tbf: Implement delayed release of a downlink TBF

Currently a DL TBF is immediately closed, when the LLC queue is
drained. This will lead to a new DL assignment if data is received
afterwards. In addition, it is not possible to keep the PACCH open
to poll the MS for UL establishment requests there.

GSM 44.060, 9.3.1a suggests to delay the release of an inactive TBF
for some time (max 5s).

This commit mainly changes create_new_bsn() to send LLC dummy
commands as filler if no LLC data is available until keep_open()
returns false. The keep_open() functions returns true unless a
configurable time has passed after the LLC data store drained. By
default, that time is not set which causes keep_open() to always
return false, so that delayed release is effectively disabled.

The following VTY commands are added:
  - dl-tbf-idle-time <1-5000>    to set the delay in ms
  - no dl-tbf-idle-time          to disable delayed release

Ticket: #556
Sponsored-by: On-Waves ehf

1 files changed, 1 insertions, 0 deletions

diff --git a/src/bts.h b/src/bts.h
index e8459957..3d74b759 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -133,6 +133,7 @@ struct gprs_rlcmac_bts {
 	uint32_t alloc_algorithm_curst; /* options to customize algorithm */
 	uint8_t force_two_phase;
 	uint8_t alpha, gamma;
+	uint32_t dl_tbf_idle_msec; /* hold time for idle DL TBFs */
 
 	/* TBF handling, make private or move into TBFController */
 	/* list of uplink TBFs */