GSM P&O TCH Call Drop & Solutions

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Internal Use Only

GSM P&O TCH Call Drop & Solutions

Special Subject GuidebookVersion: V2.0

Released by:

GSM Network Planning & Optimization Dept.

Engineering Service Division

ZTE CorporationGSM Network Planning & Optimization Dept. Special Subject GuidebookVersion introduction:

Version Date Writer Assessor Amendment records

V1.02005-12-1Ma JunhuaMa Junhua

V2.02009-3-12Hu YixiaoMa JunhuaDocument format standardization

Key words TCH call dropcall drop due to RF losscall drop due to handover failurecall drop due to LAPD link failureAbstract: This guidebook mainly describes types and causes of TCH call drop and the handling procedures.Abbr.: References: Contents 11Statistical points & types of TCH call drop

11.1Call drop due to RF loss

21.2Call drop due to handover failure

41.3Call drop due to LAPD link failure

52Causes for different types of call drop

52.1Main causes for call drop due to RF loss:

62.2Main causes for call drop due to handover failure

72.3Main causes for call drop due to LAPD link failure

83Flow of problem handling

104Typical cases:

104.1High call drop rate at Cell 1, 3 of BTS A (of a region)

114.2High call drop rate at Cell B-3 of a region

Figure contents1Fig 11 Call drop due to RF loss

3Fig 12 T3103 expiresinter-cell handover failure

3Fig 13 T3107 expiresintra-cell handover failure

4Fig 14 T8 expiresinter-BSC handover failure

9Fig 31 Flow of handling call drops

1 Statistical points & types of TCH call dropIn mobile communications, call drop refers to the phenomenon that calls are lost or interrupted due to some reasons after TCH assignment. Call drop brings a lot inconvenience to subscribers, which is also the problem that subscribers complain about most.Statistical point of TCH call drop: BSC sends CLEAR_REQ message to MSC, the channel currently occupied by MS is TCH.Call drop falls into the following types:1 Call drop due to RF loss (i.e. radio link fault);2 Call drop due to handover failure;3 Call drop due to LAPD link failure.1.1 Call drop due to RF loss

Fig 11 Call drop due to RF lossCall drop due to RF loss consists of two parts: A. DL failure: according to GSM regulations, there is a timer S (T100), which is given an initial value when the MS starts talking, that is radio_link_time out. The value is broadcasted on BCCH. Each time when MS cant decode an SACCH message (4 SACCH BLOCK), S is deducted by 1. Each time when MS decodes an SACCH message, S is increased by 2. But S wont exceed the initial value defined by radio link_time out. When S is 0, MS will quit connecting to network and enter idle mode. Hence call drop happens once.B. UL failure: the parameter for inspecting UL failure is link fail. When BTS cant decode an SACCH message, a counter in HDPC (whose max value is defined by link fail) is deducted by 1; when BTS decodes an SACCH message correctly, the counter increments by 2 (the value shall not exceed what defined by link fail). When the counter shows 0, BTS stops sending DL SACCH and starts timer rr_t3109 (rr_t3109>T100)at the same time. When MS T100 expires, MS returns to idle mode, hence call drop occurs. BTS releases radio channel when timer rr_t3109>T100 expires. BSC needs to send a message Clear-request to MSC.Either of UL and DL fails, BTS and MS will stop sending SACCH to each other, thus force the counterpart to release TCH. Each time link_failhappens on TCH, RF_LOSSES_TCH increments.1.2 Call drop due to handover failurePrinciple of call drop due to handover failure: After the original cell received Handover Command or Assignment (corresponding handover control timer is started by BSC), MS hasnt accessed into the target cell, nor has it returned to the original cell. i.e. MS not only failed to occupy TCH in target cell and send Handover Complete or Assignment Complete, but also failed to return to TCH in original cell and send Handover Failure or Assignment Failure, thus it is disconnected with network. At the same time, the BSC-controlled handover control timer expires, and MSC will be notified to clear the release, and it will count this abnormal call as call drop due to handover failure.Handover control timer in BSC is classified into: T8 expires (inter-BSC handover), T3103 expires (inter-cell handover under one BSC), T3107 expires (intra-cell handover).Fig 1-2 illustrates call drop due to handover failure.

Fig 12 T3103 expiresinter-cell handover failure

Fig 13 T3107 expiresintra-cell handover failure

T3103 expiresinter-cell handover failure

Fig 14 T8 expiresinter-BSC handover failure

1.3 Call drop due to LAPD link failureExplanation: when LAPD link breaks, calls on carriers will be interrupted. BSC counts after it receives message showing that LAPD link is broken.2 Causes for different types of call drop2.1 Main causes for call drop due to RF loss:(1) Weak coverage area, poor signal; (2) Interference: like internal interference caused by inappropriate frequency planning and external interference;(3) Unreasonable radio parameters: Minimum receive level is set too low, which leads to MS suffering from call drop in weak coverage area. Unreasonable setting of NCC Permitted: in some networks, the serving cell and adjacent cells may adopt different NCC, which requires input of NCC adopted by adjacent cells in NCC Permitted. Once the setting is not reasonable, adjacent cells of an NCC wont be detected by MS, which will result in handover nonoccurance and call drop. Value of radio link timer is set too small (radio link will time out soon), which leads to call drop when it expires due to sudden deterioration in radio environment; if its set too large, use ratio of TCH will decrease. Unreasonable power control parameters: such as unreasonable power control threshold for level and quality, which may cause MS power to decrease when signal and quality are poor. Unreasonable FH parameters: wrong setting of Maio leads to co-channel interference within a site. Definition of adjacent cell data is set wrong or not complete, which disables MS to improve signal quality by handover, hence call drop due to signal deterioration. Unreasonable handover parameters, which disables MS to improve radio quality through handover when necessary, hence call drop is resulted. Congestion in adjacent cells: it causes call drops, because MS is unable to improve radio quality through handover. the congestion problem shall be solved first.(4) Hardware fault: such as too low output of power amplifier, big difference between in transmission power of different carriers, faults of carrier transmitter, combiner and divider.(5) Antenna system fault: such as difference in down-tilt and azimuth of two antennas; too large feeder VSWR; overlarge coverage caused by unreasonable antenna height or down-tilt; all these will cause skip-zone coverage, thus creates isolated-island effect and call drop is resulted.(6) MS malfunction: for example, power-fail due to poor contact of battery, etc..2.2 Main causes for call drop due to handover failureWe need to take into consideration optimization of handover success rate (especially that of success rate of outgoing handover) when trying to solve problem about call drop due to handover failure.(7) Interference: like internal interference caused by inappropriate frequency planning and external interference.(8) Hardware fault: clock malfunction in target cell or serving cell, too low output of power amplifier, big difference between in transmission power of different carriers, faults of carrier transmitter, combiner and divider.(9) Unreasonable settings of radio parameters Co-BCCH and co-BSIC in target cell, high outgoing handover failure rate is caused, thus call drop is resulted; Inappropriate adjacent cell relation or wrong adjacent cell data, high outgoing handover failure rate is caused, thus call drop is resulted; Unreasonable settings of handover parametersPingpong handover will be caused, thus call drop is resulted.2.3 Main causes for call drop due to LAPD link failure(1) BTS transmission problem: like interruption in transmission and unstable transmission (on and off), etc.;(2) BTS hardware problem: like unreliable E1, problems about CMM board and backboard connection lines;(3) BSC hardware problem: like problem about LAPD processing module.3 Flow of problem handlingChecking of radio parameters and hardware is advised for locating problems.Flow of handling call-drop problem:(10) Analysis of performance report: to confirm the cause of call drop in cell with high call drop rate; (11) For call drops due to different reasons, make analysis accordingly;(12) If there are more call drops due to RF loss, the following procedures are recommended: Check radio parameters: adjust the unreasonable settings of radio parameters; Check indicators like BER and level of idle interference band, reduce or eliminate radio interference; Check if coverage problem exists through DT. For weak coverage, focus on hardware problem checking; for skip-zone coverage, focus on checking power parameters, handover parameters and antenna down-tilt, etc.; Check and eliminate equipment fault; change panels with problems; Check antenna system, especially the parts with problems.(13) If there are more call drops due to handover failure, the following procedures are recommended: Check radio parameters: adjust the unreasonable settings of radio parameters; Check indicators like BER and level of idle interference band, reduce or eliminate radio interference; Check and eliminate equipment fault; change panels with problems;(14) If there are more call drops due to LAPD link failure, the following procedures are recommended: Investigate equipment problem at BSC; Investigate BTS transmission; Investigate hardware problem at BTS.Basic flow of handling call drops:

Fig 31 Flow of handling call drops 4 Typical cases:4.1 High call drop rate at Cell 1, 3 of BTS A (of a region)Problem description

Performance indicators at OMCR show high call drop rate at Cell A-1, A-3.Problem analysis

Get detailed performance statistics, investigate call drop cause, its found that the main cause for call drop is RF loss. Check radio parameter settings, its found that Cell A-1, A-3 co-channel, and its confirmed that co-channel interference exists, thus call drop is resulted.Problem solution

Adjust frequency, eliminate interference. Number of call drop times is obviously reduced.Date Site Cell location area (LAC-CI)No. of TCH call dropsNo. of TCH occupancy times(including HO)TCH call drop rate (including HO)(%)

2005-05-20Region X_A _1_DV2LAC8411-CI251766611615.68

2005-05-21Region X _A _1_DV2LAC8411-CI25176328703.67


After frequency adjustment



DateSiteCell location area (LAC-CI)No. of TCH call dropsNo. of TCH occupancy times(including HO)TCH call drop rate (including HO)(%)







4.2 High call drop rate at Cell B-3 of a regionProblem description

Performance indicators at OMCR show high call drop rate at Cell B-3.Problem analysis

Get detailed performance statistics, investigate call drop cause, its found that the main reason is handover failure. Check data of cell frequency and adjacent cells, no problem is found. Trace signaling, its discovered that UL level is stronger, but UL quality is rather poor, interference level of idle TCH indicated in RF resource is 5, as shown in the following figure. Therefore, its confirmed that external interference exists in the cell.

Problem solution

Try to apply out-of-band frequency (usable frequency given by operator according to the external interference at the region). After frequency adjustment, number of call drops reduced greatly, indicators display normal, and the problem is solved.DateSiteCell location area (LAC-CI)No. of TCH call dropsNo. of TCH occupancy times(including HO)TCH call drop rate (including HO)(%)

2005-05-23Region X _B-3_DV2LAC8411-CI25458196083.12







This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

_1188738290.vsdA1

BSC

BTS:TRX

MS

ASSIGNMENT COMMAND

CHANNEL ACTIVATE

A2

CHANNEL ACTIVATE ACK

SET T3107

T3107 Timeout

_1203681143.vsd

BTS

BSC

Measure Report

MSC

Measure Result

Connection failure

Ms

Clear Request(Radio Interface Failure)

_1301899418.vsd

A cell suffers from high TCH call drop rate

Call drop of handover failure

Call drop of LAPD

Analysis of call drop types

Call drop of RF loss

If radio parameters set reasonably?

If interference exists?

An equipment hardware fault?

Any fault with BTS transmission?

Any equipment hardware fault?

Any hardware fault at BTS?

Eliminate hardware fault

Any antenna system fault?

Eliminate transmission fault

Any coverage problem?

Improve coverage

Any hardware fault at BSC?

Eliminate hardware fault

Eliminate antenna fault

Complete

Eliminate equipment fault

If radio parameters set reasonably?

If interference exists?

Check& eliminate interference

Adjust radio parameters

No

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

_1203680963.vsdA1

BSC

Old BTS:

MS

HANDOVER COMMAND

CHANNEL ACT

A2

CHANNEL ACT ACK

SET T8

New BTS

HANDOVER COMMAND

T8 Timeout

_1188738262.vsdA1

BSC

Old BTS:

MS

HANDOVER COMMAND

CHANNEL ACT

A2

CHANNEL ACT ACK

SET T3103

New BTS

HANDOVER COMMAND

T3103 Timeout

GSM P&O TCH Call Drop & Solutions

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