Kevin Purcell
Shaun James said in response to Houston Hall's observation:
"It is my understanding that if you use Airport Express with the new Airport, you will revert to 802.11g performance" The fact is, whether you have a G or an N network, your entire wireless network reverts to the speed of the slowest connected wireless device.
As another reader has pointed out this isn't true. It's more complicated than just a no but it is a no all the same.
An older g AP does drop to 802.11b (or rather 802.11 "Classic" 1/2MBs + 802.11b 5.5/11Mbs) because the standard didn't define a proper mixed compatibility mode.
But "protection" and "mixed mode" compatibility is built into 802.11n standard from the start. They knew they would have to work alongside networks running b and g.
For interoperability, 802.11n supports three modes: Legacy mode (for an n AP talking to a/b/g clients on a network); Mixed mode (for an n AP talking to n clients on a network amongst a/b/g clients); and Greenfield mode. (for an n AP talking only to n clients).
In legacy mode an n AP does drop back to b/g speeds. But most people using a n AP (if they have n clients) will be running in mixed mode which doesn't force everyone to b/g connections. It tries to deal with each connection type "natively".
This will cause some slowdown from the "potential" top speed as the slower clients take more time to move their bits on the channel. And that's time when n clients can't move their bits. How bad this is depends on whether the AP divides time between legacy and high throughput transmissions.
For a more detailed view see "802.11 Wireless Networks: The Definitive Guide, Second Edition" by Matthew Gast
http://www.oreilly.com/catalog/802dot112/
The free chapter "A Peek Ahead at 802.11n: MIMO-OFDM"
http://www.oreilly.com/catalog/802dot112/chapter/ch15.pdf
is particularly useful in understanding early draft 802.11n changes. It's not current any more (the draft is a moving target) but most of the ideas from both TGnSync and WWiSE proposals MAC and PHY have been merged into the current draft standard.
Read the sections on "Channels, radio modes, and coexistence", "Aggregation and bursting" and "Protection". And search for "Legacy" through the document. Caution: this is technical!
Glenn Fleishmann has a great interview with Matthew Gast which discusses all these issues (from a layman's POV) in WNN Podcast 18. If you want to understand this better listen to this podcast. It will dispel a lot of myths about wireless modes.
http://www.wifinetnews.com/archives/006925.html
