模式是在可选择的高吞吐量模式，没有延伸模式的那种兼容性，只被希望去提供802.11n那种高吞吐量的形式。

Addresses a pure network of 802.11n APs and clients, taking full advantage of the high-throughput capabilities of the 11n MIMO architecture.
用来解决一个纯粹的802.11nAP与客户端的模式，这种架构下能够获得802.11n能够提供的最大的吞吐量（信道容量）.