# IEEE 802.11ac

## Summary


IEEE 802.11ac is a Wi-Fi standard for Very High Throughput (VHT) applications. It's a significant improvement over the earlier IEEE 802.11n while remaining backward compatible. It's designed only for the 5 GHz band. 

It provides higher data rates, reaching a maximum of 6.9 Gbps. Explicit beamforming and MU-MIMO are two important features of 802.11ac that improves network capacity and efficiency. 

It has a migration path towards IEEE 802.11ax.

## Discussion

### What are the typical use cases for 802.11ac?

Video consumption is on the rise. A 720p uncompressed video at 60 frames/sec needs 1.3 Gbps. A H.264 lightly compressed video needs 70-200 Mbps. IEEE 802.11n offer a theoretical 600 Mbps but practical rates available for application are lot less.

The number of devices in the home or office is also increasing. There's a need for multiple devices to connect to the same access point and utilize the channels more efficiently. This is particularly true for bring-your-own-device (BYOD) scenarios where each employee may bring multiple Wi-Fi devices to the office. 

In general, 802.11ac aims to provide high data rates for video streaming, low latency experience and more efficient multiplexing of multiple clients. 


### What are main features of 802.11ac contributing to higher data rate?

While 802.11n offers a maximum data rate of 600 Mbps, 802.11ac can offer 10x data rate due many improvements: 

    + **Channel Bonding**: Each channel is 20 MHz wide but with 802.11ac we can combine 8 of these to obtain 160 MHz channel for a single client. If available, contiguous channels are easily combined although standard defines 80+80 MHz mode to combine non-contiguous channels.
    + **Higher Modulation**: The use of 256QAM is possible, 4x denser than 64QAM of 802.11n. This means that 4x more bits can be carried per symbol.
    + **Spatial Streams**: Up to 8 spatial streams are possible, although Wave 2 certification covers only 4 spatial streams.The maximum data rate of 6.9 Gbps is obtained when using 160 MHz channels, 256 QAM, eight spatial streams and 400 ns guard interval. A handy reference comparing 802.11n and 802.11ac rates relative to SNR and RSSI is available online.


### How is 802.11ac able to achieve better efficiency?

For better channel utilization and network efficiency, the following are useful:

    + **5 GHz Band**: The use of 2.4 GHz band is avoided where interference is higher due to cordless phones, microwaves and other devices. The 5 GHz band is cleaner.
    + **Enhanced RTS/CTS**: To avoid collisions due to the use of wider channels, RTS/CTS mechanism is extended.
    + **A-MPDU**: For higher MAC layer throughput, all MAC frames are sent as Aggregate MAC Protocol Data Unit (A-MPDU), which was introduced in 802.11n for selective use.
    + **MU-MIMO**: With Single-User Multiple-Input and Multiple-Output (SU-MIMO), only one client could send/receive at a particular time. Multi-User Multiple-Input and Multiple-Output (MU-MIMO) is able to multiplex multiple clients at the same time, thus reducing latency and improving overall network efficiency.
    + **Beamforming**: Due to the use of MIMO and multiple antennas, beamforming is possible. Transmission is steered towards each client. This is made more efficient via explicit feedback from clients–using Null Data Packet (NDP)–for better channel estimation.

### What are some trade-offs involved in using 802.11ac?

Decision to trade-off one metric with another will depend on real-time network conditions. Moving from 40 MHz to 80 MHz aggregate bandwidth will increase data rate, but since the same power is spread across many more subcarriers, range will reduce. Obtaining a free channel 160 MHz wide is also difficult, especially in enterprise use cases. It's easier with 80+80 MHz mode but this requires twice as many RF chains. 

Moving from 64QAM to 256QAM is possible only over short distances (good signal-to-noise ratio) since the constellation is tighter and more sensitive to errors. 

Beamforming using Explicit Compressed Feedback (ECFB) gives a precise channel estimate but this feedback comes with a lot of overhead. Beamforming and MU-MIMO become less effective when clients are moving. 

In terms of spatial streams, each stream requires its own antenna and RF chain. Although 8 streams are defined in the standard, often this is impractical in mobile devices since each antenna must be sufficiently spaced. 


### Could you explain 802.11ac MU-MIMO?

Since 802.11n, SU-MIMO allows routers to send/receive multiple streams of data to/from clients. MU-MIMO is introduced in 802.11ac in the downlink. An access point can send data to multiple clients at the same time. For example, with three clients A, B and C, two streams may be sent to A, one stream to B and one stream to C simultaneously. Clients receiving single streams need not have multiple antennas or RF chains, which is often the case with small devices and smartphones. 

With MU-MIMO, we get better network capacity utilization because a single client with not much to send can be multiplexed with other clients. Because multiple clients can be allowed to receive at the same time, latency also drops. Even non-MIMO clients will benefit since they can access the channel more easily. More device clients can be supported on the Wi-Fi network. 

In 802.11ac, MU-MIMO is available only in the downlink. Another limitation is that access points might fallback to SU-MIMO if they detect that clients are moving, since MU-MIMO may not work well. 


### What are 802.11ac Wave 1 and Wave 2?

The idea of identifying two "waves" of 802.11ac products was to allow vendors to release their first 802.11ac products into the market quickly. Wave 2 is complex due to MU-MIMO, 160 MHz bandwidth support and four spatial streams. By defining Wave 1 without these features, we can still benefit from having 256QAM and 80 MHz bandwidth support that 802.11n lacked.

First Wave 1 products started arriving in 2013. Certification for these also started in mid-2013. From 2014, all new products started supporting Wave 1. Early Wave 2 products arrived in 2015. Certification for Wave 2 started in mid-2016. 

## Milestones

Sep  
2008

Work on 802.11ac standardization formally commences with the approval of Project Allocation Request. 

2012

Draft 2.0 of 802.11ac is released in January. A refined draft 3.0 is released in May. 

Jun  
2013

Wi-Fi Alliance announces certification process for 802.11ac Wave 1 products. 

Dec  
2013

**IEEE 802.11ac-2013** standard is published. Along with IEEE 802.11ad-2012 that was standardized a year earlier, both are the result of the **Very High Throughput (VHT)** study group. 

2014

Quantenna becomes the first to have a chipset that's capable of 4x4 MU-MIMO but without 160 MHz support. In April, Qualcomm Atheros also starts offering chipsets for MU-MIMO. In general, 802.11ac Wave 2 support arrives in 2014 whereas 2013 saw only 802.11ac Wave 1 support. 

Jan  
2015

At CES 2015, more chipsets and products capable of 802.11ac Wave 2 are announced. 

May  
2015

What's probably the world's first 802.11ac router with MU-MIMO support, Linksys releases its EA8500. For best results, clients also need to support MU-MIMO. Some existing devices might support it with a firmware upgrade since their underlying chipsets have MU-MIMO support already. An example of this is ASUS RT-AC87U router released in August 2014. 

Jun  
2016

Wi-Fi Alliance announces certification process for 802.11ac Wave 2 products. 

2018

CNET identifies some of the best 802.11ac routers. This includes brands Asus, Synology, D-Link and Netgear. Asus has multiple routers highly recommended by CNET.