Thursday, May 20, 2004
Tom's Networking: WiFi PDAs' Dirty Little Secret
Posted by Pat Logsdon in "HARDWARE" @ 04:00 PM
"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).
Even though it's not an SD interface per se...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. 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."
As always, Tom's has done an excellent job of dissection and testing. The tests in this article are done with an HP 2210 with a SanDisk SD WiFi card, a Dell Axim X5 Advanced with a SanDisk WiFi card, an Asus A716, and a Palm Tungsten C. All of these devices use the 400MHz Intel PXA255 processor (except the Axim, which uses the PXA263 chip), but they all use different methods of getting the signal to the CPU. The device with the highest throughput had a speed almost three times faster than the next closest device. To find out which device "won", you'll need to read the article.
Hint: :mecry:
Even though it's not an SD interface per se...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. 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."
As always, Tom's has done an excellent job of dissection and testing. The tests in this article are done with an HP 2210 with a SanDisk SD WiFi card, a Dell Axim X5 Advanced with a SanDisk WiFi card, an Asus A716, and a Palm Tungsten C. All of these devices use the 400MHz Intel PXA255 processor (except the Axim, which uses the PXA263 chip), but they all use different methods of getting the signal to the CPU. The device with the highest throughput had a speed almost three times faster than the next closest device. To find out which device "won", you'll need to read the article.
Hint: :mecry:
