802.11ac 'gigabit Wi-Fi' starts to show potential, limits
- 07 October, 2013 19:12
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.
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. [For more details, see "11ac will be faster, but how much faster really?"]A
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.
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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.A
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 Clint 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.
Another result is that 11ac access points can service a larger number of clients than 11n access points.
"We have replaced several dozen 11n APs with 11ac in a high-density lecture hall, with great success," says University of Delaware's Mike Davis. "While we are still restricting the maximum number of clients that can associate with the new APs, we are seeing them maintain client performance even as the client counts almost double from what the previous generation APs could service."
Other features of 11ac help to sustain these capacity gains. Transmit beam forming (TBF), which was an optional feature in 11n is mandatory and standardized in 11ac. "TBR lets you concentrate' the RF signal in a specific direction, for a specific client," says Mark Jordan, director, technical marketing engineering, Aruba Networks. "TBF changes the phasing slightly to allow the signals to propagate at a higher effective radio power level. The result is a vastly improved throughput-over-distance."
A second feature is low density parity check (LDPC), which is a technique to improve the sensitivity of the receiving radio, in effect giving it better "hearing."
The impact in Wi-Fi networks will be significant. Broadcom did extensive testing in a network set up in an office building, using both 11n and 11ac access points and clients. It specifically tested 11ac data rates and throughput with beam forming and low density parity check switched off and on, according to Brown. A
Tests showed that 11ac connections with both TBR and LDPC turned on, increasingly and dramatically outperformed 11n and even 11ac with both features turned off as the distance between client and access point increased. For example, at one test point, an 11n client achieved 32Mbps. At the same point, the 11ac client with TBR and LDPC turned "off," achieved about the same. But when both were turned "on," the 11ac client soared to 102Mbps, more than three times the previous throughput.
Aruba found similar results. Its single-stream Galaxy S4 smartphone reached 238Mbps TCP downstream throughput at 15 feet, 235Mbps at 30 feet, and 193Mbps at 75 feet. At 120 feet, it was still 154Mbps. For the same distances upstream the throughput rates were: 235Mbps, 230M, 168M, and 87M.
"We rechecked that several times, to make sure we were doing it right, says Aruba's Jordan. "We knew we couldn't get the theoretical maximums. But now, we can support today's clients with all the data they demand. And we can do it with the certainty of such high rates-at-range that we can come close to guaranteeing a high quality [user] experience."
There are still other implications with 11ac. Because of the much higher up and down throughput, 11ac mobile devices get on and off the Wi-Fi channel much faster compared to 11n, drawing less power from the battery. The more efficient network use will mean less "energy per bit," and better battery life.
A related implication is that because this all happens much faster with 11ac, there's more time for other clients to access the channel. In other words, network capacity increases by up to six times, according to Broadcom's Brown. "That frees up time for other clients to transmit and receive," he says.
That improvement can be used to reduce the number of access points covering a given area: in the Broadcom office test area, four Cisco 11n access points provided connectivity. A single 11ac access point could replace them, says Brown.
But more likely, IT groups will optimize 11ac networks for capacity, especially as the number of smartphones, tablets, laptops and other gear are outfitted with 11ac radios.
Even 11n clients will see improvement in 11ac networks, as University of Delaware has found.
"The performance of 11n clients on the 11ac APs has probably been the biggest, unexpected benefit," says Mike Davis. A "The 11n clients still make up 80% of the total number of clients and we've measured two times the performance of 11n clients on the new 11ac APs over the last generation [11n] APs."
Wi-Fi uses Ethernet's carrier sense multiple access with collision avoidance (CSMA/CA) which essentially checks to see if a channel is being used, and if so, backs off, waits and tries again. "If we're spending less time on the net, then there's more airtime available, and so more opportunities for devices to access the media," says Brown. "More available airtime translates into fewer collisions and backoffs. If an overburdened 11n access point is replaced with an 11ac access point, it will increase the network's capacity."
In Aruba's in-house testing, a Macbook Pro laptop with a three-stream 11n radio was connected to first to the 11n Aruba AP-135, and then to the 11ac AP-225. As shown in Chart #2, "11ac will boost throughput in 11n clients," the laptop's performance was vastly better on the 11ac access point, especially as the range increased.
These improvements are part of "wave 1" 11ac. In wave 2, starting perhaps later in 2014, new features will be added to 11ac radios: support four to eight data streams, explicit transmit beam forming, an option for 160 Mhz channels, and "multi-user MIMO," which lets the access point talk to more than one 11ac client at the same time.
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