Friday, February 6, 2015

Should you select 802.11n or 802.11ac access points for your project?

The answer, as always, is that it depends on your project’s requirements and constraints.  

802.11ac is only available on the 5 GHz band – for 2.4 GHz, both 802.11n and 802.11ac access points will only provide 802.11n service on the 2.4 GHz band.    The 802.11ac standard is being introduced in multiple phases, called “waves” by the industry.    802.11ac wave 1, which is what all AP manufacturers currently have on the market, is very similar to 802.11n at 5 GHz. 

The two main (and cumulative) improvements in 802.11ac wave 1 are as follows:

  • 802.11ac wave 1 allows for the use of 80 MHz channel sizes, as opposed to 20 MHz or 40 MHz channel sizes with 802.11n.   This provides over double the effective throughput as compared to 802.11n using 40 MHz channel sizes.
  • 802.11ac wave 1 introduces the “256-QAM” modulation scheme, which allows for a 33% improvement in throughput compared to 802.11n by using a more complex encoding scheme to stuff more data into the same period of time.
As with all technology improvements, there are limitations:

  • Since the portion of the 5 GHz spectrum available for Wi-Fi is fixed by the FCC, as the channel sizes increase, the total  number of independent channels decrease.   Accordingly, you need to ensure that you either have only a few access points, or an appropriate channel reuse pattern, when using larger channel sizes.   When using the UNII-2 and UNII-2e bands (typical for enterprise-grade access points), there are 25 independent channels @ 20 MHz channel size, 10 independent channels @ 40 MHz channel size, and 5 independent channels @ 80 MHz channel size.   When using consumer access points (e..g consumer routers/access points), the  UNII-2 and UNII-2e bands are not supported due to DFS (subject of a future blog), there are 9 independent channels @ 20 MHz channel size,  4 independent channels @ 40 MHz channel size, and only 2 independent channels @ 80 MHz channel size.    By comparison, the 2.4 GHz band only has 3 independent channels @ 20 MHz channel size.
  • 256-QAM modulation requires that the signal strength between the access point and the client device must be very high (i.e. a signal-to-noise ratio of >29 dB), which is only achievable in practice when the client device is less than 10 – 15 feet away from the access point without any obstructions (e.g. walls) or external sources of radio frequency interference.
If you have the available Internet bandwidth, a reasonably clean radio frequency environment at 5 GHz,  and you want your network to have a 3 - 5 year lifespan while supporting all of the new Wi-Fi client devices that will emerge during this time frame, you should invest in 802.11ac wave 1 now for your deployments.   Otherwise, you should continue to deploy 802.11n, but expect to upgrade that network in 18 – 36 months to take advantage of the more radical improvements expected with 802.11ac wave 2.

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