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RE: [RPRWG] CRC check in each node?






Regards,
Devendra Tripathi
CoVisible Solutions Inc.
(formerly VidyaWeb, Inc)
Pune, India 
Tel: +91-20-433-1362

> -----Original Message-----
> From: owner-stds-802-17@xxxxxxxxxxxxxxxxxx
> [mailto:owner-stds-802-17@xxxxxxxxxxxxxxxxxx]On Behalf Of David V. James
> Sent: Monday, June 25, 2001 9:08 PM
> To: ieee 802.17 list; OLSSON,FREDRIK (A-SantaClara,ex1)
> 4) Do packets have to be checked, to uncover their bad CRCs,
>    before being forwarded?
>    NO:  The strategy at the receiver's CRC checking hardware
>         is to compute the valid CRC while the initial portion
>         of the packet is passing through. When the CRC is reached,
>         the following algorithm is performed:
> 
>         #define CRC_STOMP 0X55555555  // A possible CRC_STOP value
>         if (checkCRC!=packetCRC) {
>             errorCount+= (packetCRC!= checkCRC^STOMP);
>             packetCRC^= STOMP;
>         }
> 
>    For those unfamiliar with "C", the CRC_STOMP value is a
>    constant to be defined by this committee; the value shown
>    is not necessarily the best value. If the packet's
>    CRC is bad, but has not yet been "stomped", then an error
>    count for that link is incremented. In any case, a packet
>    with a bad CRC is always "stomped", by replacing the "bad"
>    CRC with a particular bad CRC, namely the good CRC exclusive-OR'd
>    with the STOMP value.
> 
>    Now while its possible for the "stomped" value to be generated
>    by a coincidental transmission error, the chance of this is
>    quite small, so that only a small number of errors (1 in 2**32)
>    would be incorrectly logged.
> 
>    Note that operations (2) and (4) are logically independent
>    actions, so neither interferes with the operation of the other.
> 
> DVJ (David Vernon James)

One other mechansim used is just to invert the recomputed CRC.

Regards,
Devendra Tripathi
CoVisible Solutions Inc.
(formerly VidyaWeb, Inc)
Pune, India 
Tel: +91-20-433-1362