Bandwidth Mode vs. Streams

[thetoad30]

All,

I thought I understood router streams and bandwidth mode on new N routers.

I had always thought that one stream = one channel = 20 MHz mode on the router in 2.4 GHz. I also thought that 2x2 meant that you could have two streams, and that automatically meant 40 MHz mode (2 channels at 20 MHz each).

Apparently, this is not correct, as you can have a 3x3 router with 20 or 40 MHz mode.

Can anyone please point me to, or explain to me, how these two terms are separate and how they interact with each other? Especially now with 2x3 radios (how does that even work?) and the newer 3x3 radios coming out?

Thank you very much.

 

[thiggins]

Bandwidth mode has nothing to do with the number of 802.11n streams. Let's define some terms.

A channel is a frequency that routers are set to use, like a channel on a TV.

20 and 40 MHz modes are channel bandwidths that can be set on 802.11n products. All 802.11 channels (a,b,g,n) are 20 MHz wide. You get a 40 MHz channel bandwidth by grouping two channels together.

Channel bonding was first introduced by Atheros in its "Super-G" technology. The technique is now part of the 802.11n spec.

2x2, 2x3, etc. refer to the number of transmit and receive chains in a radio. The number of transmit channels is the determining factor in the number of 802.11n streams supported.

A single transmit chain can support single stream N, two transmit chains support dual-stream N and three transmit and receive chains are required to support three stream N.

FAQ 11 in this article shows the maximum link rates supported by combinations of streams and channel bandwidths. Note that "2x3" products do not support three-stream N. You need three transmit channels (and three receive channels).

Link rates represent the raw connection speed. They do not represent the actual throughput delivered to the user. FAQ 1 in the same article shows typical throughput delivered by different 802.11 technologies.

The chart does not yet show three stream throughput because testing has shown results to be highly dependent on the combination of router and client used. Initial results can be found here and here.