New subject: OpenBSC with 2.5G or 3G mobile devices support? / Tested Phones

5 Jul 2009

Hello Harald,

On Sun, 5 Jul 2009 04:42:20 +0200, "Harald Welte" &lt;laforge(a)gnumonks.org&gt;
wrote:
...

 where did you find this XOR?  I just searched through 04.07 and 04.08 and
 didn't find any indication thereof.  Sure, the padding bit sequence ix 0x2b, so
 all spare bits have to be padded with that.   But where did you get the XOR
 from? 
I started to search deeper because I was a bit confused how the Message
Decoding Tool from Joachim Goeller interpreted the rest octets. I looked
at some phone firmware how they do it and found it there. Later I also
found a good explanation at http://csn1.info (they sell a tool which
can be used to generate message parsing source code for various
specifications).

...
  Also, this would mean that if we put explicitly 0x2b
into the padding of
 our messages, then it would result in all-zero 0x00 on the air, since

 0x2b xor 0x2b == 0x00 
The XOR is actually only used for "L" and "H", basically it means
that
if a bit at an "L"/"H" position is different from the padding
sequence,
its an "H". At least this is how I understand it.

...
  I'm now looking at an actual abis trace from a
production cell with GPRS
 enabled, and it has

 "0x80, 0x00, 0x80, 0x0b" 
Here is how I would interpret it according to GSM 04.08, 10.5.2.34:

  Selection Parameter:

   byte 0: 0x80 ^ 0x2B = 0xAB, (bit 7) -> H  (15 bits for parameter follow)

  Power Offset:

   byte 2: 0x80 ^ 0x2B = 0xAB, (bit 7) -> H  ( 2 bits for parameter follow)

  System Information 2ter Indicator:

   byte 2: 0x80 ^ 0x2B = 0xAB, (bit 4) -> L  (has no parameters)

  Early Classmark Sending Control:

   byte 2: 0x80 ^ 0x2B = 0xAB, (bit 3) -> H  (has no parameters)

  Scheduling if and where:

   byte 2: 0x80 ^ 0x2B = 0xAB, (bit 2) -> L

  GPRS Indicator:

   byte 2: 0x80 ^ 0x2B = 0xAB, (bit 1) -> H ( 4 bits for parameter follow)

...
  During early start of the BTS, SI3 rest octets are set
to 

 "0x80, 0x00, 0x83, 0x2b" 
The only difference is that at this stage GPRS is not yet set:

  GPRS Indicator:

   byte 2: 0x83 ^ 0x2B = 0xA8, (bit 1) -> L

Best regards,
  Dieter
-- 
Dieter Spaar, Germany                           spaar(a)mirider.augusta.de

Re: OpenBSC with 2.5G or 3G mobile devices support? / Tested Phones