The IMSI Manager is
the Demux process that selects the Session Manager instance based on the Demux
algorithm logic to host a new session for 2G/3G/4G subscribers for SGSN/MME.
The IMSI Manager maintains the IMSI-SMGR mapping for SGSN (2G/3G) and MME (4G)
subscribers. The mappings maintained for all registered subscribers are
synchronous with the Session Managers.
When the incoming
attach rate is high at the IMSIMGR in a short span of time, the CPU consumption
is very high and affects the normal processing activities of the IMSI Manager.
At times this can lead to an IMSI Manager crash. Overload control methods are
devised through this feature enhancement to keep the IMSI Manager CPU under
This feature is
enabled by default.
Overload control is implemented on both SGSN and MME call flows. Attach rate
throttling(network overload protection) is implemented in IMSI Manager to cap
the rate at which new requests are accepted by SGSN and MME. This feature helps
us process the incoming new subscriber requests (for example ATTACH/ISRAU) at a
configured rate, therefore the HLR and other nodes are not overloaded. The SGSN
and MME have separate pacing queues in the IMSI Manager to monitor the incoming
rate of requests and have a separate network overload configuration as well.
For the SGSN, the
following requests are paced using the pacing queues:
(with IMSI , L-PTMSI ,F-PTMSI)
In the MME, new
connections are setup for the following events:
All types of
attach – IMSI, local GUTI, foreign GUTI, mapped GUTI, emergency and so on.
Inter-CN node TAU request requiring context transfer from old MME/SGSN
TAU request with
foreign GUTI or mapped GUTI
initiated forward relocation request via Gn/S10/S3
With this feature
enhancement when the incoming attach rate is high, the pacing queue becomes
full and the further requests are either dropped or forwarded to Session
Manager. The Session Manager in turn sends the reject response based on the
configuration. When network overload protection action is set as "reject", the
IMSI Manager has to forward overflowing requests from the pacing queue to
Session Manager through a messenger call to send back error response. The IMSI
Manager spends more time on messenger read and write. The IMSI Manager CPU
reaches high values when the incoming call rate is very high (both SGSN/MME)
though the network overload protection is configured. To ensure that the IMSI
Manager CPU is under control, the IMSI Manager reduces certain messenger
activities on reaching the default CPU threshold of 70%. This threshold value
is fixed and this feature is enabled by default. This value is currently
non-configurable. The IMSI Manager drops the overflowing requests from the
pacing queue when the CPU crosses 70% mark instead of rejecting the request.
Every IMSI Manager instance monitors its CPU usage independently and actions
are taken according to the CPU usage.
Relationships to Other Features
Attach throttling feature will have an impact due to this feature
enhancement. Once the CPU reaches the threshold of 70%, the messages will be
dropped (irrespective of configured action).
Troubleshooting IMSI Manager Overload Control
New statistics are introduced as a
part of feature which can be viewed in the Debug mode. The operator can use
these statistics to find the number of requests dropped due to overload.
These counters are
available for both MME and SGSN separately.
due to pacing queue with High Imsimgr CPU
Apart from the
statistics listed above, SGSN Network Overload protection statistics which were
only available in the
statistics are now available as a part of
statistics imsimgr all. The show output is realigned for better
readability. Debug logs are also provided to display the current CPU usage.