Re: [OT] aal5 pdu CRC

From: Jose M Rodriguez (josemi_at_freebsd.jazztel.es)
Date: 01/27/05

  • Next message: Jose M Rodriguez: "Re: [OT] aal5 pdu CRC" 
    To: freebsd-net@freebsd.org, Harti Brandt <harti@freebsd.org>
Date: Thu, 27 Jan 2005 16:32:37 +0100

    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
_______________________________________________
freebsd-net@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-net
To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org"
  • Next message: Jose M Rodriguez: "Re: [OT] aal5 pdu CRC"

    Relevant Pages

    • Re: [OT] aal5 pdu CRC
      ... which requires that the PDU trailer come at the ... The trailer must be on the END of a cell. ... PDU lenght and crc is decode from a fixed ptr on the ... > JMR>Can someone confir if the CRC covers the PAD? ...
      (freebsd-net)
    • Re: [OT] aal5 pdu CRC
      ... JMR>> JMR>get some free time to work in uadsl, ... which requires that the PDU trailer come at the ... JMR>> recalculate PDU CRC JMR>itself, ... JMR>> JMR>Can someone confir if the CRC covers the PAD? ...
      (freebsd-net)
    • [OT] aal5 pdu CRC
      ... Every usb adsl modem implementation I see doesn't obey aal5 pdu encoding ... cell. ... I think that the modem must rework this and generate two cells on the ... I doubt that the modem recalculate PDU CRC ...
      (freebsd-net)
    • Re: [OT] aal5 pdu CRC
      ... which requires that the PDU trailer come at the begin of a fresh ... The trailer must be on the END of a cell. ... I think that the modem must rework this and generate two cells on the ... I doubt that the modem recalculate PDU CRC ...
      (freebsd-net)