Implement downgrade to DL MCS1-4 when USF for GPRS_only MS

In previous status, if USF for GPRS-only MS was selected, then EGPRS
TBFs were skipped and either a GPRS TBF was selected or a Dummy Block
was sent. That means the behavior was unfair towards EGPRS TBFs, because
sometimes they were skipped in favor of GPRS ones.
This patch imporves the situation in the above mentioned USF scenario, by
first, under specific conditions, allowing selection of an EGPRS TBF and
then forcing it to transmit in EGPRS-GMSK (MCS1-4) so that the
USF-targeted MS can still decode the USF, while at the same time
providing more fairness by allowing the EGPRS TBF to transmit data.

The specific conditions mentioned above are, mainly, related to the fact
that once a DL data block has been sent, and hence a BSN was assigned to
it, it cannot be retransmitted later using another MCS, since lower
MCS1-4 wouldn't be able to contain higher MCS RLC payload.

The set of conditions could be expanded in the future by also selecting
the EGPRS TBF if retransmition is required and the block to be
retransmitted was originally transmitted as MCS1-4.

Related: OS#4544
Change-Id: I9af23e175435fe9ae7b0e4119ad52fcd4707b9ca

1 files changed, 1 insertions, 0 deletions

diff --git a/src/bts.h b/src/bts.h
index 6f757158..fd520671 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -339,6 +339,7 @@ public:
 	void set_max_cs_ul(uint8_t cs_ul);
 	void set_max_mcs_dl(uint8_t mcs_dl);
 	void set_max_mcs_ul(uint8_t mcs_ul);
+	bool cs_dl_is_supported(CodingScheme cs);
 
 	GprsMsStorage &ms_store();
 	GprsMs *ms_by_tlli(uint32_t tlli, uint32_t old_tlli = GSM_RESERVED_TMSI);