ExMoto_BSS_Overload_control_ver1.pdf

7/30/2019 ExMoto_BSS_Overload_control_ver1.pdf

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BSS Overload Control

Abstract

The purpose of this document is to provide guidelines to the customer while dealing with overload

conditions in the BSS and steps to be taken to minimize the service impact.

Revision: 2.0

Date: 25th

October 2012

Issued by: INDIA CARE


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TABLE OF CONTENTS

1 BACKGROUND.............................................................................................3

2 PROBLEM DESCRIPTION............................................................................3

3 SYMPTOMS ..................................................................................................3

4 PROBLEM RECOVERY ................................................................................4

5 RECOMMENDATIONS .................................................................................5


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1 BACKGROUND

High traffic generated due to events like cricket matches, natural calamities and New

Years Eve can cause overload condition in the BSC. Under extreme conditions, the call

processing is severely impacted and the BSC can experience service failures.

The document helps to detect overload conditions and suggests actions to be taken to

bring the BSC and sites back into service.

The document also provides recommendations/guidelines to be implemented in advance,

when a high spike in traffic is anticipated in the network.

2 PROBLEM DESCRIPTION:

Analysis of the field data shows the most of the overload conditions are due to high paging

rates that occur suddenly. GSM paging requires paging all cells under a location area.Thus, if a page arrives from MSC at a single MTL-LCF, this card will duplicate this page

and send it to all RSL-LCFs. A RSL-LCF upon receiving this page message, will

duplicate it and send it to all RSLs assigned to that LCF. This way, a single page message

is send to all BTSs.

RSLs are engineered to operate at 25% utilization under normal operating conditions. If a

traffic surge occurs, the offered load can easily drive the link utilization far beyond the

engineered operating point. This exhausts the HDLC transmission buffers and fills up themailboxes and as a result pages are dropped as paging queue is full.

At this stage with the MSC repeating pages due to no response from the mobile, a sizeable

number of pages would be by IMSI which uses up twice as much paging capacity as a

TMSI page. The problem is compounded by repeat paging by IMSI and repeated call

attempts by the calling party and lack of AGCH capacity to handle the increased traffic.

3 SYMPTOMS

The paging overload condition builds up with in very short a period so its important that

problem is detected on the onset itself.

Following are the typical symptoms seen in BSCs and sites belonging to the same LAC

under extreme overload conditions Alarm 224, safe test audit failure

DRI OOS


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NIU OOS

RSL OOSMTL transitions

LCF resets

The surge in paging load will cause non paging messages like heartbeat msg, audit msg,

db cross download msg, call setup msg , handover msg etc to be dropped. When the

messages are discarded, reset/OOS of devices occur and call processing is affected.

4 PROBLEM RECOVERIES

A number of actions must be taken to reduce the incoming signaling to the BSC

depending on the severity of the problem.

Lock all MTLs in the MSC.

Lock all the BTS at the BSC.

Unlock all MTLs in the MSC and check if MTLs transition to B-U at BSC.

Unlock the BTS site one by one every 3-5 minute while checking the MTL

status. Disable global paging, if enabled at the MSC.

Disable Repeat Paging at MSC.

Reduce paging load by disabling SMS delivery reports.

In the worst case it may be required to bar incoming calls and SMS in the LAC

which is overloaded.

These actions will quickly bring the paging load to a manageable level.

In GSR 8 Software, RSL congestion Control feature is introduced to control pagingoverload. The feature will help to detect onset of congestion and reduce paging traffic till

the congestion abates.

RSL congestion control feature is enabled by default in the software.

For LCF congestion control there are two configurable parameters rsl_lcf_congestion_thi

and rsl_lcf_congestion_tlow. It is recommended not to change the default values of these

two parameters which are 60% and 25% respectively.


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5 RECOMMENDATIONS

Following are the actions to be implemented in the network when huge surge in traffic is

expected. These are only guidelines and its recommended to fine tune the parameter

settings based on the actual call traffic in the network.

Its recommended to reassess the LAC dimensioning of the whole network and make

any changes, if required based on the current call model.

Add RSLs in sites were RSL is congested.

Check MTL utilization and add MTLs if required.

Equip OMF processor in advance to reduce the load on BSP.

Increase the number of SDCCH in cells, wherever possible.

Make sure congestion relief is enabled.

Enable dynamic reconfiguration of SDCCH where call setup blocking is more than

0%. Enable extended paging to smooth out the small peaks in paging load.

Enable fast call setup using tch_usage_threshold parameter.

Channel requests for call setup will be assigned directly to TCH minimizingSDCCH usage. For SBCCH cells, the threshold is calculated considering the

TCH resources in outer zone only.

Note: The SMS_INIT_ON_TCH_FCS stat needs to be enabled when FCS is used

for consistency in the CSSR formula.

Reduce maximum retransmission value max_retrans to 2 retransmissions or one

retransmission.

Channel requests will only be repeated upto M+1 times where M is equal tomax_retran, broadcast on BCCH system information.

Valid range of max_retrans is 0 to 3

0 Maximum [1] retransmission

1 Maximum [2] retransmissions




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Change the parameter tx_integer to a higher value for e.g. 12

After the first channel request has been sent without reply, the next one is sent after

a wait period, made up of a random number of RACH slots. This wait period is in

the set (S, S+1 . . . S+T 1), where T is a value dependent on tx_integer (broadcast

on BCCH system information) and S is a preset number dependent on the value of T

and multiframe type.

Reassign high traffic carrying sites and distribute load equally among LCFs

Avoid equipping RSLs and MTLs on same LCF GPROC

Turn on RACH flow control and TCH flow control for cells were heavy congestion

is expected. This may be turned on only if the subscribers are classified to different

access classes.

The recommended parameter settings are as below.

flow_control_t1 10000

flow_control_t2 15000

tch_flow_control 1

tch_busy_critical_threshold 100

tch_busy_norm_threshold 90

ssm_normal_overload_threshold 60

ssm_critical_overload_threshold 70

ccch_load_period 12RACH_load_period 4

RACH_load_type 0

RACH_load_threshold

see

below

The rach load threshold is dependent on SDs in the cell and as rule can

be calculated as.


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SDs rach_load_threshold

8 10016 200

24 300

32 400

Switch off device audit tests for DRIs and NIU for sites with heavy congestion.

The Safe test audit is a low priority task and if a radio is under heavy processor

utilization, then DRI safe test audit failures occur.

For example, we want to turn off the relevant device audits for site 2:

device_audit 2 off dri 0 0 0

device_audit 2 off msi 0 0 0

device_audit 0 off rsl 2 0 0 (Note this command can

only be executed at the BSC for this device: RSL)

Turn on MSC Overload flow control by setting bss_msc_overload_allowed

parameter to value of 1.

This mechanism allows the MSC to notify the BSS that it is becoming overloaded

and to reduce the amount of information being sent to the MSC from the BSS.

When this parameter is enabled, the BSS bars one access class at a time if it receivesan OVERLOAD message.

Increase repeat paging timer at MSC to a higher value. This is to allow the original

page to be transmitted and to reduce the adverse impact of IMSI paging during

onset of paging overload. It would be better if repeat paging/global paging could be

turned off at the MSC.

In addition to the above actions, further precautions may be taken at the MSC in

consultation with the MSC vendor.

END OF DOCUMENT

Documents

ExMoto_BSS_Overload_control_ver1.pdf