Test show 802.11ac 'gigabit Wi-Fi' potential and limits
Real world experience shows the promise
By John Cox | Network World US | Published: 14:32, 07 October 2013
Vendor tests and very early 802.11ac customers provide a reality check on "gigabit Wi-Fi" but also confirm much of its promise.
Vendors have been testing their 11ac products for months, yielding data that show how 11ac performs and what variables can affect performance. Some of the tests are under ideal laboratory-style conditions; others involve actual or simulated production networks. Among the results: consistent 400M to 800Mbps throughput for 11ac clients in best-case situations, higher throughput as range increases compared to 11n, more clients serviced by each access point, and a boost in performance for existing 11n clients.
Wireless LAN vendors are stepping up product introductions, and all of them are coming out with products, among them Aerohive, Aruba Networks, Cisco (including its Meraki cloud-based offering), Meru, Motorola Solutions, Ruckus, Ubiquiti, and Xirrus.
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The IEEE 802.11ac standard does several things to triple the throughput of 11n. It builds on some of the technologies introduced in 802.11n; makes mandatory some 11n options; offers several ways to dramatically boost Wi-Fi throughput; and works solely in the under-used 5GHz band.
It's a potent combination. "We are seeing over 800Mbps on the new Apple 11ac-equipped Macbook Air laptops, and 400Mbps on the 11ac phones, such as the new Samsung Galaxy S4, that [currently] make up the bulk of 11ac devices on campus," says Mike Davis, systems programmer, University of Delaware, Newark, Delaware.
A long-time Aruba Networks WLAN customer, the university has installed 3,700 of Aruba's new 11ac access points on campus this summer, in a new engineering building, two new dorms, and some large auditoriums. Currently, there are on average about 80 11ac clients online with a peak of 100, out of some 24,000 Wi-Fi clients on campus.
The 11ac network seems to bear up under load. "In a limited test with an 11ac Macbook Air, I was able to sustain 400Mbps on an 11ac access point that was loaded with over 120 clients at the time," says Davis. Not all of the clients were "data hungry," but the results showed "that the new 11ac access points could still supply better-than-11n data rates while servicing more clients than before," Davis says.
The maximum data rates for 11ac are highly dependent on several variables. One is whether the 11ac radios are using 80 Mhz-wide channels (11n got much of its throughput boost by being able to use 40 MHz channels). Another is whether the radios are able to use the 256 QAM modulation scheme, compared to the 64 QAM for 11n. Both of these depend on how close the 11ac clients are to the access point. Too far, and the radios "step down" to narrower channels and lower modulations.
Another variable is the number of "spatial streams," a technology introduced with 11n, supported by the client and access point radios. Chart #1, "802.11ac performance based on spatial streams," shows the download throughput performance.
"Typically, if the client is close to the access point, you can expect to lose about 40% of the overall raw bit rate due to protocol overhead acknowledgements, setup, beaconing and so on," says Mathew Gast, director of product management, for Aerohive Networks, which just announced its first 11ac products, the AP370 and AP390. Aerohive incorporates controller functions in a distributed access point architecture and provides a cloud-based management interface for IT groups.
"A single [11ac] client that's very close to the access point in ideal conditions gets very good speed," says Gast. "But that doesn't reflect reality: you have electronic noise,' multiple contending clients, the presence of 11n clients. In some cases, the [11ac] speeds might not be much higher than 11n."
A third key variable is the number of spatial streams, supported by both access points and clients. Most of the new 11ac access points will support three streams, usually with three transmit and three receive antennas. But clients will vary. At the University of Delaware, the new Macbook Air laptops support two streams; but the new Samsung Galaxy S4 and HTC One phones support one stream, via Broadcom's BCM4335 11ac chipset.
Tests by Broadcom found that a single 11n data stream over a 40 MHz channel can deliver up to 60Mbps. By comparison, single-stream 11ac in an 80 MHz channels is "starting at well over 250Mbps," says Chris Brown, director of business development for Broadcom's wireless connectivity unit. Single-stream 11ac will max out at about 433Mbps.
There are some interesting results from these qualities. One is that the throughput at any given distance from the access point is much better in 11ac compared to 11n. "Even at 60 meters, single-stream 11ac outperforms all but the 2x2 11n at 40 MHz," Brown says.