Re: [lp-wan] ACK compression

[Olivier Gimenez <[e-mail]>]

From: Pascal Thubert (pthubert) <[e-mail]>
All I can say is that in order to work, you need the last bit of the compressed form to be the same as the elided bits.
So your "While the scissors are not on an L2 Word" in step 4 will not give the appropriate result if the scissor is already on the word boundary. In that case you need to go to the next boundary.
I suggest a step 3.5 which is to move the scissors right after the first while series.

Hi,

I do not understand why it does not work; in the following SCHC ACK example the Bitmap is on 10 bits, L2 words are 8 bits
|  3   bits |   1 bit | 1 bit | 1 bit |      10 bits             |
| Rule ID |  DTag |   W   |C=0   |1 1 0 0 0 0 0 0 0 0|

Compressed SCHC ACK would be
|  3   bits |   1 bit | 1 bit | 1 bit | 2 bits |
| Rule ID |  DTag |   W   |C=0   |1 1       |

This SCHC ACK is aligned to L2 word. The scissor has been put on the word boundary and I do not understand why it does not work: the sender is able to know that last 8 tiles are missing as it knows that the 2 first are OK and it send 10 tiles.

Sorry for the mistake on examples, here is the correction with 17 tiles in the bitmap


   Figure 20 shows an example where L2 Words are actually bytes and where the original Bitmap contains 17 bits, the last 6 of which are all set to 0 due to missing tiles.

   |---- SCHC ACK Header ----|--------      Bitmap     --------|
             |-- T --|-M-| 1 |
   +--- ... -+- ... -+---+---+---------------------------------+
   | Rule ID |  DTag | W |C=0|1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0|
   +--- ... -+- ... -+---+---+---------------------------------+
           next L2 Word boundary ->|

            Figure 20: SCHC ACK Header plus uncompressed Bitmap - Trailing 0 compression

   Figure 21 shows that the last 6 bits are not sent.

   |---- SCHC ACK Header ----|CpBmp|
             |-- T --|-M-| 1 |
   +--- ... -+- ... -+---+---+-----+
   | Rule ID |  DTag | W |C=0|1 1 1 1 1 1 1 1 1 1 1 |
   +--- ... -+- ... -+---+---+-----+
           next L2 Word boundary ->|

       Figure 21: Resulting SCHC ACK message with Compressed Bitmap - Trailing 0 compression

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