VHT
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VHT

802.11ac

• 802.11ac defines Very High Throughput (VHT) enhancements below 6 GHz.
• The technology will only be used in the 5 GHz frequency bands where 802.11a/ n radios already operate.
• 802.11ac will take advantage of the greater spectrum space that the 5 GHz U-NII bands can provide.

802.11ac promises Gigabit speeds using four major enhancements:

1. Wider Channels 802.11n introduced the capability of 40 MHz channels, which effectively doubled the data rates. 802.11ac brings us the capability of 80 MHz and 160 MHz channels. This is the main reason that enterprise 802.11ac radios will  operate at 5 GHz as opposed to the 2.4 GHz ISM band.
2. New Modulation 802.11ac will provide the capability to use 256-QAM modulation, which has the potential to provide a 30 percent increase in speed. 256-QAM modulation will require a very high signal-to-noise (SNR) ratio to be effective.
3. More Spatial Streams 802.11ac radios could be built to transmit and receive up to eight spatial streams. In reality the first several generations of 802.11ac chipsets will use one to four spatial streams.
4. Improved MIMO and Beamforming While 802.11n defined the use of single-user MIMO radios, Very High Throughput (VHT) introduces the use of multi-user MIMO (MU-MIMO) technology. An access point with MU-MIMO capability could transmit a signal to multiple client stations on the same channel simultaneously if the client stations are in different physical areas. 802.11ac will utilize a simplified beamforming method called null data packet (NDP) beamforming.
   The 802.11ac technology will hit the streets in two waves.
   
   

• Wave 1: Chipsets take advantage of 256-QAM modulation and 80 MHz wide channels. Most of the AP hardware uses 3×3:3 radios.
• Wave 2: Chipsets are expected to be able to use MU-MIMO and 160 MHz channels. The AP hardware will also most likely be 4×4: 4 radios.

802.11ad

• 802.11ad defines Very High Throughput (VHT) enhancements using the much higher unlicensed frequency band of 60 GHz.
• The higher frequency range is big enough to support data rates of up to 7 Gbps.
• The downside is that 60 GHz will have significantly less effective range than a 5 GHz signal and be limited to line-of-sight communications as the high frequency signal will have difficulty penetrating walls.
• The 60 GHz Wi-Fi technology has the potential to be used for wireless docking, wired equivalent data transfers, and streaming of uncompressed video.
• To provide seamless transition when roaming between the 60 GHz frequency band and legacy 2.4 GHz or 5 GHz bands, a “fast session transfer” feature was added to the specification.
• Currently there is a debate about whether or not 60 GHz Wi-Fi will be used in high-density deployments with a fallback to 5 GHz. This VHT technology also required the adoption of a new encryption mechanism.
• There was concern that the current CCMP encryption methods may not be able to properly process the higher anticipated data rates.
• CCMP uses two chained-together AES cryptographic modes to process 128-bit blocks of data. The 128-bit blocks of data must also be processed “in order” from the first AES cryptographic mode to the second mode.
• The 802.11ad amendment specifies the use of the Galois/ Counter Mode Protocol (GCMP), which also uses AES cryptography. However, GCMP calculations can be run in parallel and are computationally less intensive than the cryptographic operations of CCMP.

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.