Dropping LLC frames due the TBF destruction

Andreas Eversberg andreas at eversberg.eu
Tue Nov 5 06:55:29 UTC 2013

Holger Hans Peter Freyther wrote:
> * there is no indication to the SGSN for dropped frames/octets?
> * can't the DL-TBF be re-associated/updated after the assignment
>   is done again?

hi holger,

there is one issue about failed downlink TBF: the PCU does not do any
retry. when the assignment is sent to the phone and it fails, there is
no retry according to the specs. i assume that this is a task for the
SGSN. osmo-sgsn/openggsn does not resend any LLC frame in case of
LLC-DISCARDED message, if i look at the source code of libosmogb.
(LLC-DISCARDED is forwarded at bssgp_rx_llc_disc() of gprs_bssgp.c, but
i don't see where it is handled.)

what actually happens is that TCP layer will do the resend, but this is
a problem, especially at the border of coverage where many downlink
assignments get lost.

i have once enabled the debugging at ipaccess BTS and saw that the PCU
itself does the retry of lost downlink assignments (4 or 5 times).



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