GO NAST3009 E01 1 GSM TCH Congestion and Solutions 37

GSM TCH Congestion & Solutions

ZTE university

Training goals

To know the signaling flow of TCH occupation and relevant counters;

To know causes of TCH congestion; To know the process of investigating TCH congestion;

To know the common methods of handling TCH congestion

Contents

TCH occupation signaling & relevant countersCauses of TCH congestionProblem investigation processCommon methods of solving TCH congestionTypical cases

What is TCH congestion? TCH congestion means MS applies for TCH during the access process; MSC will send Assignment Request signaling to BSC after it confirms MS’ application for TCH; BSC will search for suitable TCHs; if no TCHs are available, BSC will refuse the request, and record congestion as one time.

Congestion often brings inconvenience to subscribers, thus it is the most complained problem . High TCH congestion rate has harmful effect on call establishment rate, handover success rate and call drop rate.

TCH congestion & relevant counter

KPI name TCH blocking rate Indicator definition

（ TCH congestion times/TCH call attempts)*100%/

Counter formula

(C900060020+C900060031+C900060043+C900060047+C900060022+C900060033+C900060045+C900060049)*100%/ (C900060019+C900060030+C900060042+C900060046+ C900060021+C900060032+C900060044+C900060048)

TCH congestion & relevant counter

Counter Counter Description

C900060019 Number of voice TCH/F seizure attempts for assignment

C900060020 Number of voice TCH/F seizure failure for assignment

C900060021 Number of voice TCH/F seizure attempts for handover

C900060022 Number of voice TCH/F seizure failure for handover

C900060030 Number of data TCH/F seizure attempts for assignment

C900060031 Number of data TCH/F seizure failure for assignment

C900060032 Number of data TCH/F seizure attempts for handover

C900060033 Number of data TCH/F seizure failure for handover

TCH congestion & relevant counter

Counter Counter Description

C900060042 Number of voice TCH/H seizure attempts for assignment

C900060043 Number of voice TCH/H seizure failure for assignment

C900060044 Number of voice TCH/H seizure attempts for handover

C900060045 Number of voice TCH/H seizure failure for handover

C900060046 Number of data TCH/H seizure attempts for assignment

C900060047 Number of data TCH/H seizure failure for assignment

C900060048 Number of data TCH/H seizure attempts for handover

C900060049 Number of data TCH/H seizure failure for handover

Signaling of TCH congestion

Upon receiving “Assignment Request” from MSC, BSC will search for suitable TCHs.

If no usable TCHs are available, BSC will send a “Assignment Failure” message to MSC, and the system records congestion as one time.

Contents

TCH occupation signaling & relevant counters

Causes of TCH congestion Problem investigation process Common methods of solving TCH congestion

Typical cases

Causes of TCH congestion

TCH channelCongestio

n

High dense traffic exceeds BTS capacity

Low TCH usability due to hardware problem

Large Traffic burst

Too large coverage, isolated island effect

Unreasonable setting of radio parameters

Problem with adjacent cells

Contents

TCH occupation signaling & relevant counters

Causes of TCH congestion Problem investigation process Common methods of solving TCH congestion Typical cases

Process of handling TCH congestion

A cell with high TCH congestion rate

TCH availability is low?

Any problem with

adjacent cell?

Caused by too many

handovers?

Check radio

parameters

Isolated-effect exists due to

too large coverage?

If the BTS

reaches its max

configuration?

Expand the BTS with enough TRXs

Investigate hardware

Investigate adjacent cells

Optimize HO parameters to

reduce HO

Reduce coverage and eliminate the

effect

Adjust parameters

Lower BTS power, increase down-tilt to abate congestion

Yes

Yes

Yes

Yes

Yes Yes

Due to high traffic density

Contents

TCH occupation signaling & relevant counters

Causes of TCH congestion Problem investigation process Common methods of solving TCH congestion

Typical cases

Common methods of solving TCH congestion

Trafficcontrol

Open HR

Controlcoverage

Solutions to

congestionexpansion

Common methods of traffic control

Control cell selection

Control cell reselection

PBGT HO ， Macro-micro HO, Traffic HO

Control coverage range

C1， CBA， CBQ

C2， CRO， CRH

TRX static Power class, Down tilt

Control Handover

Traffic control－ cell selection Cell selection

When MS is open or it enters coverage from blind area, it detects the cell’s signal and stays synchronous with the cell, then it scans BCCH in different cells and select one to reside according to cell’s priority and the principle C1>1.

C1: parameter for cell pathloss judgment principle C1 = RXLEVEL－ DL‑RXLEV_ACCESS_MIN－ MAX(MS _TXPWR_MAX -

P ) RXLEVEL: MS receive level average DL‑RXLEV_ACCESS_MIN: minimum MS access level MS_TXPWR_MAX： max BCCH power level P： max MS output level

Traffic control－ cell selection Cell selection priority

In PHASE2+ regulations, parameters CBA and CBQ to represent the cell’s priority are added. Through combined settings of the two parameters, we can get different selection priorities, and we can set lower priority for congested cells.

CellBarQualify

CellBarAccess

Cell selection priority

Cell reselection

status

0 0 Normal Normal

0 1 Barred Barred

1 0 Low Normal

1 1 Low Normal

Traffic control－ cell reselection Cell reselection

When C2 algorithm is open, MS reselects the cell with max C2 to reside.

whenPT≠11111C2＝ C1＋ CRO－ TO×H（ PT－ T）whenPT＝ 11111C2＝ C1－ CRO

of whichWhen x<0, H（ x）＝ 0；When x0, H（ x）＝ 1

CRO: cell reselection offsetTO: temporary offset (temporary correct value of C2. “temporary” means the value works on C2 just during a period, which is decided by PT.)PT: penalty time

With the adoption of C2, cell selection range can be adjusted. Flexible

setting of C2 can reduce cell real coverage and

congestion.

Traffic control－ handover based on layers PBGT HO

Through setting PBGTHoLayer and NCellLayer, we can control whether the handover can be carried out among undefined layer, same layer different frequency band, upper layer, and lower layer, thus we can reach flexible control over traffic distribution.

Traffic HO Through setting parameters: layer relation-TrafficHoLayrCtl

(same layer, upper layer, lower layer), frequency band TrafficHoFreqCtl and NCellLayer, we can contol the layer and frequency band for target cell of traffic handover, and traffic distribution can be controlled flexibly as well.

Macro-micro HO Macro-micro handover is to handover the MS moving with slow

speed from macro cell layer to micro cell layer. The micro cell mentioned here is just a concept in logic. In this example, DCS1800 cell can be regarded as micro cell, and the macro-micro handover can only be carried out to adjacent cells on lower layer. Use （ MacroMicroHoThs ） to control the difficulty of handover to lower layers.

For specific parameters, please refer to relevant

technical guidebooks.

Traffic control－ coverage control Control antenna down-tilt

Static power classStatic power

classstatic RF power

step

Max real transmitting powerPn

0 Maximum output power1 Maximum output power – 2dB2 Maximum output power – 4dB3 Maximum output power – 6dB4 Maximum output power – 8dB5 Maximum output power – 10dB6 Maximum output power – 12dB

Fast expansion of TRX is not necessary;Increase capacity at the cost of speech quality;Flexible setting of cell layers and configuration;Some terminals (MS) can not support HR.

Note radio interference at areas with HR open;Note subscribers’ sensitivity of speech quality;Note setting of HR threshold;Note rate of terminals supporting HR.

Half Rate Features of HR

Application of HR HR application in area with burst traffic HR application at area with dense traffic HR application at areas with lower-end subscribers

Half Rate

FR

HR

1 2 3 12 13 14 24 25 26

TCH/F TCH/F TCH/F SACCH

One speech channel TCH includes 26-frame multi-frame, time is 120ms.

idle

1 2 3 12 13 14 24 25 26

TCH/H1 TCH/H2 TCH/H1 SACCH1 SACCH2TCH/H2 TCH/H2 TCH/H2 TCH/H1

Half Rate Activating HR solution achieves fast expansion of wireless

network, relieves network intense capacity, effectively solves partial traffics congestion, and can be supplement for urgent expansion solution.

Proportion of the MS supporting HR codec Using HR will lower the speech quality during conversation in

worse radio C/I areas HR solution suits for low-value areas with outburst heavy

traffics

The ZTE BSC avoids the occurrence of HR

fragmentations to reserve continuous and complete

timeslot for FR allocation and data services. In addition, the BSC can

gather the fragmental HR channels by internal cell handover, which is re-packaging mechanism for ongoing half rate calls.

In poor radio environments, the speech quality of the HR

decreases.The ZTE BSS monitors the strength

of radio signals and speech quality to dynamically switch between the HR and FR for a subscriber. In this way, the

network capacity increases with the satisfying service quality.

Two HR subscribers share 16 kbps

transmission of Abis interface; therefore, the transmission in

Abis interface is not affected.

HR solution increases the capacity and spectrum

efficiency.

Half Rate

Full name Preferred speech version (half)

Description

When implementing the CS channel allocation policy,describe the preferred half-rate speech version. Halfrate version 1 refers to HR, and version 3 for AMR

Value RangeNot specify the preferred version, Half-rate version 1, Half-rate version 3

Half Rate

Full name Low priority of channel select priority

DescriptionWhen implementing the CS channel allocation policy,BSC describes the channel select strategy while TCH allocation

Value RangeNo change, Full rate first, Half rate first, Only full rate allowed, Only half rate allowed

Half Rate

Full name High priority of channel select priority

DescriptionWhen implementing the CS channel allocation policy, BSC describes the channel select strategy while TCH allocation

Value RangeNo change, Full rate first, Half rate first, Only full rate allowed, Only half rate allowed

Half Rate

Full name Dynamic ts

DescriptionYES: Representing dynamic timeslot; NO: Representing fixedly configured timeslot

Value Range Yes/No

Flow of network expansionGather each cell’s traffic report of a

week

Traffic (actual)>traffic

(theoretical)

No expansion need

Filter out each cell’s max traffic volume to

be the base of expansion calculation.

Look up in Erl B, obtain the cell’s

theoretical busy hour traffic Erl

(theoretical)

Open certain percent of HR, calculate

theoretical traffic to be supported

Traffic (actual)>traffic HR

(theoretical)

Calculate number of TRX needed for

expansion (actual need)

TRX(actual need)>max number of

TRX allowed

Add new BTS Cell split

Open HR for expansion

Complete expansion plan

Yes

No

No

Rate of open HR shall not be too high

Contents

TCH occupation signaling & relevant counters

Causes of TCH congestion Problem investigation process Common methods of solving TCH congestion Typical cases

Typical case 1TCH congestion rate at an overseas BTS was shown higher than usual after it's been swapped with ZTE equipment.

Problem descriptionFrom the dynamic data management, we observed that all FR TCHs have been occupied, while a lot of HR TCHs were idle.

Problem analysis Through signaling analysis, we found congestion just

occurred on assigning FR TCH. Basically, it was confirmed that the assignment failure was caused by congestion due to lack of FR TCH ;

After checking the channel assignment parameters of MSC, BSC and cells, we found the system takes the first speech version assigned by MSC as default; after most TRXs were configured with HR TCH, the channel assignment priority in radio parameters has not been changed accordingly;

Typical case 1Problem analysis Since most TRXs in these cells were configured with static

HR TCHs, there were just a few FR TCHs, which led to congestion due to lack of TCH/F;

The primary cause of congestion is there is still a certain number of MSs do not support HR.

Problem handlingAdjusted “ChanSelectPrio” (channel selection priority), changed the default “No Select” to “half Rate First” , The problem was solved.

Typical case 2Problem description

During optimization of a local network, the busy hour congestion rate (incl. HO) of two cells was high (10%), while that (excl. HO) was normal. And “number of TCH assignment failure” was high, which for the two cells were 89 and 61.

Traffic volume was lower than that before optimization.

Interference band was normal. Congestion rate was normal before optimization.

Problem analysisSince the congestion rate deteriorated after optimization, and only the rate with HO included went bad, so we could exclude reasons like antenna interference, hardware problem, etc., but focus on HO.

Recorded the two cells’ handover statistical performance of 15mins, we found almost all handovers from a certain cell （ CGI=**1768） to these two cells failed.

Frequent congestion due to improper parameter settings

Problem handling Checked the two cells’ HO statistics, we found

they were co-channel and co-BSIC, and they were adjacent cells to a certain cell at the same time, so almost all handovers from the certain cell to each of these two cells failed inevitably, and the failure just occurred during TCH assignment.

After the two cells’ BCCH and BSIC were adjusted, HO and congestion returned to normal.

Typical case 2

Typical case 3

Problem description Congestion rate in two cells under a certain BTS

increased suddenly during 21： 00～ 23： 00 pm, and the rate even reached 30%, while there was no congestion during other periods.

Number of call attempts and traffic volume were obviously increased when congestion occurred.

Other radio indicators were in normal status.

Serious congestion due to traffic burst

Problem analysis Checked if high dense traffic existed within coverage

of the two cells, it’s found that there was a high school dormitory building, and traffic burst occurred after school.

We checked the two cells' configuration, which has already reached the max allowed.

After checking we found HR in the two cells was off, so it’s suggested that HR be open for cell expansion.

Problem handling Obtained the two cells’ busy hour traffic of 7 consecutive days

from performance report. Found out the number of TCHs supporting traffic from Erl B. From calculation, we found that HR of 40% TCHs had to be open

to satisfy the actual traffic need. Opened the two cells’ dynamic HR, and set the threshold as 60%. When HR was open, the congestion disappeared.

Typical case 3

UserLabel TCH available TCH traffic

TCH congestion

rate

TCH overflow times

TCH call attempts

Site77_bts1 26 24.78 23.51 612 2603Site93_bts2 25 23.48 18.17 428 2355Site77_bts1 26 24.27 23.39 589 2518Site93_bts2 25 23.14 17.95 407 2267Site77_bts1 26 24.89 28.9 737 2550Site93_bts2 25 23.72 20.89 507 2426Site77_bts1 37 29.73 0.44 13 2831Site93_bts2 35 28.42 0.26 7 2692Site77_bts1 40 30.12 0.53 15 2881Site93_bts2 36 28.14 0.11 3 2655

Questions for thinking Why congestion occurs when traffic per TCH channel reaches 0.4Erl in cells with single TRX?

What methods can be used to achieve traffic balance?