AGCH question.

Mad mad at auth.se
Thu Feb 3 14:45:16 UTC 2011


 On Thu, 3 Feb 2011 14:54:14 +0100, Sylvain Munaut wrote:
> We only send to GSMTAP recomposed L2 packets from Normal Bursts.
>
> We don't have debug support for other burst types like 
> dummy/sch/rach,
> I'm not even sure the gsmtap has a "spec" for it, or that the
> wireshark dissector has code to process it.
>

 As gsmtap supports the payload type raw GSM Um burst and all possible
 burst types like GSMTAP_BURST_ACCESS, ..._SCH and so on, it should be
 possible to send these out to wireshark.
 But I'm not sure whether the dissector already decodes them as it 
 doesn't
 do it with normal bursts.


>> The second question is about SI AGCH response, I didn´t find the 
>> algorithm to know which SDCCH channel gives the BTS to the MS to 
>> send/recv LAPDm bursts.
>
> There is none specified ... the BTS is free to allocate whatever it
> wants ... (well technically it's not the BTS itself, but other
> component of the core network).
>

 But there is at least some kind of predictability. The assigned arfcn
 and time slot is usually the same until a certain load it reached and
 a second ts or even other arfcn/ts is used. The subchannel is hard to
 predict as I've never seen a stable pattern, it looks quite random.
 At least these are my observations with some live networks.
 
 Regards,
   Mad




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