NSG- Automatic Congestion Control

Introduction

Automatic Congestion Control (ACC) is a mechanism by which NSG (SS7 node) can inform to its peer that it is in congested state(i,e, NSG is overloaded) and its peer should control the traffic load or vice-versa.

Two levels of congestion are distinguished, a less severe congestion threshold (congestion level 1) and a more severe congestion threshold (congestion level 2). If either of the two congestion levels is reached, an automatic congestion control information message is sent to its peer indicating the level of congestion (congestion level 1 or 2). Peer when receive an automatic congestion control information message, should reduce their traffic to the overload affected exchange.

The automatic congestion control information message is sent by NSG after receiving the clear-forward signal and before sending the release-guard signal for a circuit. If the overloaded exchange returns to normal traffic load, no more automatic congestion control information messages are sent. The peers then, after a predetermined time, automatically return to their normal status.

Configuration

Automatic Congestion Control can be configured via NSG WebGUI using below steps:

Configure Automatic Congestion Control parameters as present in Genral option of Configuration section under Gateway heading as explained in the below link:
NSG General Automatic Congestion Control Config
Once above step is done goto Route Management Section of TDM configuration (NSG-GUI -> Configuration -> TDM) and then configure below paramters :
- Average Call Rate Timer
- T29 Timer
- T30 Timer

and then carry on with your SS7 configuration. Once configuration is done goto Apply as present within Configuration section under Management heading and click on Generate Configuration/Generate and Start Gateway.

There are several parameters that needs to be configured for Automatic Congestion Control feature.

Parameters Description

Given below the parameters with their default values and description:

Parameters	Description	Allowed Values	Default Values	Expected/Tested Value
auto-congestion-control	This parameter is responsible for enabling or disabling Automation Congestion Control feature in NSG	Depends upon customer	By default is disabled i.e. 0	1
T29 timer (in ms)	These parameters are responsible for release of traffic during remote congestion. These timers are also responsible for determining congestion level at remote exchange	300-1500	300	1000
T30 timer (in sec)		5-10	5	5
traffic-reduction-rate	This parameter determines the rate with which traffic is continuously decremented if REL with ACL level 1 is received (at end/DPC which is already marked at ACL level 1) after T29 timers get expired but T30 timer still running	Depends up on cutomer (must be greater than 0 and less tha call control rate at congestion level 1)	10 %	10 %
traffic-increment-rate	This parameter determines the rate with which traffic is continuously incremented if no REL is received with ACL parameter present and T29 & T30 both timers get expired	Depends upon customer (must be greater than 0 and less tha call control rate at congestion level 1)	10 %	10 %
first-level-reduction-rate	This parameter determines the call block rate when REL with ACL level 1 is received at the remote end/DPC which is not congested	Depends upon customer (must be greater than 0 and maximum up to 90 %)	50 %	50 %
second-level-reduction-rate	This parameter determines the call block rate when REL with ACL level 2 is received at the remote end/DPC which is already has a congestion level of 1 or is not congested	Depends upon customer (must be greater than first-level-reduction-rate and less than 100 %)	90 %	90 %
average call rate timer (in sec)	This parameter determines the time depending upon which average call rate needs to be determine in case of congestion	Depends upon customer (must always be greater than T.29 timer )	5	5
max_cpu_usage	This parameter determines the maximum percentage of CPU utilization in order to consider self-congestion	Depends upon customer	90	90

NOTE: 'max_cpu_usage' parameter depends upon 'cpu_monitor' flag which needs to be configured by user in /usr/local/nsg/conf/freetdm.conf as shown below:

[general]
cpu_monitor=yes
cpu_alarm_action=warn
cpu_set_alarm_threshold=92
cpu_clear_alarm_threshold=82

All these parameters needs to be configured in freetdm.conf.xml which is located at usr/local/nsg/conf/autoload_configs/.

Below is sample file illustrating how these parameters can be configured.

<sng_gen>
<param name="procId" value="1"/>
<param name="license" value="/usr/local/nsg/conf/license.txt" />
<param name="auto-congestion-control" value="1" />
<param name="traffic-reduction-rate" value="10" />
<param name="traffic-increment-rate" value="10" />
<param name="first-level-reduction-rate" value="50" />
<param name="second-level-reduction-rate" value="90" />
<param name="max-cpu-usage" value="90" />
</sng_gen>

<mtp_route name="ROUTE1">
<param name="id" value="1"/>
<param name="dpc" value="2"/>
<param name="isSTP" value="no"/>
<param name="dpc.t29" value="10"/>
<param name="dpc.t30" value="50"/>
<param name="dpc.acc-call-rate" value="50"/>
<linksets>
<linkset name="1"/>
</linksets>
</mtp_route>

For reference here is the attached sample & file.

References

1) ITU-T Recommendation Q.764 Section 2.11, Signalling System No. 7 ISDN User Part signalling procedures.
2) UK ISUP Overload Controls document i.e. PNO-ISC/INFO/015 as described in BT Certification requirement document

NSG- Automatic Congestion Control

NSG- Automatic Congestion Control

Introduction

Configuration

Parameters Description

References

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