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USB vs. Bluetooth

Table 3 compares the maximum speeds achieved by Bluetooth and USB EVDO and it's clear that using a USB connection can provide a significant download speed boost.

Table 3: Max Speed Comparison of Bluetooth EVDO and USB EVDO

Tests run in Terminal C at Chicago O'Hare International Airport using a Bluetooth / EVDO gave me typical download speeds—27.1 KB/sec, 35.5 KB/sec and 29.9 KB/sec from the Speakeasy Chicago, Seattle and Atlanta test servers, respectively. But switching to a USB connection (Option 2) gave me up to 7 times (!) the download speeds (211.0 KB/sec from Chicago, 190.0 KB/sec from Seattle, and 198.8 KB/sec from Atlanta).

But why the higher download speeds using the USB connection? A quick look at the following information from Bluesoleil should help:

"The gross data rate of Bluetooth 1.1 is 1 Mbps. The protocol splits that bandwidth to support both voice and data communication. Bluetooth can support an asynchronous data channel, up to three simultaneous synchronous voice channels, or a channel that simultaneously supports asynchronous data and synchronous voice.

Each voice channel supports a 64 Kbps synchronous (voice) link. The asynchronous data channel can support an asymmetric link of up to 721 Kbps in either direction, while permitting 57.6 Kbps in the return direction or a symmetric link up to 432.6 Kbps."

So assuming that both the Treo and the Mac aren't supporting EDR (the Mac might, but the Treo probably doesn't and both need to support it to achieve EDR rates) this limits the connection to the 1.1 rates described above. A symmetric link bandwidth of 432.6 Kbps would be 54 KB/s down and up, while an asymmetric link would yield 90 KB/s down and 7.2 KB/s up.

The max speeds in Table 3 don't exceed 90 KB/s down but they are higher than 7.2 KB/s up. So it's likely some combination of asymmetric and symmetric communication is being used. But since no download speeds are above 90 KB/s, Bluetooth does indeed appear to limit the speed.

Note that using a USB connection did not improve upload speeds during any of my tests and download speed improvement was inconsistent. For example, a test at the Detroit Metro Airport didn't yield speedier downloads, but in a Panera restaurant near Toledo, Ohio, USB speeds were 2X Bluetooth's! So if download speed is important in your business, USB tethering may help, and in certain cases, a USB connection may help a lot!

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