Since the WNDR4500 review, I've been having a series of helpful discussions with NETGEAR about the results. They have helped me to understand the reason for the low 2.4 GHz dual-stream performance results (using a three stream client with only two antennas connected). I'll be rerunning the tests and updating the Charts and publishing a retest report as soon as I receive the Intel 6200 card that seems to be crawling here from the vendor.
During the discussions, NETGEAR sent me an interesting Powerpoint deck, which they have allowed me to share with you. The key point of the presentation is summarized in the slide below.
Figure 1: Laptop Performance Comparison Summary
The slide shows six notebooks that all support three-stream 802.11n. Each has three Wi-Fi antennas that are connected to an Intel Centrino Ultimate-N 6300 mini-PCIe embedded adapter. The table contains key attributes that NETGEAR found affect wireless performance when they were running their own tests on the WNDR4500.
NETGEAR found that laptop design can have a significant effect on three-stream N performance. The next to last column—Close Range Wireless Performance (3m distance)—shows maximum throughput measured between each notebook and a WNDR4500. The biggest performance gain was 225% (5 GHz downlink on a Lenovo X220i vs. Dell E5410). But the performance gain between the same pair of laptops running a different test (2.4 GHz uplink) could be a little as 33%.
The design attributes that have the greatest affect on performance are noise coupling and antenna configuration. MIMO, which is a key contributor to 802.11n's higher throughput, is affected by antenna isolation. So if a laptop has five antennas (three Wi-Fi, two 3G/4G WWAN) crammed into whatever tiny space is available in the lid that isn't taken up by screen, those antennas are going to be closer together and have lower isolation.
High frequency noise coupling, however, is just as big, if not bigger contributor to lower performance. Processors and memory clocked at multi-GHz rates don't make for good neighbors to Wi-Fi radios that operate in a similar spectrum.
The higher the noise level the radio detects, the more likely it will switch to a lower link rate, which usually produces lower actual throughput. And if that noise varies, as you'd expect it to with the variable clock rates today's notebooks use to minimize power consumption, throughput stability will also suffer as the radio switches link rates to follow the varying signal-to-noise levels.
NETGEAR included some very interesting annotated shots of four out of the six notebooks listed in the summary table. The best of the bunch is represented by the Dell Latitude E5410. The E5410 is an unsexy beast. But its relatively large footprint that accomodates its 14.1" screen leaves room for Wi-Fi friendly component placement and cable routing.
Figure 2: Dell Latitude E5410 Internal View
Although the Intel 6300 looks like it is pretty close to what is probably the Southbridge chip, the CPU and memory are a safe distance away from the 6300's relatively short antenna cables. And since there are no WWAN antennas, there is more room for the Wi-Fi antennas, which means better isolation.
Figure 3 shows the Dell Latitude E5420, a more recent 14" laptop design with a similar internal layout. But with the Intel 6300 placed further away from the lid hinge, the cables are longer and more prone to noise pickup. And with five antennas crammed into the lid vs. the E5410's three, the higher noise and lower antenna isolation take around a 10% bite out of maximum throughput vs. the benchmark E5410.
Figure 3: Dell Latitude E5420 Internal View
Next up is the Lenovo W510. This workstation-class 15.4" laptop shows that you can't judge a notebook's Wi-Fi performance by its size. Even though the chassis is a bit larger than the two Dells', Lenovo makes the design error of both longer Wi-Fi cabling and routing it right near the CPU and memory noise generators.
Figure 4: Lenovo W510 Internal View
This bad positioning and the five antennas in the notebook lid produce performance reductions from 15% to as high as 45% compared to the Dell E5410.
The last example in the NETGEAR presentation was another Lenovo, the X220i. This happens to be the notebook that I use as the SmallNetBuilder three-stream test client. The X220 series is a "thin-and-light" style with a 12.5" screen and no optical drive. So it starts out with less wiggle room for component placement.
Figure 5 shows that the X220i's design makes a similar mistake to the W510's, with long Wi-Fi cabling that runs beside the CPU/memory area for a good distance.
Figure 5: Lenovo X220i Internal View
This, plus even less room in the smaller lid for five antennas makes the X220i the worst performer of the bunch, with throughput reductions between 25 and 55% vs. the Dell E5410.