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