首先，802.11 g仅 2.4Ghz，802.11 a仅 5GHz。如果您没有双频段客户端 (a+g)，它只能连接到g或a，具体取决于您的客户端支持的频段。

802.11n 可以同时使用 2.4GHz ( gn ) 和 5GHz ( an )，具体取决于您的 AP。

2.4GHz 802.11 n ( gn ) 设备始终可以作为 802.11g 设备运行，因为n仅意味着更大的通道宽度 (40MHz) 和最多四个空间流。但如果有 802.11g 客户端，它可以恢复为只使用一个空间流。

5GHz 802.11 n ( an ) 设备也是如此，如果需要，它还可以恢复为更少的流。

请看一下：https ://en.wikipedia.org/wiki/IEEE_802.11n-2009

First, to clarify, the 802.11 standards are proposed and maintained by the IEEE, specifically the 802.11 working group.

Wi-Fi is a trademark of the Wi-Fi Alliance which provides a voluntary certification program for devices to be Wi-Fi Certified. Devices do not need to be Wi-Fi Certified to operate as 802.11 devices.

I am wondering if one buys a 802.11n conforming station, does it mean that it is able to behave as a 802.11g or 802.11a station if it exisits within 802.11a or g basic service set?

It should definitely be able to operate in a 802.11g environment as 802.11n requires 2.4GHz support.

However, 802.11a operates in 5GHz and 802.11n made 5GHz support optional, so there are two possibilities:

• If you have a single band (only supports 2.4GHz) 802.11n client, it would not be able to operate in an 802.11a environment.
• If you have a dual band (supports both 2.4GHz and 5GHz) 802.11n client, then it would be able to operate in an 802.11a environment.

I will throw out the caveat that some drivers will allow you to specify which band or bands in which a device will operate. So it logically follows that if you have configured your 802.11n dual band device to only operate in 5GHz, it won't be able to operate in an 802.11g environment (and vice versa).

If yes, can someone explain or give me references why?

Technically speaking, when you see something like 802.11g or 802.11n, these are amendments to the main 802.11 standard. They are typically not standards in and of themselves.

Periodically, the IEEE will incorporate these amendments into the main standard as a maintenance release. The current standard is 802.11-2012, which actually started with the 802.11-2007 standard and incorporated 10 additional amendments, including 802.11n.

So, to answer your question as to why, it is because 802.11a, 802.11g, 802.11n and 802.11ac are all amendments to the standard and need to take into account the standard when written/designed. This includes interoperability with previous devices that adhere to the standard.

In order to be considered Wi-Fi gear, the wireless LAN equipment must pass inspection / testing ensuring backwards compatability with legacy standards. So, when you see the Wi-Fi logo, you know it is backward compatable. More at Certification Benefits.