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It's been nearly two years since the first Wi-Fi-certified ac products emerged, implementing core features of the IEEE 802.11ac standard. Commonly known as "first wave" 11ac, those devices used wider channels, more spatial streams and higher-density modulation to reach data rates up to 1.3 Gbps -- triple the speed of comparable Wi-Fi-certified 11n products.
Yet 11ac features required to meet the standard's full potential -- speeds of up to 7 Gbps -- were omitted from the first wave certification due to immaturity and engineering challenges. The Wi-Fi Alliance is currently evaluating features to be added to "second wave" 11ac in an updated certification program that will be announced in mid-2016.
What to expect from the wave 2 Wi-Fi
The first wave of 11ac was built upon the same technologies used by 11n -- most notably, multiple input multiple output (MIMO) antennas that transmit data along several spatial streams, optionally combined with double- or quadruple-wide channels to achieve faster data rates. But unlike 11n, 11ac focuses exclusively on 5 GHz band transmission, leaving the congested 2.4 GHz band for use by older, less capable devices and other technologies, such as Bluetooth.
Similarly, the second wave of 11ac will build upon first-wave features. The maximum channel width will be doubled once again, adding support for 80+80 and 160 MHz channels. The number of spatial streams expected from access points (APs) will also be further increased, growing from three transmit and receive streams (3x3) to 4x4. These incremental enhancements have the potential to quadruple maximum data rates -- under certain favorable conditions. In particular, 160 MHz channels may not be available or even useful in crowded enterprise airspaces; they are more likely to be used inside the home to deliver high-definition video.
More importantly, the second wave of 11ac promises to deliver on a brand new technology: multi-user MIMO (MU-MIMO). Previous generations of Wi-Fi employ single-user MIMO (SU-MIMO), where each AP talks to just one Wi-Fi client at any instant in time. With the second wave of 11ac, APs will have the option of being more versatile, combining MIMO antennas, spatial streams and beamforming in varied ways to converse with several Wi-Fi clients at once.
For example, a second wave 4x4 AP might interact simultaneously with two 2x2 laptops or as many as four 1x1 smartphones, doubling or quadrupling WLAN user density. In theory, the 11ac standard supports up to eight spatial streams, but few APs are likely to be built with that many antennas. Furthermore, MU-MIMO will be introduced as an option -- that is, MU-MIMO will not be required of all wave 2 Wi-Fi-certified ac products. However, MU-MIMO and SU-MIMO Wi-Fi devices will be able to interoperate, allowing wave 2 11ac APs to start boosting WLAN user density when serving a mix of old and new 11ac clients.
Wave 2 Wi-Fi implications
Clearly, the second wave of 11ac presents many opportunities to further increase maximum data rates. However, "faster" is not always "better" -- and not always feasible in crowded airspaces such as those found in many enterprise WLANs. Network planners should consider the following implications.
- The high-density 256 quadrature amplitude modulation (QAM) encoding offered by some first wave 11ac products can only be used by APs and clients that are fairly close, such as in the same room. This benefit is therefore more likely to be realized in residential than enterprise environments, although 256 QAM might be useful to relay video to wireless displays in a conference room, for example.
- Similarly, the wider channels offered by first and second wave 11ac products can be used to boost data rates at the expense of increasing co-channel interference and reducing the spectrum available for use by nearby APs. The 5 GHz band used by 11ac may be wider than the old 2.4 GHz junk band, but as Wi-Fi use grows, even 5 GHz channels will be quickly exhausted -- especially in urban or otherwise congested settings. The second wave may tap into more 5 GHz channels, but ultimately WLAN administrators will need to think hard before using even 80 MHz channels -- much less the 160 MHz channels offered by the second wave of 11ac. In short, don't use channels that are wider than the data rates required by your apps.
- As APs with MU-MIMO emerge, WLAN administrators will have an opportunity to support many more Wi-Fi clients per AP. In particular, MU-MIMO may be attractive in locations like classrooms, conference centers, shopping malls and stadiums where there are large concentrations of users -- each of whom requires only best-effort Internet access.
- That said, MU-MIMO is not always the best choice, as it sacrifices distance and throughput for user density. An AP using SU-MIMO will reach more distant Wi-Fi clients -- with better video performance -- than that same AP using MU-MIMO. Watch for chipset manufacturers like Celeno to address this challenge by helping APs become "smart" about when to use MU- or SU-MIMO and offering channel-aware algorithms to optimize quality of experience.
- As second wave 11ac increases total WLAN capacity, it will likely expose bottlenecks throughout your network. Controller-based WLANs may require controller upgrades or support fewer APs per controller. Backhaul links (wireless or wired) may carry more traffic, gradually reaching the point where Ethernet switches will need to be upgraded. Additional MIMO antennas require more power to drive them, which may drive Power over Ethernet upgrades. WLAN management, monitoring and diagnostic tools may need 11ac updates too. As with any other network upgrade, adopting the second wave of 11ac requires capacity planning.
- Finally, enterprise WLAN administrators may want to start learning about MU-MIMO by testing consumer-grade products that have already started to emerge. However, the Wi-Fi Alliance's delayed certification program update suggests that second wave technologies need more time to mature. As a result, vendors may still need to work out interoperability kinks in beamforming and MU-MIMO. So don't begin your second wave 11ac network upgrade in earnest without Wi-Fi-certified ac products -- but do talk to your WLAN vendor about its plans to help customers migrate to the next wave of Gigabit Wi-Fi.
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