10 GO_NA19_E1_0 GSM TCH Congestion & Solutions-23

GSM TCH Congestion & Solutions

i

Contents

1 Overview ..................................................................................................................................................... 1

2 TCH occupation signaling & relevant counters ...................................................................................... 3

2.1 TCH occupation signaling ................................................................................................................. 3

2.2 Definition of TCH congestion indicator ............................................................................................ 3

3 Causes of radio network congestion ......................................................................................................... 5

4 Problem handling procedures ................................................................................................................... 7

5 Common solutions to TCH congestion ..................................................................................................... 9

5.1 Common methods for controlling traffic volume .............................................................................. 9

5.1.1 Cell selection parameters ....................................................................................................... 9

5.1.2 Cell reselection parameters .................................................................................................. 11

5.1.3 Handover based on layers .................................................................................................... 11

5.1.4 Control of cell coverage ....................................................................................................... 13

5.2 Open HR ......................................................................................................................................... 14

5.2.1 Dynamic HR switching threshold ........................................................................................ 14

5.2.2 Suggestions on HR application ............................................................................................ 15

5.2.3 Some matters to be noted in HR application ........................................................................ 16

5.3 Network expansion ......................................................................................................................... 17

5.3.1 Flow of network expansion .................................................................................................. 17

5.3.2 Principles of network expansion .......................................................................................... 18

6 Typical cases ............................................................................................................................................. 21

6.1 High TCH congestion rate at an overseas BTS after site swap ....................................................... 21

6.2 Congestion due to traffic burst ........................................................................................................ 23

1

1 Overview

Along with the development in telecommunication industry and introduction of

competitive mechanism, subscribers' demand for high network quality is increasing,

which has put the service quality of radio network at a more prominent position.

Network quality is usually reflected in the indicators like congestion rate, call drop rate

and call quality, etc.. Congestion often brings inconvenience to subscribers, thus it is

the most complained problem. Besides, network congestion rate is also one important

indicator to evaluate network operation situation. High congestion rate will affect

indicators like call drop rate, handover success rate and call establishment rate, etc..

Therefore, currently it’s of great importance to reduce system congestion and improve

network operation quality.

3

2 TCH occupation signaling & relevant counters

2.1 TCH occupation signaling

MSC will send Assignment Request signaling to BSC after it confirms MS’ application

for TCH. Channel application and assignment are shown bellow:

Fig 2-1 Flow of occupying TCH

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 with the cause of no radio resource available. Refer to Fig 2-1 for details.

2.2 Definition of TCH congestion indicator

Table 2-1 Definition of TCH congestion indicator

KPI name TCH blocking rate

Indicator

definition TCH congestion times*100%/ TCH call attempts

Counter V2 (2.97) (C11610-C11697)*100/(C11609-C11696)


4

formula V3 (6.20)

(C900060020+C900060031+C900060043+C900060047)*100%

/(C900060019+C900060030+C900060042+C900060046)

5

3 Causes of radio network congestion

1. Main causes for channel congestion are as follows:

2. High traffic density, which even exceeds the designed capacity of BTS;

3. Equipment hardware problem, like lack of usable resources or channel

congestion caused by unstable equipment performance;

4. Problems with adjacent cells;

5. Unreasonable LAC planning: if LAC boundary is set at high traffic areas or

main transportation ways, where subscribers are in great number and in frequent

movement, LAC renewal can be very frequent, which will form unreasonable

calling modes and lower system capacity as well;

6. Unreasonable setting of radio parameters: such as delay of cell reselection,

handover margin, level of outgoing handover trigger, etc., unreasonable setting

of these parameters can result in Pingpong location renewal and Pingpong

handover;

7. Burst of high traffic volume can happen in some areas (such as schools,

playgrounds) with special traffic distribution modes, which exceeds the designed

system capacity;

8. Too large coverage can cause isolated-island effect.

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4 Problem handling procedures

It’s suggested to locate the problems through checking radio parameters and equipment

hardware.

Handling procedures for TCH congestion are as follows:

1. Check if the problem cell and its adjacent cells operate normally, check TCH

usability to locate the unstable equipment. If adjacent cells work abnormally, the

problem cell will have to take part of their traffic besides its own load;

2. Check MS mobility to see if the TCH congestion is caused by excess incoming

handovers. It it’s true, we can optimize handover parameters (increase HO

Margin) to reduce number of handovers from adjacent cells to the congested cell,

so as to ease the cell from congestion;

3. Check setting of radio parameters: such as delay of cell reselection, handover

tolerance limit, level of outgoing handover trigger, etc., unreasonable setting of

these parameters can result in Pingpong location renewal and Pingpong

handover;

4. Through test of field strength, analyze if coverage is too large and if

isolated-island effect exists. When isolated-island effect happens to one cell in

an area, where predefined adjacent cells can not be detected, MS will constantly

stay with the serving cell; and normal handovers can not be triggered, in spite of

any changes on signals, and finally call drops will be resulted. To avoid this case,

two methods can be adopted: (1) adjust the antenna of the isolated cell to

eliminate the effect. However, due to the complexity in electric wave

transmission, it takes several tests to abate the effect, and it’s really difficult to

totally eliminate the effect due to high buildings. (2) define new adjacent cells

for the isolated cell. The principle for defining related parameters is:

handovers/LAC renewal from the isolated cell to normal cells has priority over

the reversed ones.

5. Congestion due to high traffic density: check if the BTS capacity configuration

reaches the max. If not, expand it with enough TRXs.

General flow for handling TCH congestion is shown in Fig 4-1:


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A cell with high TCH

congestion rate

The cell’s TCH

availability

Isolated-island

effect exists due to

too large coverage?

Reduce

coverage to

eliminate the

effect

Correct radio

parameters

Optimize

handover

parameters to

reduce

handovers

Any problem with

adjacent cell?

Caused by too

many handovers?

Check radio

parameters

Yes

Investigate

adjacent cell

problem

Investigate

hardware

problem

Low

Yes

Unreasonab

le

Yes

Caused by high traffic

density?

If the BTS reaches

its max

configuration?

Complete

Expand the BTS with

enough TRXs

Lower BTS

power, increase

down-tilt to abate

the congestion

Yes

s

Fig 4-1 Flow for handling TCH congestion

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5 Common solutions to TCH congestion

Common solutions to TCH congestion comprise:

· Adopt traffic control in the congested cell, so as to balance traffic load;

· Open HR, increase system capacity;

· Expand TRXs or split cells, so as to increase sites and increase system capacity.

5.1 Common methods for controlling traffic volume

1. control cell selection parameters;

2. control cell reselection parameters;

3. handovers based on layered cells;

4. control a cell’s real coverage.

5.1.1 Cell selection parameters

C1 is applied as standard when MS is selecting cell. It will choose the cell with largest

C1 value. According to GSM regulations:

C1=(RXLEV- RXLEV_ACCESS_MIN) - Max(MS_TXPWR_CCH-P,0)

RXLEV: level of MS receive signal;

P: the max receive power of MS;

ACCESS-MIN: the minimum receive level for MS access:

MS-TXPWR-CCH: the allowed max transmitting power for MS access into BCCH; C1

reflects the condition of MS receive level (good/bad), whose value won’t be influenced

by network deployment mode.


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Fig 5-1 Cell selection

Usually, priority of all cells should be set “Normal”, i.e. CBQ=0. In some cases, like

microcell application, dual-band network, multi-layer network, etc., operators may

favorably want MS to access into certain type of cells, we can set priority of these cells

as “Normal” and that of other cells as “Low”, or in some high traffic areas we can set

cells’ priority as “Low” to reduce their load. CBQ has no influence on selection but cell

reselection. CBQ and C2 should be used coordinately in optimization. In order to make

dual-band cell phones access into 1800M system, we can set CBQ and CBA values to

make a difference in priority of DCS1800 and GSM900 networks, so that 1800M

network will be chosen preferably (cell’s priority won’t affect cell reselection). The

relations among CBQ, CBA, cell selection priority and cell reselection condition are

shown bellow:

Table 5-1 Relations among CBQ, CBA, cell selection priority and cell reselection condition

CellBarQualify CellBarAccess Cell selection priority Cell reselection

condition

0 0 Normal Normal

0 1 Barred Barred

1 0 Low Normal

1 1 Low Normal

In order to make MS choose 1800M network, we can set 1800M cell with Normal

priority, its CBQ=0, CBA=0; set 900M cell with Low priority, its CBQ = 1, CBA = 0.

Fig 5-2 Priority in cell selection

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5.1.2 Cell reselection parameters

In accordance to GSM standards, when cell selection is to be carried out, MS will order

adjacent cells according to their C2 values and check which one fulfills the conditions

for MS residing in the cell; if conditions are fulfilled, MS will reside in the cell. Cell

reselection is based on its algorithm C2, which is shown bellow:

· C2 = C1 + CRO – TO H(PT – T), when PT 31,

· C2 = C1 – CRO， when PT=31;

CRO = CELL_RESELECT_OFFSET;

TO = TEMPORARY_OFFSET;

PT = PENALTY_TIME.

According to C2 standard, in order to reduce cell reselection in dual-band network, we

can set CRO of DCS1800 cell a large value to make C2 in DCS1800 larger than that in

GSM 900, so as to keep MS residing in DCS1800 cells. During cell reselection, if we

need some idle cells to share some traffic load with those with high traffic volume, we

can increase their CRO; conversely, when some cells suffer from high congestion rate,

we can set PT=31, reduce value of C2 in the serving cell, thus “push” away some

traffic volume and reduce TCH load. We must note that CRO can not be set over 20dB.

Example:

Suppose an area is covered by two cells simultaneously (GSM900 cell and DCS1800

cell), and the two cells’ access priority is the same, CRO of DCS1800 cell=20, CRO of

GSM900 cell=0, PT and TO of the two cells are 0, strength of MS receiving signal

from GSM900 cell is -68dBm, that from DCS1800 is -78dBm, and their minimum

access level is the same, -104dBm. Then C1（900）=-68-(-104)=36, C1（1800）

=-78-(-104)=26. MS selects GSM900 cell when it’s powered on. After a while, in cell

reselection, MS will resides in DCS1800 cell, because C2（900）=-68-(-104)+0-0=36,

C2（1800）=-78-(-104)+20-0=46.

5.1.3 Handover based on layers

From the perspective of multi-layered cells, effective traffic control and traffic balance

can also be realized through planning layers and setting relevant parameters in

dual-band network. Among the current ZTE system equipment, the layer-related and


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most commonly used handover algorithms comprise PBGT handover, traffic handover,

macro-micro handover. Traffic control in dual-band network can be reached through

these handover algorithms, which are simply described as follows:

· PBGT handover

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. For specific parameters, please refer to relevant

technical guidebooks.

· Traffic handover

Through setting parameters: layer priority-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.

Settings for relevant parameters:

Open traffic handover；

Traffic handover threshold can be set70;

Level threshold for traffic handover （TrafficLevThs）can be set 0dB;

Frequency control value （TrafficHoFreqCtl） can be set 0.

· Macro-micro handover

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.

Relevant parameters:

Set layer relations and set DCS1800 cell layer “Lower”;

Open macro-micro handover function;

Macro-micro handover threshold （MacroMicroHoThs） can be: -90～-80dBm



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Counter for Macro-micro handover threshold（MacroMicroHoN）: 2～4

5.1.4 Control of cell coverage

The main reason for some cells suffering from congestion is unreasonable planning or

non-standard installation work, which causes long coverage and large serving area to

cells and makes the cells absorb too much traffic volume, thus cell congestion is

inevitably formed.

Common methods for locating cells with congestion due to over coverage are as

follows:

· Evaluate cell coverage through DT, analyze and find out if over coverage exists;

· From TA distribution report at OMCR, get the distribution of the cell’s main

traffic TA; combining planning data, analyze and find out that over coverage

exists.

There are two main methods for controlling cell coverage and eliminating over

coverage problem.

· Adjust antenna down-tilt and antenna height;

As for antenna down-tilt, it’s 6-10°in dense urban area, 4-6°in urban area, 2-6°in

suburb, 0-4°in villages. When adjusting antenna down-tilt, we must take into

consideration factors like the distance to neighboring cells, landforms. If it’s

necessary, we can also use DT to get the down-tilt for best coverage.

· Adjust TRX static output power

Usually adjustment of TRX static output power can help achieve coverage

control, but in order not to affect indoor coverage, it’s recommended that this

method be applied only after adjustment in antenna fails to solve the problem

completely. Note that power class of all TRXs in the cell must be adjusted to be

unanimous during adjustment of TRX static output power, or UL-DL unbalance

will be resulted.

Currently, TRX static power class can be adjusted at OMCR. Its 7 classes are

listed in Table 5-2:

Table 5-2 Static power class


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static power class Actual maximum output power

static RF power step Pn

0 Maximum output power

1 Maximum output power – 2dB






5.2 Open HR

As for TCH/FS （Full rate Speech） or TCH/EFS（Enhanced Full rate Speech）, 24

frames among the 26 are used to carry speech data, 1 frame (the 13th

frame) used for

transmitting channel associated signaling SACCH (Slow Associated Control Channel),

and another 1 frame (the 26th

frame) is idle frame.

When the system adopts TCH/HS（Half rate Speech）, the multi-frame structure of air

interface won’t change. The odd frame is assigned to a MS and the even frame is

assigned to another one, the original 13th frame is the first MS’ SACCH, the original

26th frame (idle frame) is the second MS’ SACCH. In this way, the channel, which

could carry one TCH/FS or TCH/EFS channel before, can carry two TCH/HS channels

now, thus the channel capacity is doubled.

The relation between frame structures of FR channel and HR channel is shown bellow:

Fig 5-3 Relation between frame structures of FR channel and HR channel

5.2.1 Dynamic HR switching threshold

Dynamic HR exchanging threshold is that for FR mode switching to HR mode. After

the threshold is set, system will make judgment according to it, corresponding



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switching process will be triggered if conditions are fulfilled. In this way, the

percentage of HR channel in the system is in a dynamic status.

Switching threshold is used in cells. The parameter represents the percentage of TCH

in the cell, whose calculation formula is shown bellow:

According to actual network operation, default threshold recommended by ZTE is 70%.

In the process of dynamic HR application, actual traffic volume can be obtained from

performance statistics, and number of TCH needed can be obtained from ERLB. HR

switching threshold can be obtained from calculation.

5.2.2 Suggestions on HR application

Application of HR function enables fast expansion of existing network and relieves the

pressure on radio network frequency band and capacity and solves traffic congestion.

As for specific application scenarios, such as regular network expansion and dealing

with urgent burst of traffic, different strategies should be adopted. Some suggestions on

HR application are listed bellow:

· HR application in area with burst traffic

HR is most effective in dealing with burst of traffic in some areas, such as

stadiums or sports fields, campus, and big assembly or meetings, etc.. The

outstanding feature of traffic in these areas is that traffic is busy in periods and

usually comes in a burst, for example, the traffic increases during matches in

stadiums and intervals between lessons, which will have impact on network.

When traffic volume is low, TCH is in FR; when burst of traffic happens, it’s

automatically switched from FR to HR, thus congestion will be relieved and the

cost for network expansion will be saved for operators as well.

· HR application at area with dense traffic

High dense traffic often exists at dense urban areas, airports, train stations, and

squares. Along with the fast development of cities and network subscribers,

frequent network expansion will be needed at these areas.

HR can be adopted to avoid frequent network adjustment and expansion. Before

a new round of capacity expansion, we can appropriately keep dynamic or static

HR open to deal with burst of traffic volume, and combine with long-term

planning to provide operators with a more flexible expansion choice. Meanwhile,


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HR is also a network expansion scheme, when frequency resource is limited in

dense urban area and the BTS type doesn’t allow expansion. Under this

circumstance, please note that percentage of open HR shall not exceed 30%, and

try to keep the drop of speech quality within the range acceptable to subscribers.

· HR application at areas with lower-end subscribers

Considering network completion and their brand competitiveness on market,

operators are willing to provide coverage at some areas with lower-end

subscribers, like remote villages. While in most cases number of subscribers in

these areas is very thin, and the ARPU is rather low, thus operators’

input-and-output ratio is very low.

Since the lower-end subscribers' demand for speech quality is not high, only

getting through is acceptable to them, so combining with some techniques for

larger coverage, HR can be adopted to satisfy calling demand at large areas, so

as to realize low-cost coverage. Both static and dynamic HR can be adopted.

Besides, traffic burst can happen to lower-end areas too, under the circumstances

like assemblies, migration of people, etc.. HR can be adopted to solve the

problem.

5.2.3 Some matters to be noted in HR application

HR function can quickly improve network capacity. While with a view to avoid

influence on radio indicator and solve network congestion, the following matters shall

be taken into consideration:

· Interference in radio environment:

· HR has no obvious harmful effect on network indicators, but when radio

environment is bad and C/I is low, speech quality will drop more obviously than

that of FR. Therefore, we shall try to avoid using HR when environment

interference is strong;

· Subscriber’s speech quality sensitivity

· From the perspective of MOS grading, HR speech coding is inferior to enhanced

FR speech coding. Its degree of distortion is higher when handling speech with

rich frequency spectrum (eg. music). Therefore, we need to take careful

consideration when using HR at areas of high value or with high demand on



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speech quality. Generally, the use of HR shall not exceed 30%.

· Terminal(MS) ability to support

· Use of HR depends on terminal (MS) ability to support; currently, a certain

percent of terminals don’t support HR services; according to statistical result,

above 75% terminals (MS) in China support HR. It's suggested that the ratio of

FR to HR channel assignment be controlled at 6:4 in commercial systems.

Adjustment can be made in other countries and areas according to actual

situations.

5.3 Network expansion

Network expansion is based on traffic in busy hour, overflowed traffic in busy hour and

evaluation of current site distribution density to make corresponding expansion plans.

5.3.1 Flow of network expansion

Please refer toFig 5-4：


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Export each

cell’s traffic

(busy hour)

report of the

most recent

week

Calculate each

cell’s overflowed

traffic of busy

hour (traffic lost

due to

congestion)

Obtain the cell’s

actual busy hour

traffic volume

Erl (actual)

Look up in Erl B,

obtain the cell’s

theoretical busy

hour traffic Erl

(theoretical)

After certain percent of HR

is open, look up in Erl B and

obtain the cell’s theoretical

busy hour traffic, marked as

Erl (theoretical, incl. HR)

Erl(actual )>

Erl(theoretical)

No expansion

need

TRX(actual

need)>max number

of TRX allowed

make TRX

expansion

(GSM900/1800),

First make TRX

expansion

(GSM900/1800), total

number of TRX shall not

exceed the max allowed.

Complete

expansion plan

Is capacity

need

fulfilled?

Add GSM1800

BTS

Add GSM900

BTSCell split

No

Yes

Yes

No

Yes

No

Each cell’s busy hour

traffic

Total of traffic overflowed

Average call time

TRX available in the cell

Busy hour overflowed

traffic=total of traffic

overflowed* average

call time/ 3600

Actual busy hour

traffic=busy hour traffic

+ busy hour overflowed

traffic

Look up in Erl B, get

number of TRX needed

when GOS=2% and Erl

(actual) is fulfilled.

Filter out each

cell’s max traffic

volume to be the

base of

expansion

calculation.

Calculate number of

TRX needed for

expansion (actual

need)

Erl(actual)>

Erl(theoretical,

incl. HR)

Yes

Use of HR shall be within

30%.

Open HR for

expansionNo

Fig 5-4 Flow of cell expansion

5.3.2 Principles of network expansion

Principles for GSM900/1800 TRX expansion

First we need to calculate and obtain GSM900/1800 frequency resource and the max

configuration plan, which can be reached basing on frequency planning scheme.

Compare the max configuration with that required by actual traffic need to see if the

max traffic can be fulfilled.

· When the actual configuration need is under the max configuration plan, we can

consider carrying out expansion.

· When the actual configuration need is beyond the plan, we can consider carrying

out cell split or adding new sites.



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Conditions for cell split

Cell-split is aimed at macro-cells;

Macro-cells of single frequency band;

In the BTS, which the macro-cell belongs to, there is only one busy cell; the cell can be

split;

Pay attention to adjustment of antenna parameters during cell-split.

Conditions for setting up new GSM900 BTS

If the traffic need still can not be satisfied when the TRX is expanded to the max

allowed, new BTSs need to be set up;

The average distance between the BTS and those around >400m, and number of TRX

configured in the BTSs around doesn’t reach the max allowed for GSM900, in this case,

new GSM900 BTSs can be set up.

Conditions for setting up GSM1800 BTS

If the traffic need still can not be satisfied when the TRX is expanded to the max

allowed, new BTSs need to be set up;

The average distance between the BTS and those around >400m; and number of TRX

configured in the BTSs around has reached the max allowed for GSM900, new

GSM900 BTSs would make the frequency interference out of control. In this case, we

can set up new GSM1800 BTSs, and make them co-site with those of GSM900 to

absorb some traffic.

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6 Typical cases

6.1 High TCH congestion rate at an overseas BTS after site swap

Problem description:

TCH congestion rate at an overseas BTS was shown higher than usual after it's been

swapped with ZTE equipment.

Problem analysis:

From the dynamic data management, we observed that all FR TCHs have been

occupied, while a lot of HR TCHs were idle in 3 cells. After investigating TCH

configuration in the 3 cells, we found that except the BCCH TRX all the other 3 TRXs

in the cells were configured with HR TCH, while the BCCH TRX was just configured

with 3 FR TCH. Therefore, the congestion probably occurred on FR TCH.

Through signaling analysis, we found congestion just occurred on the assigned FR

TCH. Basically, it was confirmed that the assignment failure was caused by congestion

due to lack of FR TCH. From the recorded signaling, we didn’t find assignment failure

of HR 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, which led to channel assignment according to the

default, while there were only 3 FR TCH, thus TCH congestion was inevitably

resulted.

The primary cause of this problem is that certain percent to MSs do not support HR.

Problem handling:

Adjust “ChanSelectPrio” (channel selection priority), change the default “No Select”

to “half Rate First” as shown in Fig 6-1:


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Fig 6-1 Setting of ChanSelectPrio

Considering that about 10％－15％ of MSs do not support HR TCH, increase number

of FR TCH to 15% of total TCH. After parameter adjustment, congestion rate dropped

obviously.

Table 6-1 Related congestion indicators before parameter adjustment

date UserLabel

TCH

CONGESTION

KEY

TOTAL

CALLS KEY

TCH attempt

total

num(exclude

handover)

TCH

overflow total

num(exclude

handover)

2007-7-31

14:00 - 18:00

Site1_bts1 19.51 2553 3092 532

Site1_bts2 12.17 2011 2282 258

Site1_bts3 0.49 418 425 1

2007-8-1 Site1_bts1 25.39 12994 17562 4511



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date UserLabel

TCH

CONGESTION

KEY

TOTAL

CALLS KEY

TCH attempt

total

num(exclude

handover)

TCH

overflow total

num(exclude

handover)

010:00 - 24:00 Site1_bts2 14.95 10844 12880 1970

Site1_bts3 3.48 2866 3062 163

2007-8-1 10:00

- 14:00

Site1_bts1 23.9 2831 3782 937

Site1_bts2 11.89 2292 2606 307

Site1_bts3 0.63 510 523 4

Table 6-2 Related congestion indicators after parameter adjustment

date UserLabel TCH CONGESTION

KEY

TOTAL

CALLS

KEY

TCH attempt total

num(exclude

handover)

TCH overflow

total

num(exclude

handover)

2007-8-2

12:00 - 13:00

Site1_bts1 0.64 1272 1282 6

Site1_bts2 1.59 902 925 18

Site1_bts3 0 174 177 0

6.2 Congestion due to traffic burst

Problem description:

Congestion rate in two cells under a certain BTS increased suddenly during 21：00～

23：00 pm, and the rate reached 30%. Because evaluation period was during 21：00～

22：00, these two cells had great influence on BSC congestion rate.

From performance report, we could see that TCH usage rate was normal when

congestion occurred, but number of call attempts and traffic volume were obviously

increased and their increase was even doubled.

Table 6-3 Cell congestion indicator

Date UserLabel TCH

available

TCH

traffic

TCH

congestion

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 22.48 18.17 428 2355

2008-4-12 21:00 Site77_bts1 26 24.27 23.39 589 2518


24

- 22:00 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

Problem analysis:

We checked hardware warning and TCH availability rate, but no problem was found.

However, the report showed that traffic during this period increased obviously. After

observation of a week, we found that the traffic increased regularly during this period.

We doubted traffic burst happened in the area.

From testing, we found that the two cells covered a high school. After school, traffic

burst emerged apparently, it wasn't caused by reasons like abnormal calls.

Problem handling:

From planning software we found that the dormitory building area was mainly covered

by the two cells, and other cells were a bit far from the school, so it’s difficult to reach

traffic balance.

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

and the TRXs could not be expanded. After checking we found HR in the two cells was

off, so it’s suggested that HR be open for cell expansion.

Through analysis of the cell’s actual traffic, we found that the traffic undertaken by the

two cells has already reached 23～24ERL; considering the high congestion rate, we

supposed the actual traffic could be even higher. It's stipulated in Erl B that 33 TCHs

are needed to support traffic of 23～24ERL, while there were only 26 TCHs available,

and 30% of HR needed to be open to satisfy traffic need. Besides, HR needed to be

open since the congestion was caused by burst traffic.

Opened dynamic HR in the two cells, and set the HR threshold as 70%. We checked

indicators during the same period for two days thereafter, the congestion problem was

found disappeared.

Table 6-4 Congestion disappeared when HR was on

Date UserLabel TCH

available

TCH

traffic

TCH

congestion

rate

TCH

overflow

times

TCH call

attempts

2008-4-11 21:00 Site77_bts1 26 24.78 23.51 612 2603



25

Date UserLabel TCH

available

TCH

traffic

TCH

congestion

rate

TCH

overflow

times

TCH call

attempts

- 22:00 Site93_bts2 25 23.48 18.17 428 2355

2008-4-12 21:00

- 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

2008-4-15 21:00

- 22:00

Site77_bts1 40 30.12 0.53 15 2881

Site93_bts2 36 28.14 0.11 3 2655

Documents

10 GO_NA19_E1_0 GSM TCH Congestion & Solutions-23