[OT] aal5 pdu CRC

[Jose M Rodriguez]

El Jueves, 27 de Enero de 2005 16:10, Harti Brandt escribió:
> On Thu, 27 Jan 2005, Jose M Rodriguez wrote:
>
> JMR>Hi,
> JMR>
> JMR>get some free time to work in uadsl, but have a problem.  Hope someone
> that JMR>can read ITU I.363 can answer this.
> JMR>
> JMR>Every usb adsl modem implementation I see doesn't obey aal5 pdu
> encoding JMR>standards, which requires that the PDU trailer come at the
> begin of a fresh JMR>cell.
>
> That is not true. The trailer must be on the END of a cell. Before the
> trailer there will be padding bytes so that this happens. If, for example,
> you have a one byte PDU the AAL5 PDU will consist of 1 byte information,
> 39 bytes padding and 8 bytes trailer.
>

At last, rfc1483 and the received logic side seems to point that this is true. 
Most implementations sync the begin of a new PDU after detect the last cell  
by header test. Also, PDU lenght and crc is decode from a fixed ptr on the 
ENDPDU cell (lenght=cell[2]..cell[3], crc=cell[4]..cell[7]).

>  JMR>
> JMR>So, I think that the modem must rework this and generate two cells on
> the JMR>wire.  Due how this modems works, I doubt that the modem
> recalculate PDU CRC JMR>itself, So...
> JMR>
> JMR>Can someone confir if the CRC covers the PAD?  I'm begin to think that
> the CRC JMR>only covers the playload and the pdu-trailer.
>
> The CRC covers everything but the CRC. The PAD must be filled with zeros
> though.
>

This is not so clean. This pad may be take as a cell pad or as a PDU pad.  If 
this is take as a cell pad, it may not be part of the CRC (the ENDPDU cell is 
also paded from 8 to 48).

My first guest is that the pad is part of the PDU, but I really doubt that the 
modem may be able to do a full CRC reclaculation.

> harti

--
  josemi