1. A code congestion incident occurred in Cell54631 on December 22nd, resulting in a 0% success rate for AMR RAB setup. 2. Analysis found the congestion was caused by the RNC's abnormal retention of codes allocated to FDPCH channels after calls ended. 3. The RNC periodically performs code audits that clear unused codes, and these audits eventually resolved the congestion after several hours. To prevent recurrence, disabling the "SRB over HSDPA" feature is recommended.

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Author Jinwei Staff Number 116302
Department UMTS RNP Product Family:
Wireless Performance &
RNP & RNO
Update Time 2010-12-27 Product Vension:
RNC ：
BSC6810V200R011C00S
PC100SPH184.
Approver
Title： code congestion problem analysis
Phenomenon
Description:
Code congestion occurred at about 9:00am 22nd.Dec in Cell54631, resulted in
an 0% of AMR RAB Setup Success Rate. Code congestion disappeared at about
5:00 am 23th.Dec without any operation.
Alarm
Information：
None
Cause
Analysis： 1.1 AMR RAB Setup Fail Analysis
The cause of RAB setup failure was code allocation failure according to the
statistics.
1.2 Code Congestion Analysis
According to the CHR log, almost all codes were occupied; meanwhile
HS-PDSCH occupied only one SF16.

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As indicated in the row “R99 Occupy SF”, R99 occupied only a small fraction
of all codes (less than 10),
As shown in the code tree monitor below, FDPCH occupied many codes, but
during Busy Hour there were very few HSDPA users which would lead to a
much less occupation of codes by FDPCH.
Under certain circumstances, RNC would fail to release the code occupied by
FDPCH.
1.3 FDPCH Code Allocation Analysis
RNC would reserve the information of resources occupied by UE before and

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after physical channel reconfiguration which indicated by “Old” and “Current”
respectively. Discordance of codes information occupied by FDPCH maintained
in “Old” and “Current” would occur after several reconfigurations between
FDPCH and non-FDPCH.
Typical Scene:
Step 1: UE setup an HSDPA Call with SRB over H, code (256, 21) was
allocated to FDPCH.
Step 2: UE setup CS Call with SRB over DCH, so codes (256, 21) allocated to
FDPCH was released; an SF128 was allocated to R99.
Step 3: Disconnect CS Call and followed by a reconfiguration of the DCH carry
SRB, code （256，21） was allocated to DCH.
Step 4: Success of H Retry would result in SRB over H. Since code（256，21）
was occupied by DCH, （256，22） was allocated to FDPCH.
Step 5: UE terminated HSDPACall. RNC would release （256，21）which was
allocated to FDPCH for the first time instead of（256，22）which was allocated
to FDPCH for the last time. （256，22）was considered occupied by RNC since
then.
1.4 Recovery Explanation
There are two types of code audit which can check the code resource period and
release abnormal codes.
1) RNC checks the code of FDPCH of 10 cells per hour. If no FDPCH user in
cell, the code information of FDPCH would be cleared;
2) RNC checks code tree per minute. If no DCH user in cell, the code tree
would be rebuild.
So, the abnormal code congestion was dispelled by these code audits. But it may
cost long time.
Handling
Process：
FPDCH setup was necessary in the case of that a few smart UE intended to
establish SRB over HSDPA. Abnormal release of the code of FDPCH led to
code congestion and consequently causedAMR RAB setup failure.
In order to avoid the problem, the best way is to close the license switch of
“SRB over HSDPA”. The cell would not allocate the code of FDPCH and the
problem would disappear.
Impact: because cell would use more code (SF256) with same HSDPAusers, the
available code for HSDPA would decrease, it would affect throughput of
HSDPA in theory. If the HSDPA user number is low, it would occupy less code
SF256, and the impact can be ignored; if the HSDPA user number is high, the
impact is inconspicuous because the cell would allocate less HSDPA resource
for every HSDPA user.
Suggestions
and summary：
None
Attachments：

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document
link：
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