Date: Thu, 27 Jan 2005 17:06:16 +0100 (CET) From: Harti Brandt <hartmut.brandt@dlr.de> To: Jose M Rodriguez <josemi@freebsd.jazztel.es> Cc: freebsd-net@freebsd.org Subject: Re: [OT] aal5 pdu CRC Message-ID: <20050127170022.A55581@beagle.kn.op.dlr.de> In-Reply-To: <200501271632.37457.josemi@freebsd.jazztel.es> References: <200501271600.42408.josemi@freebsd.jazztel.es> <200501271632.37457.josemi@freebsd.jazztel.es>
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On Thu, 27 Jan 2005, Jose M Rodriguez wrote: JMR>El Jueves, 27 de Enero de 2005 16:10, Harti Brandt escribi?: JMR>> On Thu, 27 Jan 2005, Jose M Rodriguez wrote: JMR>> JMR>> JMR>Hi, JMR>> JMR> JMR>> JMR>get some free time to work in uadsl, but have a problem. Hope someone JMR>> that JMR>can read ITU I.363 can answer this. JMR>> JMR> JMR>> JMR>Every usb adsl modem implementation I see doesn't obey aal5 pdu JMR>> encoding JMR>standards, which requires that the PDU trailer come at the JMR>> begin of a fresh JMR>cell. JMR>> JMR>> That is not true. The trailer must be on the END of a cell. Before the JMR>> trailer there will be padding bytes so that this happens. If, for example, JMR>> you have a one byte PDU the AAL5 PDU will consist of 1 byte information, JMR>> 39 bytes padding and 8 bytes trailer. JMR>> JMR> JMR>At last, rfc1483 and the received logic side seems to point that this is true. JMR>Most implementations sync the begin of a new PDU after detect the last cell JMR>by header test. Also, PDU lenght and crc is decode from a fixed ptr on the JMR>ENDPDU cell (lenght=cell[2]..cell[3], crc=cell[4]..cell[7]). Well, you can trust me. The reference is the ITU-T recommendation in any case. The AAL5 CPCS PDU consists of: 1...65535 information bytes 0...47 PAD bytes 1 UU byte 1 CPI byte 2 length field 4 CRC with the padding done so that the sum is a multiple of 48 byte. Sure you can use fixed offsets to access the header. It's always at cell[40] for the last cell. JMR>> JMR> JMR>> JMR>So, I think that the modem must rework this and generate two cells on JMR>> the JMR>wire. Due how this modems works, I doubt that the modem JMR>> recalculate PDU CRC JMR>itself, So... JMR>> JMR> JMR>> JMR>Can someone confir if the CRC covers the PAD? I'm begin to think that JMR>> the CRC JMR>only covers the playload and the pdu-trailer. JMR>> JMR>> The CRC covers everything but the CRC. The PAD must be filled with zeros JMR>> though. JMR>> JMR> JMR>This is not so clean. This pad may be take as a cell pad or as a PDU pad. If JMR>this is take as a cell pad, it may not be part of the CRC (the ENDPDU cell is JMR>also paded from 8 to 48). JMR> JMR>My first guest is that the pad is part of the PDU, but I really doubt that the JMR>modem may be able to do a full CRC reclaculation. The CRC is computed from everything except the CRC field. PAD must be zero as must be CPI. I've written this code several times :-/. harti
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