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Optional Features - more

So all you need to do is hit the Wi-Fi Alliance Certification database, search for routers that are certified for these optional features, get one, plug it in and instantly upgrade your wireless LAN, right? I suspected that the answer was no, but needed confirmation. Fortunately, a technical contact at Atheros was able to come to my aid.

Table 1 was put together by Atheros in response to this question: Which of the optional features implemented in an 802.11n router or AP will provide speed or performance benefits if used with clients that don't support the features?

Features Certification Requirements /Benefits
20/40 MHz Coexistence Mechanisms in the 2.4 GHz Band • Required on AP side primarily to support co-existence. STA can also send intolerant bit status to AP to signal use of 20MHz channel. STA will follow AP's channel bonding and channel switching HT20/40 mechanism
• Can be used with Legacy STA devices (11a,g)
Greenfield Preamble • Required on AP and STA side to support
• Can't use with legacy STA
Short Guard Interval (SGI), 20 and 40 MHz • Primarily used on AP side to reduce Guard interval between frames to improve performance
• Helps improve data rate in Up/Down if both AP and STA support Short GI. Otherwise only Downlink performance (AP to STA) will improve
• Part of Mixed mode frame format. So while talking to legacy client will not be used. No side effects.
Space Time Block Coding (STBC) transmit • Helps improve reception and transmission if STBC is available on AP and STA devices
• Since it requires 2 or more antennas (at least 1 stream or higher) legacy devices might not support this feature
HT Duplicate Mode (MCS 32) • Tradeoff between higher throughput vs robust transmission at range.
• Need to have HT40 MHz present all time to send duplicate packet on extension channel
• Not legacy device compatible
A-MPDU (Transmit mode) • Aggregation size is independent of PER and can go up to 64KB
• With each subframe within this aggregated frame has its own error check CRC and shows improves robustness of the transmission along with increasing row throughput
• Need STA side to support A-MPDU aggregation to achieve benefits from this mechanism

I've bolded the relevant parts of each answer, but the bottom line is that only one of the optional features—Short Guard Interval—will provide a benefit when used with 802.11n clients that don't implement it. And even then, the benefit will be only for downlink.

Bottom Line

If you want to take advantage of the performance-enhancing optional features of 802.11n, you're going to have to do your homework. Don't expect any help from product makers; they make the Wi-Fi Certification logo very tiny and make you search for it on box side and bottom panels. Not that it matters for these purposes; you need the Certification certificate (Figure 3) to really see what's going on in the Options department.

802.11n optional feature throughput improvement - simulation
Click to enlarge image

Figure 3: Wi-Fi Certification Certificate with optional features

Your best bet is to search the Wi-Fi Alliance Certification database and use the advanced search to seek out routers and clients that support the options you're looking for. Unfortunately, three streams isn't among the searchable options right now, although an Alliance spokesperson said that they expect to number of streams as a search parameter by the end of the year, "if not sooner". So you'll also be able to search for single-stream products once their Certification process gets going.

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