What is 802.11AC?

802.11ac is an evolutionary improvement to 802.11n.

One of the goals of 802.11ac is to deliver higher levels of performance that are commensurate with Gigabit Ethernet networking:

  • A seemingly “instantaneous” data transfer experience
  • A pipe fat enough that delivering a high quality of experience (QoE) is straightforward

In the consumer space, the target is multiple channels of high-definition (HD) content delivered to all areas of the house. The enterprise has different challenges:

  • Delivering network with enterprise-class speeds and latencies
  • High-density environments with scores of clients per AP

What is 802.11ac?

802.11ac is about delivering an outstanding experience to each and every client served by an AP, even under demanding loads.

Meanwhile, 802.11 is integral to a hugely broad range of devices, and some of them are highly cost, power, or volume constrained. One antenna is routine for these devices, yet 802.11ac must still deliver peak efficiency. The one thing that 802.11ac has in its favor is the evolutionary improvement to silicon technology over the past half-dozen years: channel bandwidths can be wider, constellations can be denser, and APs can integrate more functionality.

By design, 802.11ac is intended to operate only in the 5-GHz band. This avoids much of the interference at 2.4 GHz, including Bluetooth headsets and microwave ovens, and provides a strong incentive for users to upgrade their mobile devices (and hotspot APs) to dual-band capability so that the 5-GHz band is more universally usable. This choice also streamlines the IEEE process by avoiding the possibility of contention between 802.11 proponents. There is barely 80 MHz of bandwidth at 2.4 GHz anyway.

As we’ve already seen, 802.11 introduces higher-order modulation, up to 256QAM; additional channel bonding, up to 80 or 160 MHz; and more spatial streams, up to eight. There is an alternative way to send a 160-MHz signal, known as “80+80” MHz.

802.11ac continues some of the more valuable features of 802.11n, including the option of a short guard interval (for a 10 percent bump in speed) and an incrementally better rate at range using the advanced low-density parity check (LDPC) forward error-correcting codes. These LDPC codes are designed to be an evolutionary extension of the 802.11n LDPC codes, so implementers can readily extend their current hardware designs.

Various space time block codes (STBCs) are allowed as options, but (1) this list is trimmed from the overrich set defined by 802.11n, and (2) STBC is largely made redundant by beamforming. 802.11n defined the core STBC modes of 2×1 and 4×2 and also 3×2 and 4×3 as extension modes, but the extension modes offered little gain for their additional complexity and have not made it to products. Indeed, only the most basic mode, 2×1, has been certified by the Wi-Fi Alliance. With this experience, 802.11ac defines only the core 2×1, 4×2, 6×3, and 8×4 STBC modes, but again only 2×1 is expected to make it to products: if you had an AP with four antennas, why would you be satisfied with 4×2 STBC when you could - and should - be using beamforming?

What 802.11ac also gets right is to define a single way of performing channel sounding for beamforming: so-called explicit compressed feedback. Although optional, if an implementer wants to offer the benefits of standards-based beamforming, there is no choice but to select that single mechanism, which can then be tested for interoperability.

Although 802.11ac will be faster then 802.11n, it is not going to give you a Gigabit of throughput. True, 802.11ac access points working with 802.11ac devices will give you faster data transmission feeds than 802.11n. Wi-Fi Certified 802.11ac can deliver data rates up to more than double those of a typical 802.11n network. Practically speaking, "this means a network can support simultaneously streaming multiple HD-quality videos to multiple devices."

Fair enough, but in practice you're not likely to see an 802.11ac reach its theoretical maximum of 1.3 Gigabit per second (Gbps). That's because the conditions you need to reach that speed requires a laboratory not your office.

To reach the highest speeds you need three data-streams, each of which can run up to 433 Megabits per second (Mbps). A typical 802.11ac access point can support up to eight data streams. Client devices must only support one.

The "unofficial" 802.11ac devices that have been shipping in 2013/2014, and the first generation of the standard 802.11ac devices aren't likely to hit these speeds even on a testbed. Some of the fastest speed recorded to date, came from a Netgear router, which hit a high of 331Mbps. That's great, but it's not gigabit great. It is, however, a lot faster than you'll see then with any combination of 802.11n gear.

802.11ac Infrastructure

What many people don't know, is that second-wave 802.11ac APs will require two, not one, Gigabit Ethernet ports. That just doubled your need for switch ports and cable runs. Oh boy!

Sure, you can get by with one port for now, but remember you're not really going to have that many 802.11ac clients in 2013/2014. Next year is when they'll start showing up in large numbers and that's when the second wave 802.11ac APs will be appearing. So, you can forget about doing a drop and replace for your existing 802.11g/n network APs. You won't be able to do it. Even with the next generation of 802.11ac you probably won't need to back them up with 10Gbps up-links.

In summary, all this means is that Gigabit Wi-Fi isn't really here. Faster Wi-Fi is but it's not really going to take off until late 2014 and when it does come deploying it is going to be expensive. We foresee all of us using 802.11n Wi-Fi for years still to come.