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Re: [Discuss-gnuradio] 8b/10b for GMSK

From: Dan Halperin
Subject: Re: [Discuss-gnuradio] 8b/10b for GMSK
Date: Sun, 11 Mar 2007 16:36:06 -0700
User-agent: Thunderbird (X11/20070103)

Johnathan Corgan wrote:
> If a received packet fails CRC because of some pattern-specific
> synchronization problem, and if the upper protocol layers cause a
> retransmission, then the re-transmitted packet will have a different
> whitened bit pattern, allowing it to go through.
> While this doesn't solve the underlying problem, whatever that may be,
> it does give a workaround for users who have to get past this issue
> while we are debugging it.  Unfortunately this workaround is only for
> people who are using higher level protocols which use retransmission to
> recover from link errors.  This of course includes TCP, which is the
> underlying protocol for the majority of users.
> We haven't decided whether this will make it into the trunk.  We'd much
> rather make a real fix.

I found out what is (most likely) going wrong here. If a packet payload
(post-whitening) matches the complement of the access code, its bits get
flipped. A packet that contains the access code is handled correctly. I
haven't tried it, but it would be interesting to see what happens if you
transmit a packet that (post-whitening) contains the entirety of a valid
packet... I'm not sure what the right fix is, so no patch...


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