go_na19_e1_1 gsm tch congestion & solutions-29

7/28/2019 GO_NA19_E1_1 GSM TCH Congestion & Solutions-29

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GSM TCH Congestion & Solutions

ZTE university



Training goals

To know the signaling flow of TCH occupation andrelevant counters;

To know causes of TCH congestion;

To know the process of investigating TCHcongestion;

To know the common methods of handling TCH

congestion



Contents

TCH occupat ion s ignal ing & relevant counters

Causes of TCH congestion

Problem investigation process

Common methods of solving TCH congestionTypical cases



What is TCH congestion?

TCH congestion means MS applies for TCHduring the access process; MSC will send Assignment Request signaling to BSC after itconfirms MS’ application for TCH; BSC will search

for suitable TCHs; if no TCHs are available, BSCwill refuse the request, and record congestion asone time.

Congestion often brings inconvenience to

subscribers, thus it is the most complainedproblem . High TCH congestion rate has harmfuleffect on call establishment rate, handover success rate and call drop rate.



TCH congestion & relevant counter

KPIname

TCH blocking rate

Indicator

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

Counter

formula

V2（

C11612-C11699）

*100%/（

C11611-C11698）

V3

(C900060020+C900060031+C900060043+C900060

047+C900060022+C900060033+C900060045+C90

0060049)*100%/

(C900060019+C900060030+C900060042+C900060

046+

C900060021+C900060032+C900060044+C900060048)



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



Common methods of solving TCH congestion Typical cases




TCH

channel

Congestion

High dense

traffic

exceeds BTS

capacity

Low TCH

usability due to

hardware

problem

Traffic burst

Too large

coverage,

isolated island

effect

Unreasonable

setting of radio

parameters

Problem with

adjacent cells



Contents



Prob lem invest igat ion process




Process of handling TCH congestion

A cell with high TCHcongestion rate

TCH

availabilit

y 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

configuratio

n?

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







Common methods of solving TCH congestion

Trafficcontrol

OpenHR

Control

coverage

Solutionsto

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 selectionWhen MS is open or it enters coverage from blind area, itdetects 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 principleC1 = 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

CellBarQualify CellBarAccess Cell selection

priority

Cell reselection

status

0 0 Normal Normal

0 1 Barred Barred

1 0 Low Normal

1 1 Low Normal

Cell selection priorityIn 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.



Traffic control－cell reselection

whenPT≠11111

C2＝C1＋CRO－TO×H（PT－T）

whenPT＝11111

C2＝C1－CRO

of which

When x<0, H（x）＝0；

When x0, H（x）＝1CRO: cell reselection offset

TO: 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

Cell reselectionWhen C2 algorithm is open, MS reselects the cell with maxC2 to reside.

With the adoption of C2, cellselection range can be

adjusted. Flexible setting of

C2 can reduce cell realcoverage and congestion.



Traffic control－handover based on layers

PBGT HO

Traffic HO

Macro-micro 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.

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.

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 class

static RF power step

Max real transmitting power

Pn

0 Maximum output power

1 Maximum output power – 2dB








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

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.

HR function



Flow of network expansion

Gather 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)>maxnumber of

TRX allowed

Add new BTS Cell split

Open HR for

expansion

Completeexpansion plan

Yes

No

No

Rate of open HR

shall not be too high



Contents

TCH occupation signaling & relevant counters Causes of TCH congestion


Common methods of solving TCH congestion Typic al cases



Typical case 1TCH congestion rate at an overseas BTS was shown higher than usual after

it's been swapped with ZTE equipment.

Problem description

From 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 priorityin radio parameters has not been changed accordingly;

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.



Typical case 1Problem handling

Adjusted “ChanSelectPrio” (channel selection priority), changed the default “No

Select” to “half Rate First” , The problem was solved.



Typical case 2

Problem 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 analysis

Since 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 andcongestion 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 therewas 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 themax 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

date UserLabelTCH

available TCH traffic

TCH

congestio

n rate

TCH

overflow

times

TCH call

attempts

2008-4-11

21:00 - 22:00

Site77_bts1 26 24.78 23.51 612 2603

Site93_bts2 25 23.48 18.17 428 2355

2008-4-1221:00 - 22:00

Site77_bts1 26 24.27 23.39 589 2518

Site93_bts2 25 23.14 17.95 407 2267

2008-4-12

21:00 - 22:00

Site77_bts1 26 24.89 28.9 737 2550

Site93_bts2 25 23.72 20.89 507 2426

2008-4-14

21:00 - 22:00

Site77_bts1 37 29.73 0.44 13 2831

Site93_bts2 35 28.42 0.26 7 2692

2008-4-1521:00 - 22:00

Site77_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 reaches0.4Erl in cells with single TRX?

What methods can be used to achieve traffic balance?

go_na19_e1_1 gsm tch congestion & solutions-29

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