Hi Henry,   This is probably the most frequent topic and there are a number of typical root causes.  Here are some of the most common:   • Client wireless device drivers While 802.11ac-capable client devices and access points have been on the market for the last 12-18 months or so, the fact is it's still an evolving technology and plenty of interoperability issues have been discovered and addressed during its adoption.  Too, early versions may not yet have supported many features defined in the standard.  Check with your vendor and install current versions of the drivers as a first step.   • Interoperability issues for non-11ac clients 11ac-capable APs advertise new information elements (IEs) in their beacon/probe responses that did not exist a couple of years ago.  And from an RF perspective, things look a little strange in the air compared to when that adapter or driver was state-of-art.  While the device may not be 11ac-capable, it needs to be able to interoperate.  The 802.11ac standard is written to be backwards compatible with b/g/a/n devices, but older drivers may not have been written or tested with 11ac in mind.  Again, consider starting with a driver upgrade as first troubleshooting step.   •Adapter Capabilities 802.11ac promises to deliver raw wireless data rates up to 1300 Mbp/s – and it can with the right hardware under the right conditions.  But the proper access point and client adapter are required – just because a device is 802.1ac-capable doesn’t mean it is able to fully exploit it.  To achieve that speed, the client adapter and the access point must be capable of supporting 80MHz-wide 4-channel bonds, 3 spatial streams, and a guard interval of 400ns.  Most USB 11ac client adapters and integrated chipsets in mobile devices on the market are 1SS or 2SS, therefore the max possible rates they can achieve will be lower.  The data rates supported by each AP are in their respective Data Sheets,  and the Tables are useful to describe what's possible for each # SS, GI, channel width, etc. Below is the link to the AP 3700 data sheet for reference.   So for example, take a 1SS-capable 11ac device with 80MHz channel.  The maximum possible 11ac DTR for that device is 433.3Mb/s.  Many Apple MacBook Air and Pro models have a Broadcom 3SS-capable card so they are the most common we see in the field capable of top DTRs.  Check the specs on your adapter, and if purchasing a new one compare the capabilities.   http://www.cisco.com/c/en/us/products/collateral/wireless/3700-series-access-point/data_sheet_c78-729421.html   • Configuration 802.11ac (like 11n) requires the WLAN to have AES encryption (or Open) and WMM. Also, 4 channels must be available and assigned to each AP for the 80MHz bond to achieve highest rates.  If using the WLC, check under DCA and verify 80MHz channel width is enabled.  If any one of these is not met then 11ac will not work.  Too, check the driver configuration of the client adapter and verify it’s set properly for 11ac. One other common quirk that comes up is how the WLC reports client 11ac rates.  It does not report the clients' rate in channel-width qualified terms,  only an MCS Index and # of SS (refer to Yilin's earlier post). So if you see M9 3SS (for example), that could mean 288.9, 600, or 1300 depnding on the channel-width configuration of the AP.  For long GI rates, we list the actual DTR value the client is using.  So when reading client details, keep in mind what the channel width is of the AP.  I may file an enhancement request for this so it's clearer. • RF Conditions All typical WiFi caveats apply for 802.11ac just like legacy standards.  Wireless is a half-duplex, shared medium environment so think ‘Hub’ (not switch) when considering throughput math.  Each addition of a new associated device divides the aggregate available bandwidth of the AP.  Even under the best of RF conditions, WiFi is lossy.  802.11 frame loss over-the-air occurs regularly and can vary greatly  depending on hardware (client and AP), interference, and distance from the AP just to name a few.  The impact this loss can have on protocol packet data performance also varies depending on the type of stream, IP stack, protocal, etc.  Realized data transfer rates will be lower than the raw wireless data rate by an appreciable margin. Also – until a few years ago when MIMO and 11n came into play, multipath was the devil.  Much of our own documentation still discusses ‘multipath mitigation’ in evil terms.  But to achieve 11n and 11ac rates, multipath is essential.  This requirement presents special challenges especially in outdoor environments where there are no convenient cubes/windows/floors/ceilings to induce MP.  For environments where MP may not be intrinsic, consider the use of external antennae with cross-polarized elements which can help induce it.   • Software Bugs If you are not running the current versions of AP or controller code and are experiencing an issue, check the Release Notes of latest images to see if any issues have been addressed.  There have been a few 11ac-specific fixes published since 7.5/7.6 first released so consider current code (7.6.110.0 as of this writing).  You can also check the Bug Toolkit on cisco.com for more up-to-date information or call TAC for assistance.   ... View more