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Wireless Features

Quantenna, QCA, Broadcom logos

It's clear by now from ASUS announcements [story] and NETGEAR leaks [story] that AC2350 and AC3200 class routers will soon be on the way. (ASUS has referred to its upcoming RT-AC87U as AC2300, but I suspect that will soon change to AC2350.)

After spending some quality time with Broadcom, Quantenna and Qualcomm Atheros (QCA) at their offices last week, I can say that this product transition will be different. These introductions are not just about higher link rate, although one of the classes has that. Instead, what knowledgeable buyers will be faced with are two fundamentally different approaches to improving bandwidth utilization.

The Problem

The Wi-Fi industry has trained us to expect that higher link rate—the "bigger number on the box—is the cure for any Wi-Fi woe. Poor range? Buy a new router with a bigger number. Video stuttering? Get a bigger, badder router. Everyone trying to watch YouTube, Netflix, etc. at the same time on an assortment of devices? Step right up to AC1900 and your problems will vanish.

In some cases, trading up to a higher class router helps. But most of the time, it just introduces another set of problems, especially when dealing with the mix of N, AC and sometimes G clients that most of us have.

The mix of client types and transition in wireless activity from intermittent bursts of small data chunks (email, web browsing, messaging) to continuous streams of video at higher data rates is the real problem. And your pain is networking companies' gain, since they are all selling $200+ top-of-line routers as fast as they can make 'em.

So how much do client mixes cause problems? How Well Do AC Routers Handle Mixed Networks? tested a mix of AC, N and A clients typically found in many networks connected to a NETGEAR R7000 AC1900 class router. Table 1, taken from that article, summarizes throughput differences measured with combinations of clients. Since the measurements were done with continuous streams of data, they are a reasonable indicator of how streaming video traffic would be affected.

Combo Class Individual Simultaneous % Difference
1 AC867 199.4 130 -35
N450 81.0 59 -25
2 AC867 199.4 120 -40
N150 12.6 12.8 0
3 AC867 199.4 140 -30
A54 16.7 6.2 -65
Table 1: Class combinations and throughput losses

Note that even a single N150 device, typically found in most smartphones and tablets currently in use, reduced throughput of the higher-class AC867 client by 30 - 35%. These tests were performed at maximum link rates and signal levels. Lower link rates caused by weaker signals could make these results optimistic.

Even switching to all AC clients may not boost throughput as much as you might expect. Testing in How Much Throughput Can You Really Get From An AC Router? showed that maximum wireless throughput is no higher than the maximum supported by the combination of your router's class and its highest-class client. In other words, if your highest class client is a 1x1 AC580 smartphone, your total wireless throughput will max out at under 200 Mbps, vs. the 300 - 400 Mbps possible with full 3x3 AC clients.

The reason for the above is the serial nature of current Wi-Fi. All versions of 802.11 to date allow only one client at a time to use the channel(s). If a client has a low link rate due to its innate capability (wireless class) or signal level, then it will need more airtime to transmit a given amount of data than a client supporting a higher link rate. This is illustrated in the simplified diagram below, which doesn't factor in framing overhead.

Airtime required to transmit 1 Gigabit of data

Airtime required to transmit 1 Gigabit of data

The contention for airtime is normally not a problem when the amount of data moved is small relative to available bandwidth. But when a slow client has to move a lot of data, it can start eating up a lot of airtime and cause problems for all other clients, regardless of higher link rates they may support.

The other thing to note about the current scenario is that bandwidth use is very inefficient. Two of the streams in an AC1750 or AC1900 router are basically unused in most current networks since most clients are single-stream (N150, AC580).

MU-MIMO

Multi-user MIMO (MU-MIMO) is intended to improve wireless bandwidth use by enabling simultaneous AP-to-client transmission. The diagram below from a QCA presentation simply illustrates MU-MIMO's effect, which is to move from one transmission to multiple transmissions in each on-air timeslot.

Single User vs. Multi User MIMO Throughput

Single User vs. Multi User MIMO Throughput
Image credit: Qualcomm Atheros

Notice that the example shows three MU-MIMO streams from a four-stream (4x4) access point (AP). Broadcom's MU-MIMO presentation, which focused on showing "real world" MU-MIMO, explained the # of streams-1 MU-MIMO client rule. It comes from the fact that economically realizable MU-MIMO devices need to use linear precoding vs. the non-linear precoding assumed in earlier MU-MIMO descriptions. Broadcom also asserts that MU-MIMO beamforming will result in lower effective per client transmit power, which in turn results in lower link rate.

The parallel transmission nature of MU-MIMO is also intended to free up airtime for "legacy" (802.11a/b/g/n) clients, increasing bandwidth utilization for even non MU-MIMO devices.

Single User vs. Multi User MIMO Throughput

Single User vs. Multi User MIMO Throughput
Image credit: Qualcomm Atheros

MU-MIMO doesn't require a 4x4 router. But the first MU-MIMO capable devices from both Quantenna and QCA are 4x4 designs. Four-stream support is available in 5 GHz only, where the additional stream bumps the maximum link rate to 1733 Mbps. Three-stream and 256-QAM support in 2.4 GHz adds 600 Mbps, for a total of 2300 Mbps if you round down, 2350 Mbps rounding up.

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