| Re: PCIe-AS - Tutorial -- Is there a CRC for the address header ? [message #933 is a reply to message #930] |
Wed, 15 September 2004 18:15   |
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| David Slogsnat wrote on Wed, 15 September 2004 16:40 |
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Every single routing byte is parity checked at each hop(parity check is only a very small overhead). If there is an error, the packet is retransmitted on link level.
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That means, in ATOLL there is protection against single bit errors, not less, not more.
AS uses the PCIe physical link layer, which is serial with 8b/10b coding. The consequence is, that a single bit error on the medium will give in many cases an invalid code word, and will thus be detected, but in other cases, gives a different code word leading to multiple data bit errors. For examplefor the later case is that a single bit error in bit e can turn D0.1- into D9.1-, which when decoded gives a two bit error (two 0 are turned into 1). Since two bits are flipped, this isn't detected with a parity bit.
From this I'd conclude, that the ATOLL protection method actually doesn't protect against all single bit errors on the medium when a serial link with 8b/10b is used, thus a parity bit doesn't seem to be the prefect solution either.
W.F.J.Müller, GSI, CBM, Tel: 2766
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