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No doubt about it, PDAs with wireless LAN support can be handy little devices. But what you might not know is how slow that wireless capability can be. This NeedToKnow will focus on testing the wireless throughput of a number of popular PDAs to show what we found to be a surprisingly wide performance range.

It's the Integration that counts!

I first stumbled upon low WiFi throughput in PDAs last fall, when testing SanDisk's SD WiFi card in an HP H2210 iPAQ running Pocket PC 2003. The tests for that review yielded best case average throughput of only about 350kbps. Both SanDisk and SyChip (who makes the card for SanDisk) said that the main factors in the low speed were software and hardware issues in the H2210's SDIO interface.

What I found after a little digging seems to confirm the SanDisk / SyChip story, though there is some conflicting info. The SD interface on Intel's PXA255 XScale processor used in the H2210 (and in many current PocketPCs) is actually an MMC interface capable of supporting only the 1 bit transfer mode at a maximium 20MHz clock rate(1) .

NoteNOTE: The PXA250 described in Reference (1) is the same as the PXA255 in its SD capability.

Even though it's not an SD interface per se, Table 1 (2) shows the MMC 1 bit mode is theoretically capable of supporting a 2.5MB/s (20Mbps) transfer rate. This should be plenty fast to support 802.11b's full 11Mbps raw data rate, as long as software overhead isn't a significant factor.

Update May 27, 2004 - An informed source told me that the H2210 actually doesn't use the PXA255's MMC interface for SD. Instead it uses a Samsung "iPAQ Companion Chip" that includes an SD Host controller capable of 4-bit SDIO operation. So it would appear that the H2210's hardware is willing but the firmware is weak...

But my experience with some of the PXA255-based PDAs that I tested yielded WLAN transfer rates far below what you'd expect for such relatively fast raw bus speeds. So it would appear that there are some software or other issues at play that limit wireless throughput.

Product Maximum Clock Speed Maximum Burst Rate
SPI Bus mode 20 MHz 2.5 MB/s
MMC 1-bit mode 20 MHz 2.5 MB/s
SD Card
SPI Bus mode 25 MHz 3.125 MB/s
SD 1-bit mode 25 MHz 3.125 MB/s
SD 4-bit mode 25 MHz 12.5 MB/s
Table 1: MultiMediaCard and SD Card Clock Speed and Burst Rate (2)

Table 1 also shows that a true SD interface capable of supporting 4 bit mode at 25MHz has a maximum bus speed of 12.5MB/s (100Mbps)! Even if software overhead is a significant factor, the 4 bit vs. 1 bit transfer should yield improved throughput since 4X the data is moved per transfer. Note that the new crop of PDAs based on Intel's PXA27x processor family - such as Dell's X30 line - can support 4-bit SDIO(3) .

Not all PDAs use the SD bus to connect either an internal or external WiFi module with the host processor, however. You'll see shortly that this seems to help move 802.11b speeds up to more normal rates.

(1) Using SDCard and SDIO with the Intel PXA250 MMC Controller Application Note

(2) Host Design Considerations: Application Note NAND MMC and SD-based Products

(3) Intel PXA27x Family Processor FAQ

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