GPRS Typical Cases Analysis
ZTE university
Purposes
After learning the course, you will: Be familiar with GPRS trouble-shooting methods Command GPRS CQT&DT OMC analysis methods Command GPRS signaling tracking analysis methods
Contents
ATTACH FailurePDP Activation FailureWAP FailureFTP FailureOther Failures
Overview ATTACH Failure
Attach Failure FeaturesAttach Failure Features Attach success rate is low and delay is longAttach success rate is low and delay is long
Reasons for Attach Failure
Subscriber’s Failure MS doesn’t support GPRS or has no GPRS MS isn’t compatible with network’s
equipment MS accessing authority is restricted Too frequent cell reselection
causes instable attach
or untimely
Network Failure
Network has no GPRS
Network hasn’t been allocated
GPRS resources or GPRS resource
isn’t enough
Signaling data transfer failed
Weak coverage Cell has co-channel interference, adjacent interference or
external interference Cell congestion causes untimely response to attach request Busy system or slow response impacts logon efficiency
Radio Environment
Failure
Overview ATTACH Failure
Solutions for Attach Failure turn off SGSN authority;
use one-step accessing mode;
check coverage;
prevent frequent cell reselection;
trouble shoot to improve C/I; check RACH or AGCH configuration;
check GPRS ENABLED setting;
check Gb links load and adjust NSEI configuration;
GPRS core network modification.
MS cannot logon GPRS ATTACH Failure
Failure Description: In one network, subscriber fails in
MS GPRS Attach. MS sends Attach request, then
SGSN sends back attach accept
message, but BSC sends LLC
unknown information signaling.
Failure Description: In one network, subscriber fails in
MS GPRS Attach. MS sends Attach request, then
SGSN sends back attach accept
message, but BSC sends LLC
unknown information signaling.
Solution:
Turn status of P-TMSI from
disable to enable and then the failure
is solved.
Failure Analysis: According to signaling flow, BSS is in
charge of TBF flow establishment, and
then MS exchanges signaling with SGSN;
in normal flow, MS sends Attach complete
to SGSN. Related BSC data have no problem. From global test we can see that the
same failure occurs in sites controlled by
all BSC that connect with SGSN, so we
initially estimate it is SGSN problem. After checking we see modification that
is status of P-TMSI is changed from initial
enable to disable that causes P-TMSI
can’t allocate and subscriber can’t use
GPRS
ATTACH Failure
Problem: frequent cell reselection (60021→60332→60021) causes too
long ATTACH delay (14.55s).
Solution: raise 60021CRH from 8dB to 10dB, adjust
60332RXLEVACCESSMIN from 10dB to 12dB.
ATTACH Failure
ATTACH Failure
Problem: network sends Packet Access Reject message to MS as the response to
Packet Resource Request message, which includes “Wait_Indication” whose value is
T3172; MS starts up T3172 when receiving Packet Access Reject message, during T
3172 operation, network doesn’t allow MS to reattempt packet access in one cell. This
problem is caused by tight radio resource.
Solution : expand statistic PS channel.
ATTACH Failure
Contents
ATTACH FailurePDP Activation FailureWAP FailureFTP FailureOther Failure
Overview PDP Activation Failure
PDP Activation
Failure Features
PDP Activation
Failure FeaturesLow success rate of PDP activation, PDP
activation failure, long PDP activation time
Low success rate of PDP activation, PDP
activation failure, long PDP activation time
Reasons for PDP Activation Failure
MS Failure:
APN set incorrectly; OoS mistake; PDP address mistake; MS isn’t compatibly
with network
Radio Network Failure: Weak radio
network signal; Interference; Unavailable radio resource or
congestion.
Core Network Failure:
DNS translation failure;
GTP failure between SGSN and
GGSN
Frequent Route Update:The area is located on
edge of LAC, route
update causes
unsuccessful PDP
activation or too long
delay
Overview
Solutions for PDP Activation Failure check coverage;
prevent frequent cell reselection;
trouble shoot to improve C/I;
check RACH or AGCH configuration;
check Gb links load and adjust NSEI configuration;
set T3168 timer reasonably;
check GPRS parameter setting, set BSC parameter DRX_TIMER_MAX r
easonably;
turn on SGSN cache to shorten DNS translation time;
check GGSN processing function, improper DHCP or RADIUS server con
figuration or powerless GGSN processing ability may cause slow PDP activ
ation or even PDP activation failure.
PDP Activation Failure
PDP Activation Failure
Problem: PDP activation failure is caused by less 20281AGCH after reselect from
20022 to 20281.
Solution: control 20281 coverage extension and increase number of 20281AGCH.
PDP Activation Failure
PDP Activation Failure
Problem: due to continuous twice cell reselection (CELLID: 10071 Channel:2—
CELLID: 111 Channel:90—CELLID: 114 Channel:512), PDP failure is caused by n
o available timeslot during a long period of time.
Solution: raise 60021CRH from 6dB to 10dB.
PDP Activation Failure
PDP Activation Failure
Problem: PDP activation failure is caused by no PS channel.
Solution: increase 8540 site’s statistic PS channel and adjust MFR from 5 to 2.
PDP Activation Failure
PDP Activation Failure
Problem: PDP activation delay is occasionally too long in CQT. 7 abnormal dela
ys occur in 100 times PDP activations in 10 nodes, which is shown in the followin
g tables.
PDP Activation Failure
PDP Activation Failure PDP Activation Failure
PDP Activation Failure
Failure
analysis:
PDP Activation Failure
From the test result we can see that time is mainly consumed below Gb interface as well as betwe
en Radius and WAP gateway. The sixth time consumption between GGSN and Radius is relativel
y long, because the first Accounting Request message sent by GGSN isn’t received by WAP gate
way due to the lost of Accounting Request message in transfer or Radius abnormal processing.
The other six failures are due to too long time consumption below Gb interface as well as betwee
n Radius and WAP gateway. GGSN sends Accounting Request message to Radius, waits for resp
onse from Radius and starts up corresponding wait timer; corresponding WAP gateway doesn’t r
esponse after receiving Accounting Request message. Then wait timer is overtime because GGS
N doesn’t receive response, the Accounting Request message, and then GGSN resends Accountin
g Request message; this time corresponding WAP gateway gives response and GGSN receives A
ccounting Response message transmitted by Radius, and GGSN begins to response to MS PDP a
ctivation request.
TBF resource is released due to overtime timer caused by too long time consumption on Gi interf
ace, therefore MS reestablishes TBF while receiving PDP activation accept message, which furth
er increase time delay on radio interface.
Therefore we can see that the fundamental reason for too long time delay in PDP activation is too
slow WAP gateway response.
Contents
ATTACH FailurePDP Activation FailureWAP FailureFTP FailureOther Failures
Overview WAP Failure
WAP Failure Features:
low success display rate of WAP front page;
long display time of WAP front page;
low success refresh rate of WAP pages;
long refresh time of WAP front page.
WAP Failure Reasons:
Radio coverage problem, interference, cell res
election failure;
Data configuration mistake on two ports of G
b interface;
Too long WAP gateways connection;
WAP parameter setting mistake.
Solutions for WAP Failure:
Optimize WAP gateway;
Optimize GPRS core network;
Check coverage;
Prevent frequent cell reselection;
Carry out trouble-shooting to improve C/I;
Check Gb links load and adjust NSEI configuration.
Slow Download Rate of WAP Picture & Ringtone
Problem: through analysis we see RLC data retransfer ratio is high, Gb link l
oad reaches as high as 64% in test period from OMC statistic data that causes
low download rate.
Solution: readjust NSEI to decrease Gb link load and improve GPRS CQT d
ownload rate.
WAP Failure
WAP Logon Failure
Problem: WAP logon test in ** hotel, after tracking Gb interface we figure out th
at on uplink MS sends Get (URL=http://wap.monternet.com) and network replies a
PDU with the contents of “Your request for a service could not be fulfilled, please t
ry again or contact your operator if the Failure persists”. This problem is caused b
y failures in moternet server.
Solution: require cooperation from core network engineer through negotiation.
WAP Failure
Too Long WAP Logon and Refresh Delay
WAP Failure
Too Long WAP Logon and Refresh Delay
M
I
S
C
Accounting Res.
Accounting Req
Access Accept
Access Request
Active PDP
context accept
Active PDP
context request
Attach accept
Attach request
Ack
Reply
get
Ack
Connect reply
GPRS
核 心网
Wap
网关
Connect
SP
get
Reply
P
C
U
R
A
D
I
U
S
WA
P test signaling flow
Problem: through signalling analysis
we figure out gateway response time
prolongs between CONNECT to CONNE
CT REPLY and between GET to REPLY;
GET request needs to retransmitted and
there is even no response; long signaling
delay occurs especially between GET an
d REPLY and sometimes even reaches t
ens of seconds that largely impacts MS a
ccessing rate to WAP;
Solution: core network engineer
optimize WAP gateway.
WAP Failure
Problem: after MS gets required webpage, WAP gateway replies “URL NOT
EXIST” message, which is the response of WAP gateway after it receives “servi
ce temporarily interrupted” message from SP server.
Solution: this test will be deleted in CDS by processing software and isn’t rega
rded as an attempt test.
Feint Event WAP Failure
Contents
ATTACH FailurePDP Activation FailureWAP FailureFTP FailureOther Failures
Overview FTP Failure
FTP Download Failure Features: High RLC retransfer ratio
TBF drop and longtime transfer interruption
Slow FTP download rate
Overview FTP Failure
Reasons for FTP Download Failure
Network resource
utilization status
Lack of PDCH and
unreasonable logic
channel setting
Impact of cell reselection
rate to GPRS data transfer
Impact of coding mode
Impact of
RLC/MAC block
retransfer ratio
Weak server cell signal
and severe interference
Impact of C/ I to data
transfer
Impact of TBF and
its parameter setting
T3168 and T3192 are set
unreasonably; T3198
and BS_CV_MAX are set
unreasonably.
Overview FTP Failure
Solutions for FTP Download Failure Optimize FTP server;
Adjust CS proportion dynamically;
Optimize coverage;
Prevent frequent cell reselection;
Carry out trouble-shooting and improve C/I;
Set T3192 timer reasonably;
Check Gb links load and adjust NSEI configuration;
Turn on CS3 and CS4.
FTP Download Failure
Problem: after MS reselects Cell41921, DL TBF hasn’t been established that
causes long-time unavailable downlink data transfer and finally PDP drop. We
check cell’s GPRS KPI during the test period and figure out downlink timeslot
congestion rate is relatively high that is the reason for no DL TBF establishment.
Solution: increase the number of the cell’s statistic PS channel.
FTP Failure
FTP Download Failure
Problem: it is Inter-PCU Cell Reselection when MS reselects from CI1892 to
CI481 that causes long-time interruption of data transfer and finally PDP drop.
After checking we figure out NSEI value in CI1892 doesn’t comply with that of
surrounding sites.
Solution: adjust NSEI value in CI1892.
FTP Failure
FTP Download Rate Low
Problem: frequent cell reselection during FTP download process that causes
long-time suspend of TBF. Cell’s busy TCH makes it’s hard for MS to apply PS
channel and average download rate is only 8.11kbit/s。
Solution: expand 30553 and 30152 statistic PS channel, adjust
30553RXLEVACCESSMIN from 10 to 14.
FTP Failure
FTP Download Failure
Problem: level decrease to -94dBm when MS
reselects from 43372 to 42093 and then from
42093 back to 1531 and then frequently
reselects 1531; after a while PDP drops.
Solution: control coverage of 42093 and 1531.
FTP Failure
FTP Download Failure
Problem: MS frequently reselects cell 1483→18621→741 after it carries out
inter-LAC reselection from 83 to 1483 that causes long-time suspend of TBF and
finally PDP drop. We can see from the map there are elevated roads and no
master control cell beneath.
Solution: adjust 1483 CRH from 6 to 10, confirm master control cell in the area.
FTP Failure
FTP Download Rate Low
Problem: PDCH that MS obtained isn’t stable and PS is in great demand that c
auses slow DL FTP rate.
Solution: increase the number of CI2631 statistic PS channel.
FTP Failure
BS_CV_MAX Set Unreasonably
FTP Failure
Problem description:
Data transfer rate in one cell in one GPRS network is relatively low; the cell is in
suburb and has low traffic.
Failure analysis
GPRS radio parameter BS_CV_MAX is set as 15; the area has low traffic and no
channel congestion. BS_CV_MAX is GPRS cell option parameter that helps system
to know in advance when MS ends uplink transfer and improve uplink resource utiliz
ation ratio. If value of BS_CV_MAX is set too low, MS may retransmit RLC block dat
a before network sends back confirmation message and radio resource occupation r
atio may be too high; if it is set too high, then sliding window program efficiency will
be too low and data transfer rate will get slower.
Solution:
Decrease set value of BS_CV_MAX to improve retransfer ratio. After adjust the v
alue of BS_CV_MAX to 6, transfer rate is improved obviously.
FTP Download Rate Low FTP Failure
FTP Download Rate Low
Problem: in repetition test we figure out that in one same place, on 52484, C
1=25, C2= 21; the status from 15:40:33.764 to 15:53:586 lasts 20s and then t
he value of C2 raises to 53. The venue of repetition test is an intersection wher
e large car flow exists and the roads are congested, so PET= 20s that we pre
viously set is far more lesser than the time that we stick in the traffic jam.
Solution: set the cell’s PET as 200s and then the possibility of cell reselection
decreases greatly.
FTP Failure
FTP Download Failure
FTP transfer drop goes with RAU Failure every time pass through RAC
area. After detailed analysis, we figure out data error on SGSN is the reason
for this route update failure.
These three parameters are related to GPRS route update:
(1) home location of each MSC on SGSN
(2) home location of each BSC in MSC
(3) RAC area that each cell belongs to
Normal GPRS MS route update can only be insured when the above 3
parameters comply with hardware in the existing network.
FTP Failure
FTP Download Rate Instable
Problem: In one area subscriber complains that FTP download rate varies greatly, with
the difference of more than tens of Kilobits within a very short period of time; The area is covered completely by Cell A, and this complaint isn’t appeared in
other cells, so we mainly check the difference between Cell A and other cells; We figure out Cell A’s T3168 is set as 4000ms but that of other cells is set as
1000ms. Solution:
The value of T3168 is set so high that data transfer rate is relatively high in
normal transfer; MS sends Require Packet Resource message as the response when it
receives Packet Uplink Assignment message; T3168 is started up when the
response is sent; MS doesn’t respond Packet Downlink Assignment message
when T3168 is running, therefore no downlink data are transmitted during the
time that is equal to the value of T3168, which makes low transfer rate. Modify T3168 from 4000ms to 1000ms and then FTP download rate is
stabilized and the problem is solved.
FTP Failure
Problem: vehicle drives from
North to South and passes
through the red circle covered by
Cell A; when it keeps on going to
South, level of Cell A decreases
gradually and cell reselection still
doesn’t occur when the level is
lower than -94dBm; this situation l
asts a long period and causes we
ak coverage. After checking, the c
ell reselection parameter settings
of Cell A and its adjacent cells at t
hat time is shown in the table on t
he next page.
Weak Coverage FTP Failure
Cell Name Rexlev (dBm) RXLEVACCESSMIN CRH CRO TO C2
A -103 -100 8 26 0 23
B -80 -100 8 8 0 28
C -84 -100 8 10 0 26
Solution: control the coverage of Cell High Education Scientific Research 4 (B
CCH: 560), modify CRO of Cell High Education Scientific Research 4 from 26 to
20.
Weak Coverage FTP Failure
FTP Download Failure
After sniffing and analyzing packet on Gi interface
we figure out that FTP server stops sending data be
cause it receives [RESET] message after being logg
ed on. The process of FTP download test on MS doe
sn’t stop but FTP server stops transfer, therefore test
software judges failure due to overtime.
It is server logon failure due to location update, TC
P process fails due to packet lost while logging on F
TP server; the test software shows successful logon
but failed data download.
Feint Event
FTP Download Failure
Through sniffing packet by test equipment we can see that FTP-DATA packet has been received successfully but hasn’t been turned to effect data that finally causes overflow in receiving window and the end of data transfer. But the process of FTP download test doesn’t stop either, and finally test software judges failure due to overtime. it is also the server logon failure due to location update, although the process of TCP and data transfer are normal while logging on FTP server, FTP-DATA still fails.
Feint Event
FTP Download Failure
Conclusion: The two kinds of FTP download failures mentioned previously are all caus
ed by FTP failure that appears on TCP/IP transport layer and application la
yer instead of caused by GPRS network layer failure.
From the test software we can see that data on application layer can’t be r
eceived in the two situations while logging on FTP server, and finally FTP do
wnload fail is displayed due to overtime and it is counted as FTP download f
ailure once.
Feint Event
Contents
ATTACH FailurePDP Activation FailureWAP FailureFTP FailureOther Failures
PING Failure 1
Problem: in Ping test, interval is set
from 4s to 12s, and the results vary
greatly. Through recording signalling in
core network we figure out the reason for
unstable Ping time delay are some
rubbish data packets appear in the test
which are caused by the automatic
update of antivirus software and other
application softwares.
Solution: reinstall operation system,
avoid from installing softwares
irrespective of test; turn off all programs
related to network communication in tray,
turn off Windows automatic update.
Other Failures
Problem: low cell’s C/I causes high BLER (cell BCCH C/I= 5.9). After chec
king planning data we figure out beehive 8800, 7660 and 8750 use the same BCCH. Solution: control the coverage of 8800, 7660 and 8750, adjust frequencies of 8800, 7660 and 8750.
PING Failure 2 Other Failures
MMS Transfer Failure
Problem:
The transfer success rate of MMS that larger than 15Kbits sent by Panasonic G
D88 mobile phone is relatively low; and the rate is almost 0 when the MMS is larger
than 30Kbits. But if using mobile phone of other brands, such as NOKIA 6100, the l
arge MMS can be sent both normally and fast.
Solution:
After recording signaling on GB interface we figure out Panasonic EB-GD88 mo
bile phone doesn’t completely support GSM04.60 criterion (GPRS RLC/MAC layer
criterion); it can’t support completely the site parameter settings that BS_CV_MAX=
13, 14 or 15, which is allowed in GSM04.60 criterion, and it only supports BS_CV_
MAX=1, …, 12. We suggest modifying the default value of BS_CV_MAX in the who
le network from 15 to 6.
Other Failures
MMS Receiving Failure
Problem: MS sends Get request on uplink for MMS to MMSC, but Cause val
ue in the network Reply message is 404: Not Found; it may be because MS do
esn’t pick MMS in time and MMSC deletes it after waiting for a while, then net
work replies Not Found when MS requires MMS.
Other Failures
MMS Transfer Failure
Problem: through recording signalling by signalling analyser we figure out
that MS sends PDP activation request initially, APN is CMWAP, but then MS
doesn’t send any signalling later. This situation is caused by dead MS software
process.
Solution: reinstall MS operation system.
Other Failures
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MMS Receiving Failure
Problem: through recording signalling by signalling analyser on Gi interface
we figure out that Get request is sent time after time within around 10ms after
MS has sent Get request (Get request retransmitting timer is set as 10s) , that
makes WAP gateway have no enough time to deal with MS’s Session; from
checking retransfer interval we can see this situation isn’t caused because MS
receives MMS continuously. This situation is caused by MS software failure.
Solution: reinstall MS operation system.
Other Failures
GPRS Network Accessing Failure
Other Failures
Problem description: Some subscribers reflect the network accessing rate is slow by u
sing GPRS network card.
Failure analysis: The subscriber’s SIM card classification is high and confirmation
mechanism is used on LLC layer, so network accessing rate is slow
ed.
Solution: Check the subscriber’s PS QoS, reliability; 3 represents unconfir
med, 1 represents confirmed. GPRS network accessing rate accele
rates after mode has been modified as unconfirmed.
BVC Reset and Block Failure Other Failures
Problem description: BSC Gb interface connects successfully with SGSN on NS layer, but c
onfigured PS channel’s status is “BVC reset and block”.
Failure analysis: After checking signaling, RAC is reported mistaken. Check parameter
settings of BSC and SGSN then find out the previous two parameters a
mong the four parameters of RAC in SGSN configuration are DEC, whic
h should be HEX in SGSN configuration.
Solution: Modify the parameter to HEX and then signaling go back to normal. W
e shall pay special attention that no DEC parameters but HEX paramete
rs shall be used in SGSN configuration.