For people who have looked at upgrading their network to 802.11ac, no doubt you have been perplexed by the wide range of physical layer rates advertised. Before, in the 802.11b and g era, all cards operated at just one rate – i.e. 11Mbit/s and 54Mbit/s respectively.
802.11n changed this somewhat when they introduced the concept of MIMO and spatial streams, with clients supporting multiple spatial streams effectively multiplying the “base” rate (i.e. 1-stream = 150Mbit/s, 2-streams = 300Mbit/s, 3-streams = 450Mbit/s). Dual-band operation also increased the variety of physical layer rates advertised, as many routers were adding the rates on both bands together, so it could be possible to see 150Mbit/s, 300Mbit/s and 450Mbit/s as single band routers, and 300Mbit/s, 450Mbit/s, 600Mbit/s, 750Mbit/s and 900Mbit/s as dual band routers.
The concept is much the same with 802.11ac, just that the numbers are a little different and the number of possibilities has gone up. The base single-stream rate for 802.11ac is 433.3Mbit/s (80Mhz, Short GI), with each spatial stream effectively multiplying this base rate. This is somewhat simplified as the 802.11ac makes provision for even larger bandwidth channels (160Mhz) and deeper modulation modes which can increase or change this “base” rate, but this is what is presently used in specifications and datasheets, so we’ll go with that.
The base single-stream rate for the 2.4Ghz band remains the same as 802.11n (as it is still 802.11n) and is 150Mbit/s (40Mhz, Short GI), with each spatial stream multiplying the base rate. An additional wrinkle is added, however, that there is a deeper modulation, known as TurboQAM which uses 256QAM and is only compatible with TurboQAM-supporting chipsets, which provides a theoretical throughput equivalent to four streams on 2.4Ghz but actually uses 3 spatial streams.
That might be hard to visualize from the text, so I made this table which lists the total throughput rates for a given number of streams in each band. Common market-available rates are marked in bold.
- AC600 is 1+1 (i.e. 150Mbit/s on 2.4Ghz + 433 Mbit/s on 802.11ac)
- AC750 is 2+1 configuration with 300Mbit/s on 2.4Ghz and 433Mbit/s on 802.11ac
- AC900 often means 867Mbit/s on 802.11ac and is often equivalent to AC1200 depending on number of 2.4Ghz streams
- AC1200 is 2+2 (i.e. 300Mbit/s on 2.4Ghz + 867Mbit/s on 802.11ac)
- AC1750 is 3+3 (i.e. 450Mbit/s on 2.4Ghz + 1300Mbit/s on 802.11ac)
- AC1900 appears to be 4+3, but relies on TurboQAM extension to provide 600Mbit/s on 2.4Ghz with 3 spatial streams with unchanged 802.11ac rate
- AC2400 appears to be 4+4 but relies on TurboQAM on 2.4Ghz as AC1900 does.
As a result, you need to be careful with what you buy and whether certain models are worth the upgrade.
For example, an AC1750 and an AC1900 unit are likely to perform exactly the same for most users. For users using the 5Ghz band with 802.11ac clients, they will both connect at 1300Mbit/s. The only difference comes down to the 2.4Ghz band, where theoretically, 600Mbit/s is available from the AC1900 unit, however, this is only possible for TurboQAM capable chipsets (i.e. new 802.11ac ones running in 2.4Ghz), and won’t make a difference to your existing dual or triple-stream non-Turbo-QAM 802.11n clients. In fact, dual stream 802.11n clients can only achieve 300Mbit/s on the 2.4Ghz band anyway, so the extra stream is really “wasted” on many clients.
Likewise, if you’re looking for 5Ghz performance, paying extra for an AC750 router over an AC600 router isn’t meaningful because the extra performance comes from one additional spatial stream in the 2.4Ghz band.
If you see adapters advertised as AC900, they’re almost always the same as an adapter advertised as AC1200. The reason is that those are usually earlier adapters with only the 5Ghz AC rate of 867Mbit/s advertised, rather than the marketing practice of summing both bands and then rounding up. For that matter, you can only ever be connected on one band. There is no way a single client can see both throughputs summed together!
Building your network out for maximum performance, you will need to match the number of spatial streams on your intended operating band. If you have an AC2400 router, you will need a quadruple stream capable card to make the most of the available throughput. If you use any less, you will be limited by the number of spatial streams supported by the card! As a rule of thumb, cards supporting more spatial streams are generally harder to obtain and more expensive. But it does mean that an AC1750 card and an AC1900 card operating in 802.11ac 5Ghz mode will perform (practially) identically as they both support the same number of streams and throughput in the given band.
In the future, TurboQAM may be extended to four spatial streams for a throughput of 800Mbit/s on the 2.4Ghz band, and thus four-stream routers might be able to claim another 200Mbit/s on top (i.e. AC2600) of the AC2400 designation they market with today.
There is a wide range of gear being sold as 802.11ac, and while it is true that they do support 802.11ac modulations, they vary significantly in their ability to support a number of spatial streams, which improves throughput. As a result, users need to be careful to buy the solution that works best for them – those which produce more 2.4Ghz throughput than two streams in 2.4Ghz are unlikely to be beneficial due to interference and most 2.4Ghz devices only supporting two or less streams.
Users should also be aware of the marketing practice of rounding up, and adding band throughputs together – as a result, they might not be aware that an AC1750 card and an AC1900 card operating in 802.11ac mode both have an identical maximum link rate of 1300Mbit/s. It makes it slightly confusing to an end user, who may just look at the number and think bigger is better, and pay more to receive no tangible benefit.