Date: Thu, 27 Jan 2005 16:32:37 +0100 From: Jose M Rodriguez <josemi@freebsd.jazztel.es> To: freebsd-net@freebsd.org, Harti Brandt <harti@freebsd.org> Cc: net@freebsd.org Subject: Re: [OT] aal5 pdu CRC Message-ID: <200501271632.37457.josemi@freebsd.jazztel.es> In-Reply-To: <20050127160658.K55581@beagle.kn.op.dlr.de> References: <200501271600.42408.josemi@freebsd.jazztel.es> <20050127160658.K55581@beagle.kn.op.dlr.de>
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El Jueves, 27 de Enero de 2005 16:10, Harti Brandt escribi=F3: > 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 tr= ue.=20 Most implementations sync the begin of a new PDU after detect the last cell= =20 by header test. Also, PDU lenght and crc is decode from a fixed ptr on the= =20 ENDPDU cell (lenght=3Dcell[2]..cell[3], crc=3Dcell[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=20 this is take as a cell pad, it may not be part of the CRC (the ENDPDU cell = is=20 also paded from 8 to 48). My first guest is that the pad is part of the PDU, but I really doubt that = the=20 modem may be able to do a full CRC reclaculation. > harti =2D- josemi
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