The Wireless Access Point and the device have to support 11ac, right? for instance, let's say my WAP supports 11bgn and my smartphone supports 11ac, then I will not get the 11ac benefits on my device right?
If your Access Point (WAP) only supports 802.11bgn then your 802.11ac smartphone will not see any benefit until you upgrade the Access Point. However if you have an 802.11ac AP, you will still get benefits for your legacy 802.11bgn devices. We see around 30% benefit on average and much greater at further distances from the AP.
Can I mix AP-135s (802.11n) and AP-225s (802.11ac) in a single building? We have high-density large auditoriums in our classroom buildings, in addition to staff offices and "average" density rooms.
Yes, we are recommending exactly this approach for customers who are not ready (or don't yet have the network demands) to move their entire network to 802.11ac. We recommend in this case deploying the 802.11ac APs in the areas where you have either the highest density or where the application bandwidth requirements are most challenging.
Will upgrading from .11n AP's to .11ac require new POE switches to power them?
No. Existing PoE switches can be used with most vendors' 802.11ac APs, Aruba included. If you are upgrading your switches in the future you should ensure they are capable of PoE+ (802.3at) in order to future proof them as this is likely the last generation of Wi-Fi APs that can operate on PoE (802.3af) power.
As opposed to A, B, G or N how is 802.11ac different and how will the emerging standard be different from that?
While 802.11n supported both the 2.4GHz and 5GHz frequency bands, 802.11ac has limited itself to the 5GHz band only. The Wi-Fi Alliance certification for 802.11n supports up to 3 spatial streams with 40MHz channels. 802.11ac builds on top of the 802.11n standard by extending it in the following ways: (a) expanding the channel width up to 160MHz (with 80MHz supported in the current 11ac products); (b) adopting a new 256-QAM encoding method that is 33% more efficient; (c) allowing up to 8 spatial streams (with current 11ac products supporting up to 3 spatial streams); (d) adopting a single type of Explicit Transmit Beamforming; (e) adding Multi User MIMO so that an AP can transmit to more than one client simultaneously (expected to be implemented in the Wave 2 products in 2015-2016).