# Gr-ais: bit unstuffing bug? **URL:** \ **Category:** GNU Radio\ **Created:** [August 10, 2012, 8:48pm UTC](https://www.ruby-forum.com/t/gr-ais-bit-unstuffing-bug/222083 "2012-08-10T20:48:23Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [August 10, 2012, 8:48pm UTC](https://www.ruby-forum.com/t/gr-ais-bit-unstuffing-bug/222083/1 "2012-08-10T20:48:23Z")
Hi all, Nick, In gr-ais. the bit un-stuffing is done before the de-framing, shouldn’t it be done after instead? Chris – Christian G., Embedded systems engineer. Techworks Marine 1 Harbour road Dun Laoghaire Co. Dublin Ireland Tel: + 353 (0) 1 236 5990 Web: [http://www.techworks.ie/](http://www.techworks.ie/) ---
**Author:** ![christiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/77aa72/32.png) [@christiang](https://www.ruby-forum.com/u/christiang)\ **Post date:** [August 10, 2012, 8:56pm UTC](https://www.ruby-forum.com/t/gr-ais-bit-unstuffing-bug/222083/2 "2012-08-10T20:56:34Z")
You can do it either way since unstuffing will never generate a valid HDLC framing code (“01111110”). Unstuffing before the deframing correlators means my tags’ sample numbers don’t have to be modified for the bits inserted by the unstuffing operation. –n ---
**Author:** ![christiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/77aa72/32.png) [@christiang](https://www.ruby-forum.com/u/christiang)\ **Post date:** [August 10, 2012, 9:31pm UTC](https://www.ruby-forum.com/t/gr-ais-bit-unstuffing-bug/222083/3 "2012-08-10T21:31:03Z")
On 10/08/12 19:55, Nick F. wrote: > You can do it either way since unstuffing will never generate a valid > HDLC framing code (“01111110”). Unstuffing before the deframing For what i understand, bit stuffing never generates framing code (that’s the whole point), but the bit _un_stuffing can and will if the payload contains itself a byte which as the same value as the framing code. Actually, while dumping data from ais\_parse.cc, I found that gr-ais unstuffing code do generate framing codes. that has fooled get\_tags\_in\_range (a stop was found because the AIS payload had a 01111110 in it). Well, I think I found, I’m still scratching my head… > correlators means my tags’ sample numbers don’t have to be modified for > the bits inserted by the unstuffing operation. Yes, I see the point. But you could unstuff inline, in place of the memcpy, just before calling parse(). Chris > ``` > Ireland > Tel: + 353 (0) 1 236 5990 > Web: http://www.techworks.ie/ > > _________________________________________________ > Discuss-gnuradio mailing list > removed_email_address@domain.invalid > https://lists.gnu.org/mailman/__listinfo/discuss-gnuradio > > > ``` – Christian G., Embedded systems engineer. Techworks Marine 1 Harbour road Dun Laoghaire Co. Dublin Ireland Tel: + 353 (0) 1 236 5990 Web: [http://www.techworks.ie/](http://www.techworks.ie/) ---
**Author:** ![christiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/77aa72/32.png) [@christiang](https://www.ruby-forum.com/u/christiang)\ **Post date:** [August 10, 2012, 9:45pm UTC](https://www.ruby-forum.com/t/gr-ais-bit-unstuffing-bug/222083/4 "2012-08-10T21:45:04Z")
Hmm, you might be right about that. I’ll take another look. –n