Mondo bondage

Atheros' Channel Bonding technology isn't new, since it's been used for a few years now in Atheros' 802.11a "Turbo" feature. NETGEAR's 108Mbps Technology overview paper explains that Channel Bonding works by using two radio channels in a way that they appear as a single, higher-speed channel to both the transmitter and receiver. The effect is similar to "multilink" techniques used by ISDN and some dialup ISPs, although the exact mechanics are different.

The upside is that Super-G really does provide significantly increased throughput and - unlike Nitro and Xpress - doesn't rely on mixed mode aggregate throughput gains to provide its benefit.

The downside - as Broadcom wants everyone to know - is that Channel Bonding cuts a wider swath in the 2.4GHz spectrum, allegedly interfering with all eleven channels in the 2.4GHz 802.11b/g band. As I'll show shortly, Super-G with channel bonding does use more spectrum than a single, unbonded channel (sorta makes sense doesn't it?). But first let's look at this spectrum that everyone's so stirred up about.

The truth about channel overlap

802.11b and g use eleven channels in the 2.4GHz band, spaced at 5MHz intervals. Since the commonly accepted width of each channel is 22MHz for 802.11b and 20MHz for 802.11g, both 802.11b and g are said to have three non-overlapping channels (1, 6 and 11).

Tip: Wireless networking management company Cirond argues that there are actually four channels (1, 4, 8, 11) that can be used for 802.11b and g with virtually no performance penalty.

Now if all the energy of the transmitted signal actually were contained within a 20 (or 22MHz) band, the definition of "non-overlapping" might be simpler. But reality is somewhat more complicated.

Figure 1: 802.11b Transmit Spectrum Mask
From Matthew Gast's 802.11 Wireless Networks: The Definitive Guide , used by permission

Figure 1 shows an idealized spectral plot (power vs frequency) of an 802.11b signal. To paraphrase the explanation in Chapter 10 of Matthew Gast's excellent book, this plot shows that transmitted power is reduced by 30dB below (1/1,000) the power at the center of the channel (that's what the dBr notation means) at +/-11MHz away from the channel center and 50dB below (1/100,000) at +/-22MHz away.

NOTE: The following spectrum diagrams are based on Figure 1 and are not done to exact scale. Any inaccuracies are not intentional!

Since 11b and g channels are on 5MHz spacings, two channels right next to each other (1 and 2 for example) would overlap as shown in Figure 2.

Figure 2: 802.11b adjacent channel overlap

The yellow shaded area represents the power from channel 2's signal that overlaps into channel 1's main lobe (the largest "hump" and also the frequency band that contains most of the signal's power). Since a significant amount of channel 2's main lobe overlaps into channel 1's main lobe (and vice versa), communication on both channels will suffer. Contrast this picture with the situation shown in Figure 3.

Figure 3: 802.11b "non-overlapping" channel overlap

This figure has the same scale as Figure 2, but shows signals in the "non-overlapping" channels 1, 6 and 11. Since the power from each signal doesn't magically stop at the 22MHz channel boundaries, there is still overlap between "non-overlapping" channels. But in this case, the yellow shaded area that represents channel 11's power that is overlapping into the main lobe of channel 6 is at least 30 dB lower (1/1000) than channel 11's peak power.

Put simply, channels 1, 6 and 11 are considered to be "non-overlapping" because the amount of power that does overlap is supposedly too small to significantly affect each channel's operation. Whether that's actually the case, however, depends on many other factors, including the device's Adjacent Channel Rejection (ACR) capability, and, of course, the distance between devices on different channels. By the way, although I've been using examples based on 802.11b, the situation is pretty much the same for 802.11g.

Theory, however, is nice, but what happens in the real world? That's what I'll show next.

Tags: Atheros, WiFi,

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