linux-kernel - Re: single bit errors on files stored on USB-HDDs via USB2/usb

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Message-ID: <20061210084453.13702.qmail@science.horizon.com>
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.
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