On 6/2/10 12:25 PM, Frank A. Stevenson wrote:
> Another 'quick and easy' question perhaps.
>
> The SDCCH has a fixed bit & burst rate, but because L3 messages are
> mostly processed in synchronous manner, there will be idle time when no
> messages need be sent. The BST is supposed to work at a constant power
> level, therefore these TDMA frames are filled with dummy bursts.
>
> But what happens when ciphering is enabled ? Will the dummy bursts still
> be all 0 bits, or will they be encrypted ? So far I have only had access
> to L3 logs that shows there is a time gap between individual messages,
> but do we know what those gaps contain ? I would imagine that signal
> processing on the receiving side will be easiest if the dummy bursts are
> also encrypted, otherwise handling error bits in a a received dummy
> burst becomes rather complicated.
>
>
>
Not sure if we're talking about the same "dummy bursts", but official
GSM dummy bursts have a fixed pattern:
in airprobe/gsm-receiver/src/lib/gsm_constants.h:
//Dummy burst 0xFB 76 0A 4E 09 10 1F 1C 5C 5C 57 4A 33 39 E9 F1 2F A8
static const unsigned char dummy_burst[] = {
1, 1, 1, 1, 1, 0, 1, 1, 0, 1,
1, 1, 0, 1, 1, 0, 0, 0, 0, 0,
1, 0, 1, 0, 0, 1, 0, 0, 1, 1,
1, 0, 0, 0, 0, 0, 1, 0, 0, 1,
0, 0, 0, 1, 0, 0, 0, 0, 0, 0,
0, 1, 1, 1, 1, 1, 0, 0,
0, 1, 1, 1, 0, 0, 0, 1, 0, 1,
1, 1, 0, 0, 0, 1, 0, 1, 1, 1,
0, 0, 0, 1, 0, 1,
0, 1, 1, 1, 0, 1, 0, 0, 1, 0,
1, 0, 0, 0, 1, 1, 0, 0, 1, 1,
0, 0, 1, 1, 1, 0, 0, 1, 1, 1,
1, 0, 1, 0, 0, 1, 1, 1, 1, 1,
0, 0, 0, 1, 0, 0, 1, 0, 1, 1,
1, 1, 1, 0, 1, 0, 1, 0
};
at night you actually see them flying around by the hundreds, in clear
text. This pattern, as well as other defined marker patterns, are
defined in this spec:
http://www.3gpp.org/ftp/Specs/html-info/0502.htm
All-zero will not be used I presume, since this would only complicate
matters when looking for FCCH bursts.
Cheers, M.
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