Thursday 19 April 2012

Do you need 802.11ac?

802.11ac Intro

It's a new standard from the Wi-Fi Alliance, requiring new radio hardware - new access points, new client chipsets.  802.11ac products are faster and more reliable than the old 802.11a/g/n products.  

802.11ac will remain backwards compatible with 802.11a/n devices. So your new 802.11ac clients will be able to use any current 802.11a/n wireless networks. 

The two main benefits for enterprise are speed and reliability:

  • 5GHz only and operates in cleaner airspace.
  • New modulation typically supporting up to 200 Mbps*.

* Common use with single-antenna, single-stream mobile devices in a 40MHz enterprise environment.

Real Throughput

802.11ac will offer higher bandwidth and more spatial streams. Access points will support up to 8 spatial streams (double what 802.11n can achieve) and 256-QAM modulation (four times .11n).  So the theoretical maximum goes up from 600Mbps to nearly 7Gbps.

Before you get too excited, the theoretical maximums aren't realistic.  These headline 8-channel/8-spatial stream figures aren't going to be supported in the enterprise.  Here's why:

  • Spectrum is limited, there isn't enough frequency to allow radios to bond 8 channels (160MHz) in a multi-AP environment.  Designing for 2 channel bonding (40MHz) is the de-facto.
  • Devices don't support 8 spatial streams as that would need 8 antennas (read on...).

Further reading on Wikipedia 802.11ac

802.11ac Deployment

AP Placement
802.11ac range is no different to 802.11an so you are likely to swap-out your 802.11n models for 802.11ac when the time is right, or simply add an 802.11ac module.  

You'll need Gigabit switches to support the speeds achieved by 802.11ac access points.  The access points also draw more power to run the additional radios.  So your access switches will ideally support PoE+ or 802.11at with power up to 20Watts per port. However, the AP can reduce it's capability to run on standard PoE at 15.4Watts.


Using MIMO you can add more radio-chains or 'spatial streams' to an access point.  This effectively multiplies maximum bandwidth. You'll see the MIMO and spatial stream support written as axb:c where a = transmit antenna, b = receive antenna and c = spatial streams.  802.11n access points are labelled as 2x2:2, 2x3:2, 3x3:3, 3x4:3, 4x4:4 (or simply 2SS, 3SS, 4SS).  The headline 802.11n figure of 600Mbps comes from a 4SS configuration where both the access point and client support 4SS and achieve 4 x 150Mbps.  In reality the majority of 802.11 clients are 1SS phones, tablets and netbooks. Laptops offer 2SS and 3SS, there are no 4SS clients! 
The reason there are no 4SS 802.11n devices out there is to do with throughput requirements, power requirements and antenna spacing.

802.11ac takes the MIMO theory applied to 802.11n and grows it up to 8SS.  We can assume 8SS clients will be scarce for the same reasons there are no 4SS clients.  So to make use of the extra radio streams the vendors are going to have to develop ways of maximising the AP performance.

I am interested to see how vendors will approach the MU-MIMO capability of 802.11ac Wave 2.  The idea here is that your bandwidth can be split between clients, which will be a first for Wi-Fi.  I like to think of this as the wireless hub becoming more like a switch.

Why Invest in 802.11ac?

If your 802.11an network is meeting expectations then you may not NEED to adopt 802.11ac infrastructure.  
Reasons why organisations would consider migrating to 802.11ac:
  • The access points cost the same as previous 802.11n models
  • They have high bandwidth requirements for LAN based services.
  • They want to future proof their WLAN. 
  • They have Gigabit LAN infrastructure.
  • They have PoE+ or 802.11at support on their switches.
  • They are migrating from legacy 802.11ag infrastructure.

Reasons why organisations may prefer to put 802.11ac on the back-burner:
  • They have 802.11n infrastructure which meets their current requirements.
  • They don't have Gigabit LAN infrastructure.
  • The cost of hardware refresh is prohibitive.

Final Thoughts

MU-MIMO will speed up the network in general and in busy environments will bring RF utilisation levels down - because devices spend less time transmitting and receiving.  So it's all good and hopefully those 802.11ac clients will proliferate the environment to achieve that goal.

Since Cisco decided to bring out the 3700 AP at the same price as the 3600 AP the decision around 802.11ac adoption is simple for top-end customers.  Lower-end customers may not want to deploy it based on the cost.  However, current 802.11n clients will achieve throughput speeds of over 20Mbps at the cell edge (16-QAM at around -71dBm).  So realistically, the 802.11n infrastructure will support 99% of customer needs.

Thanks for reading and feel free comment!


Some further info on 802.11ac:

  • 802.11 foundations and 802.11ac review by WildPackets. 
  • Network World article on 802.11ac by GT Hill from Ruckus.