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Date: 10 Dec 2006 03:44:53 -0500 From: linux@...izon.com To: linux-kernel@...r.kernel.org Cc: linux@...izon.com Subject: Re: single bit errors on files stored on USB-HDDs via USB2/usb_storage How the wires can cause single-bit errors is a bit beyond me; USB protects every bit on the wire well enough that communication errors should be detected. Every packet starts with an identifier byte; this contains a 4-bit packet identifier repeated twice. Some small "token" packets have an 11-bit payload (7 address and 4 endpoint bits) and a 5-bit CRC. Any corruption of those would result in USB state machine confusion and at least large data gaps. Packets with an actual data payload are protected with a CRC-16. Not quite as strong as Ethernet, but sufficient to detect all errors of three bits or less, and all burst errors of 16 consecutive bits or less. A single-bit flip can't get past a CRC-16 unless you flip at least three bits in the CRC as well. The actual pattern depends on the bit position and averages 8 bits; given the documented bit error positions and a better knowledge of the ATA-over-USB encapsulation protocol, the actual CRC changes could be computed. Now, I can imagine a USB slave controller so cheap and/or buggy that it doesn't check the CRC, but I'd think that most would. Checking a CRC is hardly a novel challenge. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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