- This topic has 11 replies, 1 voice, and was last updated 6 years, 10 months ago by Manmayur.
21st August 2013 at 14:50 #69198GPRSGuest
I’m GPRS optimization enginner and a new member in this forum.
Well, I am facing a major problem, i have a BSC suffering from high UL_TBF_Drop_Rate (around 10%) cause T3169 Timer expiry (around 96%), and high DL_TBF_Drop_Rate (around 15%) cause T3195 Timer expiry (around 95%).
Please advice me how to proceed and what are different parameters i must check and change to fix this problem
your reply is appreciated21st August 2013 at 19:18 #69199pixGuest
Could you please post the definition of those 2 timers ?
10% is not so high, so it might actually be a radio issue. Reduce your initial coding scheme, enable link adaptation, and check the (speech) HO QUALITY % on the same frequency as the one carrying the GPRS.
pix21st August 2013 at 22:09 #69200RexGuest
What is your SDCCH DROP? If it is high and GPRS is configured on BCCH TRX than you should check BCCH planning. How many cells are suffering on that BSC? What is your HO Uplink quality rate?
Rex22nd August 2013 at 10:01 #69201GPRSGuest
Thanks for replying me,
T3195 : Linked to TBF and TFI, it starts when N3105 reaches to maximum value, and when it expired the wetwork releases TFI resource
T3169: Response to USF tolerance timer, On expiring, the USF is released (which means it can be reassigned to another mobile station)
But it’s not really clear for me this definition
– NW is Siemens
– All freq of the cell is carrying GPRS
– Avg of 10 % of TBF drop rate at Cell level (apprx all cells)
– HO_Quality % at BSC level is around 7%
PLZ ur comment22nd August 2013 at 16:27 #69202pixGuest
ok, sounds like real radio issue on GPRS. You should focus on decreasing the Coding Scheme (CS), in order to make it more robust.
1/ CS-INIT = CS-1 rather than CS-2
2/ LINK ADAPTATION (LA) = ON
3/ Is your EGPRS activated (or GPRS only) ?
4/ Experiment increasing OR decreasing the PDCH INACTIVITY TIMERS. I’m not sure which one will actually decrease your TBF DROP% the most.
5/Ensure you allow 4 PDCH or 5 PDCH per TBF
6/ Maybe decrease the MAXIMUM CODING SCHEME (MAX_CS) to CS-3 rather than CS-4
7/ Prefer the TBF on BCCH FREQ (you should find such settings somewhere, which will favour tbf establishment on BCCH freq until it’s full, then the next estab will be on non-BCCH freq)
That’s about it, on top of my head.
Please let me know how it goes. Don’t forget to tell me if EGPRS (EDGE) is activated or not (MCS-1 to MCS-9)
pix23rd August 2013 at 08:03 #69203ManmayurGuest
Hi to all ,
What exactly is LAPm connection that mobile establishes with BSC during call setup ?
Will anyone explain please ?
With thanks in advance..23rd August 2013 at 17:35 #69204pixGuest
LapDm is the Layer-2 of the Air interface. LapDm is used to carry signalling channels towards the right “ports” in the BTS.
It’s all very transparent to the user, and to us, telecom engineers.
LapDm can trigger timers based on T200 timers, and that can be used to detect dropped calls.26th August 2013 at 16:14 #69205ManmayurGuest
Hi Pix ,
Thank you so much for the answer . It was like some sort of an eye opener for me when you did say “port” .However I am a starter hence did not get when you said “It’s all very transparent to the user, and to us, telecom engineers.” and
Will you explain something about T200 timer and the way we can monitor the timer to detect dropped calls please ..
With thanks and regards ..31st August 2013 at 21:13 #69206pixGuest
It’s transparent because it’s just a “transport protocol”, meaning there’s not much room for improvement,IMO.
Some ppl here have been experimenting with longer T200 duration, and it seems to improve drop rate.
There is a mecanism in the LapDm : if the SACCH is not decoded succesfully in one occurence, then the timer T200 is started. At expiry, if SACCH are still not decoded, then a counter “n” is incremented by 1.
When n reaches the value set by N200, then the lapDm consider the session as “terminated”, which is counted as a “call drop cause radio” by the BSC.
This mechanism works independantly in UL and in DL, as far as I remember.
It works in parallel to the “typical” Radio Link Failure mechanism.
pix15th September 2013 at 15:49 #69207GPRSGuest
I was on vacation
Well, for the cells suffering from high DL_TBF_drop (%), i changed below parameters as follows:
1/ CS-INIT = CS-2
2/ IMCSUL8PSK = MCS2 (Initial MCS uplink 8PSK)
3/ IMCSULGMSK = MCS2 (Initial MCS uplink redundancy GMSK)
3/ INIMCSDL = MCS3(Initial MCS downlink)
4/ LINK ADAPTATION (LA) = TRUE
5/ Both EGPRS & GPRS are activated
6/ Uplink Incremental Redundancy = TRUE
Unfortunately, all these changes did’nt bring any results, drop rate still the same, knowing that all cells frequency are clean (no interference)
PLZ, ur advice
thx15th September 2013 at 16:20 #69208pixGuest
1/are the drops occuring in GPRS or in EGDGE ?
2/ are they occuring at any time of the day (proportionally to traffic), or are they especially located around the busiest hours of the day?
3/ reduce the maximum MCS possible from MCS9 to MCS7 *and* from CS4 to CS3.
4/ try pushing the GPRS allocation to another TRX (right now, either your GPRS is going on BCCH trx or it is not.. whatever you have chosen until now –> do the opposite)
You may also suspect a BSC issue ? In such case, you might consider doing a BSC “gprs module” restart during the night. Have you already contacted your technical support, check if there isn’t any bug in the software release you are using ?
If nothing worked, then it’s gonna be complicated to help you remotely. There are several things one can do, but that requires heavy analysis.24th September 2013 at 18:43 #69209ManmayurGuest
Thanks so much for your answer Mr. Pix..
I am a lot clear now..to be fully aware ,will have to study a lot.
With thanks again.