Cisco Aironet Access Point Deployment Guide
the Intel card or open a case with Intel or the laptop manufacturer for a possible remedy. During the AP 3600 beta trials, Cisco observed differences in performance with different notebooks using the Intel 6300 card.
Note: Sometimes it can be difficult to reliably maintain a 3SS link, since it is easy for the client to rate‐shift out of the 3SS mode. The ability to maintain a 3SS link varies with the quality of the client and the test environment.
The AP 3600, with its extra radio per band, can use the extra redundant radio to beamform (thanks to ClientLink 2.0) and uses this to maintain the advantage of 3SS links. ClientLink 2.0 can also improve the overall performance of 802.11n clients using one, two, and three spatial streams and legacy .11a/g clients.
Beamforming in ClientLink 1.0 and 2.0
ClientLink 1.0 was first introduced with the AP 1250 and the AP 1140; it is a method to create a stronger signal on the downlink side for 802.11a/g clients by hearing the clients on the uplink and then adjusting the transmitter timing so the signal appears much stronger at the client end.
This feature was previously user configurable. However, starting with 7.2 code stream, it is now on by default and is not user configurable because there is no benefit to disabling it.
The AP 3600 fully supports ClientLink 1.0 for 802.11a/g clients but has a greater advantage, for it also supports all 802.11n clients including one, two, and three spatial stream clients. This capability is called ClientLink 2.0. There is a distinct advantage with ClientLink 2.0 over the 802.11n enhanced beamforming specification, because ClientLink 2.0 works with all clients today and does not require any client sounding or support. (See Figure 55.)
Figure 55: ClientLink 2.0
With beamforming technology, changing the timing of two transmitters creates a stronger signal for the receiver (for example, a client device). This is referred to as constructive interference. If the opposite happens, and the signals cancel each other out, it is called destructive interference. (See Figure 56.)
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