GSM 2G

1. SD EST STUDY AND POSSIBLE
CAUSES
DRC RNPO DEPARTMENT

2. INDEX
1. CCALLS Channel allocation attempt counter (on SDCCH).
2. CMSESTAB Successful MS channel establishments on SDCCH.
3. CESTCHACTIV Number of SDDCH establishment failure that occurs under channel allocation
and channel activation. Please, note that this counter is stepped also in case of SDCCH
congestion.
4. CESTIMMASS Number of SDCCH establishment failure due to time-out after sending
Immediate Assignment, timer T3101 expired.
5. NLALOCTOT Total number of LU attempts in the LA
6. CSIMMASS Number of CS IMMEDIATE ASSIGNMENT messages sent on the CCCH
7. CNROCNT Number of Accepted Random Accesses.

3. Main SD establishment
Reasons of Low SD estabishment
1. Low Availability
2. Too frequent Periodic Registration
3. Wrong SDCCH Dimensioning
4. Location Area Border Cell
5. Border Cell
6. High SMS Usage
7. Cell Broadcast Used
8. IMSI Attach/Detach in Use
9. High Ratio of Random Accesses
10.Low Signal Strength on Down or Uplink
11.Poor Quality on Down or Uplink
12.Over-heard random access burst from another co -channel / co –BSIC cell
13.The A interference SCCP connection problem
14.The abnormal behavior of “MS”(No response of immediate assignment and keep sending random access
)

4. Overview
According to statistical data records, low (89 %) S DCCH establishment success rates have been observed in
AFC radio network. An interesting point here is that, the problem is observed on all of the BSCs and many of
the cells; even tough the other key performance indicators (KPIs) related to SDCCH and TCH performance,
such as drop rates, drop reasons and handover reasons seem quite good.
In Ericsson GSM systems, CMSESTAB counter is incremented once, when a successful SCCP connection is
established between BSC and MSC; SCCP connection confirmed message arrives to BSC from MSC.
CCALLS, the call attempt counter is incremented at every attempt to allocate a SDCCH in a resource type in
the cell, regardless of whether the allocation succeeded or failed. Allocation attempts are made at signaling
connection setup, at handover and at assignment.

5. Executive Summary (I)
Based on our investigation we can ensure that main issue for LOW SDCCH ESTAB is the T3101 which
could the case of Phantom RACH.
There are some random access bursts (cause value Location updating) accepted by the BTS, and BSC
allocates SDCCH resource in the BTS for signaling connection setup, and BTS sends out Immediate
assignment to the “MS”, after that there is no response from the “MS”, which causes the Timer
3101(no receiving of Establish Indication) in the BSC expired, consequently the IMMEDIATE
ASSIGNMENT procedure failed. The more no-response from the “MS”, the bigger difference between
counters CCALLS and CMSESTAB. The reason to this can be Phantom random access bursts or a MS
that is not behaving correctly)
By checking SD drop and TCH drop reason, most of drops are due to other in SD and Sudden loss of
connection. Where Sudden loss of connection an be related to Subs behavior , RF reason and HW.
While 16% of drops are related to RXLEV less than -100 Dbm.

6. It was noticed also that BasCongo regions having better performance in terms of SD estab, than KIN,
this proof will neglect the coverage reason who is affecting the T3101 release
An audit and comparisons between Parameters of satellite interface cells and normal will be carried on
hour NW.
99% of Rach attempts are related to LAU, we do suggest to reduce the T3212 parameter controls the
Periodic Registration interval. A shorter interval, together with implicit detach supervision initiated in the
MSC, will reduce the amount of unnecessary paging, but it leads to more signalling on the SDCCH,
thereby affecting the SDCCH dimensioning. The recommended initial setting of T3212 is 40, that is,
registration every 4th hours, while we have 6 hours on our NW.
Executive Summary (II)

7. SD ESTAB VS CCALLS VS CMSESTAB
Low Value of SD ESTAB is due to the difference between CCALLS and SD ESTAB, in other hand it
was noticed that we do have 11% of failure is due to CESTIMMASS, where CESTIMMASS is
Number of SDCCH establishment failure due to time-out after sending Immediate Assignment,
timer T3101 expired

8. SD ESTAB VS SD CONG
No clear relation between SD CONG and SD ESTAB, even though we have low value of
SD CONG SD estab is always 89 %

9. SD ESTAB VS SD DROP
No clear relation between SD DROP and SD ESTAB, even though we have low value of SD DROP
SD estab is always 89 %

10. SD ESTAB VS Immediate assign Success rate
From Above we can ensure that there is no load who is affecting our Immediate assignment

11. Successful Immediate Assignment
it was noticed by dividing CESTIMASS by CSIMASS by that we do have 11% of failure is due to
CESTIMMASS, where CESTIMMASS is Number of SDCCH establishment failure due to time-out
after sending Immediate Assignment, timer T3101 expired. Reason To be investigate.

12. CCALLS VS CSIMMASS VS CNROCNT
CCALLS(SDCCH channel activation) increases together with CNROCNT CSIMMASS(CS IMMEDIATE
ASSIGNMENT) increases together with CCALLS, which means SDCCH allocation succeeds. RAOTHER AND
NLALOCTOT almost the same, that is mean some attempt of CCALLS are considered as phantom RACH.

13. RA Attempt percentage
99% RA Attempt percentage is mainly due to counter (RAOTHER). The counter will be stepped
for cause value Location updating (SDCCH needed). Reason to be investigate!!!

14. END