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Wireless Reviews

How It Works

I covered the basics of Quantenna's "Full-11n" technology in 2010: The Year of HD Streaming Wirelessly? Part 2. So some of this will be going over covered ground. If you really want the gory tech details, check the videos linked in the article linked above and on Quantenna's website and download the whitepaper from Quantenna (free, simple registration required).

Quantenna's technology uses 4X4 MIMO (4 transmit, 4 receive), but with only two spatial streams. (This is abbreviated 4X4X2 in the NETGEAR and Quantenna literature.) They claim this technology produces "5x the coverage and 2x the throughput" of other 802.11n products.

But two spatial streams will support only a 300 Mbps maximum link (PHY) rate (with two bonded channels, i.e. 40 MHz channel bandwidth mode). And since two streams can be accomplished with 2X2 MIMO, why does Quantenna's system incur the extra expense and complexity of 4X4 MIMO to get just two streams?

The complex answer essentially boils down to the fact that the more antennas used per stream, the higher the possible signal gain, so the higher the Signal To Noise Ratio (SNR). A high SNR yields higher effective (i.e. real) throughput with a lower bit or packed error rate (BER or PER). Figure 8 shows two spatial streams being multiplexed to four transmit antennas and recovered by four receive antennas.

4x4 MIMO with multiplexing of two spatial streams

Figure 8: 4x4 MIMO with multiplexing of two spatial streams
(courtesy Quantenna Communications)

BER/PER is particularly important, since most 802.11n implementations allow high BER/PER in order to achieve higher speed benchmarks in tests, which vendors have trained consumers to look for. But for reliable HD streaming, you need consistent throughput that is free of the second-long dropouts you get in most every other 802.11n system available today.

There is some pretty heavy-duty signal processing involved and Quantenna says its technology makes use of some of the techniques that are optional, but not commonly used in consumer 802.11n gear. Maximal Ratio Combining (MRC), for example, optimizes the received SNR by using all four antennas to recover two spatial streams.

Quantenna also makes heavy use of dynamic digital beamforming to increase gain. Beamforming works by having the MIMO receiver talk to its associated transmitter, telling it how to tweak the transmitted signal to compensate for the environment between them. Quantenna says that it obtains 12 to 25 dB of gain using this technique. Check this video if you'd like Dr. Rezvani, Quantenna's Founder, President and Chairman, to give you a primer on dynamic beamforming.

A Peek At Performance

But most of you probably don't care if NETGEAR's box uses hamsters spinning on wheels inside, as long as it gets your HD stream from server to player. So what can you expect for your $260?

Quantenna says Full-11n "delivers up to four [emphasis mine] flawless high-definition (HD) video streams at more than 100 Mbps data rates, over 100 feet, and guarantees this performance nearly 100 percent of the time through near-zero packet error rate (PER) data transfers, regardless of signal impairments and dead zones that are typical in the home".

NETGEAR takes Quantennas claim of four streams and appears to double it (!) in the diagram shown in Figure 9, which appears on the product's webpage and datasheet. But there were no actual throughput claims in any of NETGEAR's marketing information, so I asked them what the product was designed to deliver.

VERY optimistic WNHD3004 connection diagram
Click to enlarge image

Figure 9: VERY optimistic WNHD3004 connection diagram

Som Pal Choudhury, Director of Product Marketing, Consumer Networking, answered my query with the following:

Taking an average of MPEG2 1080p stream at 20 Mbps per stream ... we can stream Multiple HD streams (Up to 3) inside a above average size single family home across multiple walls. As 3D streams implies double the bandwidth (one for each eye), you can do at least ONE if using 1080P MPEG2 taking 20 Mbps average.

So NETGEAR basically said the product can provide an average of ~ 60 Mbps throughput across multiple rooms.

And what about those eight devices connected via three WNHD3004s? Well, Som said they showed four devices per adapter because of its four Ethernet ports. But "Connects Up to Four Devices" (as noted in the diagram) is different than "Streams to up to four devices simultaneously", which is not what the diagram states. And since typically there is only one display device in a room, only one of the four devices will be active at a time.

Som also said they showed two rooms because, continuing with the one display per room model above and the claim of being able to support up to three 20 Mbps HD streams, they could support HD streaming to two rooms simultaneously.

My initial tests seem to support NETGEAR's 60 Mbps claim, at least for same-room testing, but nowhere near Quantenna's 100 Mbps. I set up the pair of WHD3004's in my office about 10 feet apart with no other wireless traffic active. One WNHD3004 was connected to my LAN's switch, the other to my Acer Aspire 1810T netbook. I let the WNHD3004s set themselves up, which they did without any intervention from me, right out of the box.

I then set up an IxChariot test, using three throughput.scr scripts with the test file size changed from 100,000 to 1,000,000 Bytes, using TCP/IP traffic and ran the test for the usual one minute.

Figure 10 shows the transmit results and Figure 11, the receive.

WNHD3004 Three TCP/IP Transmit streams - same room
Click to enlarge image

Figure 10: WNHD3004 Three TCP/IP Transmit streams - same room

Both directions produced >60 Mbps total throughput and >20 Mbps per stream, which validates NETGEAR's claim in terms of maximum throughput. But remember, this is a stream over a 10 foot distance in the same room.

WNHD3004 Three TCP/IP Receive streams - same room
Click to enlarge image

Figure 11: WNHD3004 Three TCP/IP Receive streams - same room

But I want you to notice how low the throughput variation is and that there was not one single throughput dropout during the one minute test. However, most HD video is longer than a minute. So I ran a one hour test (Figure 12) and was happy to see that it too, had no dropouts.

WNHD3004 Transmit and Receive - same room - 1 hour
Click to enlarge image

Figure 12: WNHD3004 Transmit and Receive - same room - 1 hour

So, at least for same-room operation, the WNHDB3004 kit seems to meet NETGEAR's 60 Mbps design goal. The real test, however, will be how much of that throughput is lost as I test it in all six of my test locations. For those results, and the full review, you'll have to check back in a few weeks.

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