High SDCCH Assignment Failure

My network GSM Nortel V15.1.1 is experiencing the same extrange high SDCCH Failed Assigment Rates that appear only for some hours during the day and only at some cells.

I noticed the SDCCH Failed Assigment Rate is calculed by SDCCH Seizures/SDCCH Attempts*100%

Also comparing the counters when the cell was having problems and when it was not, I realized that the counter that measures the Expiration of the T3101 (set at 3 seconds) increased from 1 when it was OK to 707 when it was with problems. For this comparison I only selected one hour (12:00) at mid day.

On the other hand checking the notifications of the Site I found that the following alarm appeared at the BTS (cell) level:

Fault number : (1266) BLOCKAGE OF RADIO CHANNEL(S) IN FREE STATE
radAnoChln : (
transceiverId : 0
channelId : 1
subChanId : 0)
(
transceiverId : 0
channelId : 1
subChanId : 1)
(
transceiverId : 0
channelId : 1
subChanId : 3)
(
transceiverId : 0
channelId : 1
subChanId : 4)

indicating the TRX0 where the BCCH (TS0) and SDCCH/8 (TS1) are defined and subchannels of the SDCCH8 (0,1,3,4 experienced blockage).

Reading about this fault, says that the RadioResourseSupervision measures the channels on BUSY and FREE state but do not understand what is the reason yet. I’ll investigate.

Also noted that the CSSR was degraded, Network and Cell. And that the SDCCH attempts were at least doubled, but the SDCCH seizures maintained the same as when the cell was OK.

How/where did you do the trace to detect via the LAU procedure?

In my case is not due to BCCH/BCC problems.

Has anybody noticed if this SDCCH drop case has anything to do with cell having EGPRS configured? Did changing the T3101 timer have any positive affect?

Hi,

if anybody knows abt t3101 then plz share the view i have modified the t200 value it has given improvement in sd completion …

Hi,

in my network drop due to t200 was max so after increasing its value i got the improvement.

It is not a network problem. Some phones missbeheave causing those abnormal failures.

well, that’s your philosophy… but actually what is the point of CSSR ?
One number to show maximum amount of info about your network.
If your CSSR takes only one indicator (tch assign succ %) then it’s quite limited.
If your CSSR takes more indicators together, then – ok – it might not show “only” the CALL s.s.r, but it gives a good idea on how well-tuned is your network.

CSSR + CDR = wide overview of QoS
those are really just “management” KPI, they are absolutely useless for troubleshooting 🙂

Hello,

Do you know what is the exact “cause” indicator for SDCCH Drop ? I assume you have a split for the SDCCH Drop, such as “Radio drop”, “Timer Expiry”, “Hardware Drop” etc…?
(indics are vendor dependent, which vendor are you working with ?)

What is the value of SD Drop Rate ? (split ?)
What is the amount of SDCCH erlang at busy hour ?

Thanks to your actions, we can safely say that:
– interference are not an issue
– hardware in the BTS is not faulty (not so sure yet)
– no other QoS indicators are degraded

Path balance ?
There is still a possibility that all calls too far away are getting a SDCCH drop due to poor path balance. While all calls close enough are kept until TCH calls is established. However, which such assumption, you should notice high call drop rate as well.

What you should try to do is a drive test over there, perform several call setups and try to obtain a sdcch drop 🙂 Move around in the cell coverage.

Try to send very long SMSs to extensively test the SDCCH.