go_na20_e1_1 gsm network sdcch assignment analysis-62

7/15/2019 GO_NA20_E1_1 GSM Network SDCCH Assignment Analysis-62

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SDCCH Assignment Analysis

& Solutions

Zte university



Contents

Overview

Analysis of signaling and counters related to

immediate assignment

Radio parameters Instructions on checking of SDCCH assignment

failure

Typical cases on SDCCH assignment



Definition of SDCCH

SDCCH: the Standalone Dedicated ControlChannel is used to transmit information likechannel assignment, which falls into the followingtwo types:

SDCCH/8: the standalone dedicated control channel; SDCCH/4: the SDCCH that is combined with CCCH.

In brief, the following processes shall be taken intoconsideration in the process of occupying SDCCH:

Location update, periodical location update; IMSI attach/detach

Call setup

SMS



Signaling flow of immediate assignment



Counters related to SDCCH assignment &

corresponding signaling messages V3

C900060242

Number of

SDCCH

assignment

success

Function:

After BSC sends out the immediate assignment message (IMM_ASS),this counter counts the number of successful MS accesses to thecorresponding SDCCH.

Sampling:

when BSC receives the correct EST_IND or the message of assignmentcomplete.

C900060243

Number of

SDCCH

assignment

failure

Function:

After BSC sends out the immediate assignment message (IMM_ASS),this counter counts the number of failed MS accesses to the allocatedSDCCH.

Sampling:

when BSC receives the wrong EST_IND, or when T3101 expires.



SDCCH assignment success rate

KPI SDCCH assignment success rate

Definition Number of successful SDCCH assignments*100/(Number of successful

SDCCH assignments + Number of failed SDCCH assignments)

Counter formula

V2 C11644*100%/( C11644+ C11645)

V3 V6.2 C900060242*100%/(C900060242+C900060243)



Difference: Random access success rate

Definition: Number of successful randomaccesses / Number of random accessrequests*100%

Number of random access requests

Definition: MS applies for a channel in the idle mode. Trigger point: it counts the message of CHANNEL

REQUIRED received by BSC from MS. (A1)

Number of successful random accesses

Definition: BSC successfully assigns a dedicatedchannel for MS.

Trigger point: it counts the message of IMMEDLATE ASSIGNMENT sent from BSC to MS. (A2)



Contents

Overview

Analysis of signaling and counters related to



failure




Analysis of Channel Request cause

Establishment Cause




Establishment Cause (continued)




Summary on Establishment Cause Emergency call

Call re-establishment

Paging response（MTC）

Mobile originating call（MOC）

Location update（LOC）

Other access causes

One-step access

LMU service

MBMS service



Channel Required



Channel Required

Request Reference

RA（Random access reference）: it continues to use the Cause and

Random Reference in the Channel Request.

Byte 3 and 4 (T1, T2, T3): receive the frame number(42432) of the

burst pulse.



Channel Required

Access DelayThe estimated TA

Physical Context

including Rxlev_UL



Immediate AssignmentPage Mode = same as before

Packet Response Type and Dedicated mode or TBF

Downlink assignment to mobile in Ready state: no meaning

TBF or dedicated mode: this message assigns a dedicated mode resource

PR Type: immediate assignment procedure for RR connection establishment

Channel Description

Type = SDCCH/8[0]

Timeslot Number: 1

Training Sequence Code: 0h

ARFCN: 104

Request Reference

Random Access:

Establish Cause: E0h = Originating call and TCH/F is needed, or originating call

and the network does not set NECI bit to 1Random Reference: 12h

N32: 13h; N51: 1Fh; N26: 0Dh

Timing Advance: 1 = 0,6 km

Mobile allocation



Establish Indication




T represents the sub-channel number.




Information on layer3: CM SERVICE REQUEST

LOCATION UPDATING REQUEST

IMSI DETACH

PAGING RESPONSE

CM RE-ESTABLISHMENT REQUEST

NOTIFICATION RESPONSE

IMMEDIATE SETUP

RR INITIALISATION REQUEST




CM SERVICE REQUEST Originate call

Emergency call (Access statistics show that emergency

call is not included in MOC )

SMS Supplementary service

Group call establishment

Voice broadcast call



Access counters

Basic measurement

Counter Number Counter name

C900060001 Number of MTC access requests

C900060002 Number of MTC access successes

C900060131 Number of CM SERVICE REQ of MOC

C900060136 Number of MOC access requests

C900060137 Number of accesses due to paging response

C900060236 Number of MOC access successes



Access counters

Radio access measurement (I)Counter Number Counter name

C901110001 Number of invalid access requests

C901110003 Number of successful process for MOC access

C901110006 Number of successful process for MTC access

C901110008 Number of call re-establishment access requests

C901110009Number of successful process for call re-

establishment access

C901110010 Number of call re-establishment access success

C901110011 Number of emergency call access requests

C901110012Number of successful process for emergency call

access

C901110013 Number of emergency call access success

C901110014 Number of LOC access requests

C901110015 Number of successful process for LOC access

C901110016 Number of LOC access success

C901110017 Number of access requests due to other causes

C901110018 Number of successful process for other causes’ access

C901110019 Number of access success of other causes



Access counters

Radio access measurement (II)C901110020 Number of LMU Establishment access requests

C901110021 Number of successful process for LMU Establishmentaccess

C901110022 Number of LMU Establishment access success

C901110023 Number of accesses due to location update

C901110024 Number of accesses due to CM SERVICE REQ

C901110026 Number of Emergency Call (CM SERVICE REQ)accesses

C901110027 Number of SMS (CM SERVICE REQ ) accesses

C901110028 Number of supplementary service (CM SERVICE REQ)accesses

C901110029 Number of accesses for LCS (CM SERVICE REQ )accesses

C901110031 Number of accesses due to call re-establishment

C901110032 Number of accesses due to IMSI de-activation

C901110033 Number of accesses due to other causes



Contents

Overview Analysis of signaling and counters related to



failure




TxInteger

Before response to the previous “channel request”is received, MS waits for a period of time at

random and sends the request again after

expiration. TxInteger is to decide the random

waiting time. The interval (number of timeslots) from MS originating

the immediate assignment to the transmission of the

first “channel request” message is a random number

among { 0,1,…,Max（T,8）-1 }. The interval (number of timeslots) between two

consecutive “channel request” is a random number

among {S,S+1,…,S+T-1}.



TxInteger

T(Number of

timeslots

Of TxInteger)

S

(CCCH is NOT

combined with

SDCCH)

S

(CCCH is

combined with

SDCCH)

3, 8, 14,50 55 41

4, 9, 16 76 52

5,10,20 109 58

6,11,25 163 86

7,12,32 217 115

TxInteger Number of

timeslots (T)

0 3

1 4

2 5

3 6

4 7

5 8

6 9

7 10

8 11

9 12

10 14

11 1612 20

13 25

14 32

15 50



MaxRetrans

Because RACH is a ALOHA channel, in order toimprove MS access success rate, the network

allows MS to send several Channel Request

messages before it receives the Immediate Assign

message. The max number of Channel Requestssent by MS is decided by MaxRetrans.

MaxRetrans Max number of retransmission

0 1

1 2

2 4

3 7



TaAllowed

It represents the max TA allowed for access to thecell.

It is used to filter out fake accesses.



RachAccessMin

New parameter for iBSC 6.20.100e Used to filter out fake access, but not

recommended because it will affect the paging

performance.



Contents



Radio parameters Instructions on checking of SDCCH

assignment failure




Explanation on common causes of SDCCH

assignment failure

MS frequently originates location update due topoor downlink quality;

Improper setting of Tx-Integer;

High SD assignment failure rate due to LAPD

delay

Co-channel/co-BSIC interference

Uplink interference

Overshooting



Improper setting of Tx-Integer

The default of Tx-Integer is 14, which is also the

max value.

Usually, the one-way signaling transmission delay

at Abis interface is 60ms~100ms; there should be

a delay of about 240ms from MS originates

Channel Request till it receives Immediate Assign.

When the transmission link delay is long, while

TxInteger is set with a small value, it will result in

MS sending too many access requests. However,

MS only responds to the first Immediate Assign it

receives.




Flow chart of repeated assignment failure

Channel Request

Channel Required

M S

Channel Active

Channel Active Ack

Imm Assign(OK) Imm Assign Cmd

B T S

Channel Request(Re-Send）

TxInteger

Lapd

Delay

Channel Required

Channel Active

Channel Active Ack

Imm Assign Cmd Imm Assign(Fail)

MS change

to SDCCH

B S C



LAPD delay

Possible causes of LAPD delay Application of LAPD 1:4 multiplexing will lead to the situation that

several BCCH TRXs are multiplexed on one LAPD, which willcause heavy flow on the LAPD and hence delay.

Heavy flow on LAPD leads to delay. For example, improper LACdivision will lead to large amount of paging and hence LAPD flow

control. Transmission equipment fault leads to loss of messages on LAPD

or long LAPD delay. These phenomena are often accompaniedwith SDCCH assignment failure.

The transmission equipment’s own delay, such as the delay

caused by satellite transmission at Abis interface. Impact of PS service: PS service is more sensitive to network delay.

Any LAPD delay will leads to re-transmission of PS servicemessage, which increases the flow on LAPD and causes longer LAPD delay, then a malicious circle will be resulted.



Co-channel & co-BSIC

Two cells have same BCCH and same BSIC The Channel Request sent by MS is received by two

cells and they assign SDCCH at the same time, but MS

can only accept one SDCCH, therefore, one of the two

cells will inevitably experience SDCCH assignment

failure.

For RACH coding, first add in 6bit color code, which is

obtained through taking mod2 of 6bit BSIC and 6bit

parity checking code. Therefore, co-BCCH and co-BSIC

may cause the BTS to incorrectly decode MS access

bursts to other sites, which will lead to SDCCH

assignment failure



Co-channel & co-BSIC

Two cells have same BSIC and the TCH Arfcn of one cellis same as the BCCH Arfcn in the other cell.

The handover access request occurring on the TCH timeslot is

received as Channel Request by the other cell, which thereafter

performs assignment. This certainly leads to SDCCH assignment

failure. It’s stipulated in protocols that the MS-started handover access

information and the random access request share the same format,

which is AB frame; the difference is that the handover access

information content (RA) in one handover started by MS is the

same, and the FN is in consecution. Signaling related to this problem displays that the RA is the same,

TA is in consistence and FN in consecution. It’s confirmed that all

the large amount and consecutive Channel Requests are fake

accesses caused by handovers between co-channel cells.



Overshooting

If the coverage of cell is too large, the DL Rxqual at the cellmargin will be poor. In this case, BTS can receive ChannelRequest sent by MS, but MS can not receive Immediate

Assign sent by BTS, for BTS is more sensitive than MS,

If the coverage of cell is too large, the cell may share

channel and BSIC with the cell which is far away. Solution to overshooting:

Adjust the engineering parameters of antenna to limit the cellcoverage.

TA_allowed can effectively decrease SDCCH assignment failures

caused by overshooting. The side effect it brings is that the distantMS is not able to access network. Therefore, the threshold of TA_allowed shall be set a bit higher than the cell’s actual coverage.Besides, we should take into account the transmission distance of repeater when calculating the cell coverage range.



Uplink Interference --- Fake Access

BTS receiving sensitivity is -112dbm~-125dbm. If the random access signal

strength received by BTS is lower than BTS sensitivity, it usually is confirmed to

be interference. The interference can be decoded as random access, which iscalled as fake access, and will definitely lead to SDCCH assignment failure.

Another feature of fake access is that TA is larger than that needed for the actual

coverage range.

Solution: TA_allowed

Note:① RachAccessMin is not recommended to use

② As for TA-allowed, the corresponding name used by Nortel is RNDACCTIMADVTHRESHOLD,

whose description is as follows: adjust the parameter according to the cell’s actual coverage range.

Fake RACH request can be filtered out through setting proper threshold, therefore unnecessary

SDCCH assignment can be avoided. Test results prove that if TA-allowed is set 35Km for cells with

small coverage radius, fake RACH (the system demodulate the noise into RACH pulse by mistake)accounts for almost 30% of all RACH requests. After rndAccTimAdvThreshold is changed to 2, fake

RACH is totally filtered out.



Frequent location update started by MS

If MS needs to make location update, while theradio environment is poor, it will retransmit

Channel Request with the cause of location

update again and again, but it can never receive

Immediate Assign message. The frequent location update will cause

fluctuations in SDCCH assignment indicators.



Frequent location update started by MS

Number of SDCCHassignment

successes

Number of SDCCHassignment failures

SDCCH assignmentsuccess rate

Number of

MOC access

requests

Number of

MOC access

successes

Number of

MTC access

requests

Number of

MTC access

successes

Number of

SDCCH

occupation

attempts (for

assignment)

(MOC+MT

C)

assignment

success rate

(MOC+MT

C)

proportion

Reference

indicators



Troubleshooting instructions

Check TxInteger of the problem cell, along with LAPDdelay observed from signaling.

Check whether the LAPD link of BCCH TRX in the problemcell is multiplexed with that of other cells.

Check whether any of the adjacent cells have same Arfcn

and BSIC with the problem cell. Check whether the value of counter “number of access

attempts due to other causes” is big. If so, and the counter “number of access successes due to other causes” is zero,it is possible that “handover access” on other TCH TRXsare decoded as “channel request” by the problem cell.

Error Report with Channel Number 0x88 is available in themplog file.



Troubleshooting instructions

Check SDCCH allocation KPIs and transmissionalarms.

If SDCCH &TCH assignment indicators are all bad,

the problem shall be related to radio environment.

Analyze signaling and check if Channel Requestwith large TA, if so, fake access exist and

TA_allowed restriction can be used.



Contents




failure


LAPD d l C 1 L t f



LAPD delay—Case 1: Large amount of

paging

Problem description: It’s found through performance

analysis that ZTE BSC3 has low SD assignment success

rate, which is only about 60% on late busy hours.

Problem analysis:

It’s observed that all the cells are experiencing high SD assignment

failure rate, so impact from radio parameters is excluded.

Indicators of other BSCs are normal; the SD assignment success

rate is low in only BSC3 and the Siemens BSC, both of which are

under MSC7.

The paging success rate in MSC7 is also very low; as the traffic

volume increases, the amount of paging increases as well.

LAPD d l C 1 L t f



LAPD delay—Case 1: Large amount of

paging

Adjustment measure:

Add one LAC under MSC7. After the adjustment, the SD assignment

success rate of BSC3 returns to normal, reaching above 95％.

50%

60%

70%

80%

90%

100%

0

20000

40000

60000

80000

100000

3月10日 3月11日 3月12日 3月13日 3月14日 3月15日

BSC3 SDCCH指配成功率对比

SDCCH指配成功次数 SDCCH指配失败次数 SD指配成功率



LAPD delay—Case 2: Satellite transmission

Problem description: 4BTSs are under BSC01, butbelong to different peripheral modules. The SD

assignment failure rate of the 4BTSs reaches as

high as 50%.

The time stamp shows that it takes an average of 0.58s to successfully activate a channel.



LAPD delay—Case 2: Satellite transmission

How to confirm that two Channel Requests arestarted by the same call attempt?

They should have the same Establish Cause;

The same Access Delay;

The frame number interval corresponds to the setting of TxInteger:

Calculation formula: FN＝T1*26*51+((T3-T2)mod 26)*51+T3



LAPD delay—Case 3: Transmission equipment fault

Problem description: Massive SDCCH assignmentfailures occur in 3 cells of a site, accompanied

with lots of SDCCH allocation failures.

Problem analysis: SDCCH allocation failure

usually means transmission equipment fault. After checking mpLog printing, there are lots of LAPD

Errors.

Also There are a lot of transmission alarms.




A cell’s ordinary SDCCH assignment failure rate remains at around

20% and hits 30% in busy hour. However, other KPIs(such as TCHassignment failure rate, handover success rate) are all good.

Problem analysis: After analyzing the cell’s signaling, we find thereusually are Channel Request messages appearing in couples in thecell (with the same TA and cause). The Imm Assignmentcorresponding to the first Channel Request was successful, but theone corresponding to the second Channel Request failed.




Problem analysis: Tx-Integer=12, which means “channel request”

retransmission interval is 109~128

FN of the first Channel Request is 964; that of the second

Channel Request is 1086; there is a difference of 124 frames.

It’s confirmed that the two Channel Requests are sent by the

same MS.

Solution: change Tx-Integer to be 14. After the

adjustment, the SDCCH assignment failure rate drops

to below 10%.

Access of interference signal Case 1: TA



Access of interference signal—Case 1: TA

exceeding the actual coverage range

Problem description: the SDCCH assignmentsuccess rate in a cell is very poor.

Time Alias

11644(Number of

SDCCH Assignment

Success)

11645(Number of

SDCCH Assignment

Failure)

2007-4-26 19:15 Cell A 191 15

2007-4-26 19:30 Cell A 190 24

2007-4-26 19:45 Cell A 177 33

2007-4-26 20:00 Cell A 192 26

Access of noisy signal—Case 1: TA exceeding the



Access of noisy signal—Case 1: TA exceeding the

actual range

Problem analysis: analyze ABIS signaling; the TA of

failed random access Immediate Assign failure is asfollows; the neighboring sites are near each other ,

with a distance less than 1 Km.

Serial No. TA CauseCorresponding time stamp for sending Immediate

Assign

1 35 MOC 06-08-55.3752 36 MTC 06-08-55.562

3 35 MOC 06-08-55.984

4 34 MTC 06-08-56.578

5 32 MOC 06-09-11.640

6 30 MTC 06-09-24.546

7 27 MTC 06-09-38.031

8 27 MTC 06-09-38.578

9 27 MTC 06-09-39.10910 0 MOC 06-09-57.171

11 24 MOC 06-09-57.828

12 10 MOC 06-11-15.406

13 2 MOC 06-12-12.781

14 0 MOC 06-12-52.671

15 0 MOC 06-12-53.218

16 1 LAC update 06-15-13.140

Access of noisy signal Case 2: the Rxlev



Access of noisy signal—Case 2: the Rxlev

lower than BTS sensitivity

Problem description: A cell’s SDCCH assignmentfailure rate keeps high, but the TCH assignment

rate is acceptable.

SDCCH

assign

successfulnumber

SDCCH

assign

failurenumber

SDCCH

assign

failure rate

TCH

Assignment

SuccessNumber

TCH assign

failure

number

TCH assign

failure rate

14479 4490 23.63 4678 122 2.54

Access of nois signal Case 2 the R le



Access of noisy signal—Case 2: the Rxlev

lower than BTS sensitivity

Problem analysis: The Physical Context carried by

Channel Required message reports the Rxlev of random

accesses, in which we find lots of Channel Request

messages whose Rxlev is -135dbm(0x87).



Co-BCCH & co-BSIC— Overshooting

Problem description: the SDCCH assignment failure rate

in many cells exceeds 25%. Process procedure:

After all the hardware is changed, the problem still exists.

Through signaling trace we find that the co- BCCH/co-BSIC

signals received when TA=20 lead to SDCCH assignment failure. Based on the above finding, re-plan the BSIC of more than 10

cells in the network. After the re-planning, coverage of the cells

returns to normal.

Solution:

Temporary solution: the CMM of cells with high reset failure rateenables the clock to reset, which lead to synchronous malposition

of SDCCH timeslot.

Ultimate solution: to avoid co-channel/co-BSIC.



Co-Channel & co-BSIC—Handover

Problem description: a cell experiences a suddenincrease of SDCCH assignment failure rate in

busy hour; the TCH assignment indicators are

good.

Cell ID PmdatatimeSDCCH assign

failure rate

TCH assign

faliure rate

Cell A 19:00-20:00 15.85 0.68

Cell A 21:00-22:00 12.78 0.71

Cell A 20:00-21:00 11.27 1.36



Co-Channel & co-BSIC—Handover

Problem analysis: Through signaling trace, we find that there is a

large number of continuous random accesses; these ChannelRequests have the same RA, TA, and consecutive frame numbers.

Solution: After checking frequency planning, we find there are co-

channel & co-BSIC cells which are located 14km away from the BTS.

After re-planning of frequency, the problem disappears.



Weak coverage

Problem description: The SDCCH assignment failure rate in a cell reaches

58% in busy hour, and TCH assignment failure rate

56%; handover success rate in only 20%.

Network performance statistics of fore-and-aft daysdisplay that the TCH assignment failure rate, call drop

rate and handover failure rate have remained high.

UserLabel Handover success rate(%)

SDCCH assignfailure rate

TCH assignfailure rate

Cell A 20 58.67 56.19



Weak coverage

Problem analysis:

DT result shows that the problem cell not only experiences weakcoverage, but also overshooting and co-channel interference.

Signaling trace shows a large number of abnormal accesses of

consecutive Channel Requests with TA =63.



Consecutive LOC update request

Problem description: some sites at LAC boundaries and suburb

experience sudden increase of SDCCH assignment failure rate, whichmoves in no certain pattern; while other indicators of the cell are quite

normal.

Problem analysis:

The basic measurement data shows that LOC access attempts and

failures count for a large proportion of the SDCCH assignment failures.



Consecutive LAC update access request

Problem analysis:

Signaling analysis shows that MS continuously starts Channel

Requests with cause of LAC update, which all end in failure.

go_na20_e1_1 gsm network sdcch assignment analysis-62

Documents

channel assignment

corresponding sdcch

allocated sdcch

dedicated channel

message of channel

immediate assignment

message of assignment

process of occupying