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

[Dieter Spaar]

Hello Harald,

On Sun, 5 Jul 2009 04:42:20 +0200, "Harald Welte" <laforge at gnumonks.org> 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 at mirider.augusta.de