diff options
author | Pau Espin Pedrol <pespin@sysmocom.de> | 2021-03-26 20:44:48 +0100 |
---|---|---|
committer | Pau Espin Pedrol <pespin@sysmocom.de> | 2021-03-31 17:39:46 +0200 |
commit | 54e6450293f2b5880004f49e76dc4d6f4645c299 (patch) | |
tree | 625fb3f5969dabc696bf1073981e556f9a3d7231 | |
parent | 0b998b15da4bb4e4c6ce35316218c47daa190004 (diff) |
sba: Document AGCH_START_OFFSET after some experimental tests
Related: OS#5020
Change-Id: Id1460207be25750aeb5c1d7af2fac6591cf5e424
-rw-r--r-- | src/sba.c | 20 |
1 files changed, 18 insertions, 2 deletions
@@ -33,8 +33,24 @@ #include "pdch.h" #include "pdch_ul_controller.h" -/* starting time for assigning single slot - * This offset must be a multiple of 13. */ +/* TBF Starting Time for assigning SBA. + * See TS 44.060 12.21 "Starting Frame Number Description". + * According to spec, k=0 (offset=4) "should not be used, so as to leave time + * for the MS to analyze the message and get ready to receive or transmit". + * Hence, k=1 (offset=8-9) is theoretically the minimum offset which could be + * used. + * + * However, it was found, empirically, that it takes around 30-40 FNs time for + * the Immediate Assignment message created with this "TBF Starting Time" to + * travel from PCU->BTS and be transmitted over CCCH on the Um interface. Hence, + * we must account for this delay here, otherwise the MS would be receiving eg. + * a TBF Starting Time of FN=40 while the Imm Assigned containing it is sent in + * eg. FN=50, which would be too late for the MS to answer to it. + * + * The situation described above, can even get worse on high BTS load for AGCH, + * since the Immediate Assignment will queue in the BTS waiting to be + * transmitted one after the other, hence increasing the required delay. + */ #define AGCH_START_OFFSET 52 |