GSM TCH Assignment Failure & Solutions

Zte university

Training goals

This guidebook mainly describes TCH allocation failure and investigation of the problem.

Contents

TCH Assignment Failure & Investigation Relevant Counters & Assignment success Rate Typical Cases

Definition of TCH assignment failure

It refers to the number of TCH call attempts after MS occupies SDCCH;

The statistics counted is the number of “Assignment Request” sent from MSC to BSC.

number of busy hour TCH call attempts (excl. HO)

It refers to the number of calls when MS fails to occupy TCH after it seizes SDCCH.

The statistics gathered is “Assignment Failure + Clear

Request (radio interface failure)”. Allocation failure may be

caused due to lack of TCHs or problems on lower layers.

Number of busy hour TCH assignment failures (excl. HO)

MS fails to access into TCH after it sends out request for TCH

assignment to BSC.

Definition of TCH assignment failure

BSC

Assi gnment Command

Channel Acti vati on

Channel Acti vati onAck

Assi gnment Fai l ure（1）

MSC BTS Ms

Assi gnment Request

Assi gnment Fai l ure（2）

Assi gnment Command

SABM

UA

Assi gnment Compl eteAssi gnment Compl eteAssi gnment Compl ete

Assi gnment Fai l ure（3）

Remarks:Assi gnment Fai l ure（1）：BSC can’ t compl ete MSC’ s request f or TCHAssi gnment Fai l ure（2）：BTS can’ t compl ete MSC’ s request f or TCH

Assi gnment Fai l ure（3）：MS i s not abl e to occupy TCH assi gned by BSC

Main causes of TCH allocation failureTraffic congestion in cellsTraffic congestion in cells

Hardware problemHardware problem

Co-channel or adjacent channel interference Co-channel or adjacent channel interference

Antenna feeder problem Antenna feeder problem

Unreasonable setting of parametersUnreasonable setting of parameters

Transmission problem at A interface or Abis interface Transmission problem at A interface or Abis interface

Influence of repeater Influence of repeater

TCH allocation failureTCH allocation failure

Problem handling procedures

Check hardware Check hardware

Check if traffic congestion exists Check if traffic

congestion exists

Check if radio parameters areset reasonable

Check if radio parameters areset reasonable

Check indicators like BER, idle interference

band class

Check indicators like BER, idle interference

band class

Check antenna system

Check antenna system

TC

H a

ssig

nm

en

t fa

ilure

Contents

TCH Assignment Failure & Investigation Relevant Counters & Assignment success

Rate Typical Cases

Relevant counters_V2

C11609 total number of attempts for TCH (excl.HO)

C11653 total number of successful TCH assignments (excl.HO)

Definition: It represents the total number of attempts to acquire TCHs (seizure attempts) after all SDCCHs in cells are seized. It includes MOC and MTC, and very early assignment of TCH as SDCCH, but handovers are excluded. TCH falls into TCH/F and TCH/H.

Definition: It represents the number of received activation response messages after channels are activated for TCH assignment (signaling, speech and data).

Relevant counters_V2

C11654 total number of TCH

allocation failures (excl.HO)total number of unsuccessful TCH

assignments (excl.HO)

Definition: It represents the number of negative replies in response to TCH activation request for assignment or the number of activation acknowledge which haven’t been received (signaling, voice and data).

Definition: It represents the number of negative replies in response to TCH assignment command (signaling, speech and data), or the number of unreceived “Establish Indication” or “Assignment Complete”.

Relevant counters_V3 & TCH assignment success rate

KPI TCH allocation success rate(%)(exclude handover)

Definition Number of TCH assignment success times (excl. HO)/number of TCH assignment attempts (excl. HO)

Counter formula

V2(1- （ C11654+C11610+C11658 ） / C11609)*100%

V3

V6.0

(C100030017+C100030028+C100030036+C100030235 +C100030199+C100030210)*100%/(C100030010+C100030019+C100030030+C100030038+C100030042+C100030046)

V6.2

(C900060017+C900060028+C900060036+C900060235 +C900060199+C900060210)*100%/(C900060010+C900060019+C900060030+C900060038+C900060042+C900060046)

Contents

TCH Assignment Failure & Investigation Relevant Counters & Assignment success Rate Typical Cases

Low TCH allocation success rate in a cell

Indicator statistics at OMCR showed that TCH allocation success rate was low in Cell2 under BTS D1A.

Checked radio parameters, but no problem was found. From the dynamic management at OMCR, we found that one TRX in the cell couldn't be seized.

We confirmed the TRX has problem. After the TRX board with problem was replaced, TCH allocation success rate increased greatly and problem was solved.

1 2 3

Problem description

Problem analysis

Adjustment

Low TCH allocation success rate in a cell

Total number of TCH allocation requests

Total number of TCH Allocation success

TCH Requests success rate

Antenna connection problem

TCH allocation success rate was low in Cell1 in an area, which was below 94.0%.

From analysis of OMCR performance indicators, we found the problem was caused by the low TCH assignment success rate at air interface. From further analysis, we found the assignment failure rate of TRX3 was high (Maio=2), and DL signal was weak

Hardware engineers went to the site and checked, finding that antenna system was connected wrong (the TRX3 of Cell1 was connected to the antenna of Cell2.)

1 2 3

Problem description

Problem analysis

Adjustment

Antenna connection problem

Antenna connection problem

Change LAPD

TCH assignment success rate was low in three cells under a BTS (S111), just around 10%.

We made CQT on site and found that call completion rate was very low during idle hours, just one or two out of ten was completed. From hardware checking, no problem was found. Therefore, we confirmed it’s caused by LAPD timeslot problem.

Changed LAPD from 1:4 multiplex to non-multiplex. The TCH assignment success rate improved from 10% to 99%.

1 2 3

Problem description

Problem analysis

Adjustment

Interference within network

Performance statistics showed that TCH assignment failure rate was high in Cell2 and Cell3, which led to low TCH allocation success rate.

From the planning software, we could see the two cells were right opposite each other, and their overlapped coverage area was large, and co-channel existed, thus TCH assignment failure rate was high

After we changed frequency point in Cell3 from 119 to 101, number of TCH assignment failures in the two cells reduced greatly, from 20times/h to 1time/hour.

1 2 3

Problem description

Problem analysis

Adjustment

Discrepancy in time at BSC and OMCR

A customer reported that call completion rate exceeded 100%, almost reached115%.

Analysis of basic measurement data showed that Sum of CallProceeding and CallConfirm was 4955, while that measured by customer was 9428. We found that time at BSC was 5mins 30secs behind the time at OMCR.

We adjusted time at OMCR and renewed data of OMCR with PMDR, and the data in OMCR and signaling device conformed to each other

1 2 3

Problem description

Problem analysis

Adjustment

Questions for thinking

Which parameters can influence TCH allocation success rate?

Please introduce counters and signaling statistical points relating to TCH allocation success rate.