Cisco Cisco ASR 5000 Technical Manual
The UE is allowed to initiate the PDN disconnection procedure, but it does not delete the related SM
back−off timer.
back−off timer.
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The back−off timer does not stop the UE from data transmission or the initiation of the service
requests for activation of the user plane bearer towards the congested APN.
requests for activation of the user plane bearer towards the congested APN.
•
APN−Based MM Congestion Control
Similar to the SM procedures, the MME also has a MM back−off timer and can reject the attach procedure.
The MME should keep the subscriber data for some time after it rejects the attach procedure so that rejection
for subsequent requests for the same subscriber can be completed without interaction with the HSS.
The MME should keep the subscriber data for some time after it rejects the attach procedure so that rejection
for subsequent requests for the same subscriber can be completed without interaction with the HSS.
While the back−off timer runs, the UE should not initiate any NAS request for the MM procedure, except for
high priority access or emergency services. However, the UE is allowed to perform Tracking Area Updates
(TAUs) if it is already in connected mode.
high priority access or emergency services. However, the UE is allowed to perform Tracking Area Updates
(TAUs) if it is already in connected mode.
The MME should select a back−off timer in such a way that all of the UEs should not have the same value of
this timer, and the UEs should initiate deferred requests simultaneously. When the mobility back−off timer is
received, the UE behavior is not APN−specific.
this timer, and the UEs should initiate deferred requests simultaneously. When the mobility back−off timer is
received, the UE behavior is not APN−specific.
General NAS−Level Congestion Control
The general NAS−level congestion control is helpful in general overload conditions. It works similar to the
APN−based congestion control and has a similar concept for the back−off timer. When the back−off timer
runs, the UE can initiate detach requests, high priority requests, and TAUs (while in connected mode).
APN−based congestion control and has a similar concept for the back−off timer. When the back−off timer
runs, the UE can initiate detach requests, high priority requests, and TAUs (while in connected mode).
The back−off timer continues to run even after the UE is detached from the network. The MME should stop
the back−off timer if the MME wants to page the UE for which the back−off timer already runs, and the UE
should stop the back−off timer after it receives the paging request from the MME and initiate the service
request.
the back−off timer if the MME wants to page the UE for which the back−off timer already runs, and the UE
should stop the back−off timer after it receives the paging request from the MME and initiate the service
request.
The MM back−off timer does not affect the Cell/RAT and PLMN change. The TA change does not stop this
timer. This timer is stopped when a new PLMN that is not equivalent to the PLMN is selected.
timer. This timer is stopped when a new PLMN that is not equivalent to the PLMN is selected.
When the UE receives a handover command, it should proceed with the handover regardless of the back−off
timer status.
timer status.
If the MME rejects the TAU request or the service request with a MM back−off timer, which is larger than the
sum of the UE periodic TAU timer plus the Implicit Detach timer, the MME should adjust the mobile
reachable timer and/or Implicit Detach timer such that the MME does not implicitly detach the UE while the
MM back−off timer runs.
sum of the UE periodic TAU timer plus the Implicit Detach timer, the MME should adjust the mobile
reachable timer and/or Implicit Detach timer such that the MME does not implicitly detach the UE while the
MM back−off timer runs.
Note: The SGSN congestion control also works in similar manner as that of MME. Refer to 3GPP TS 23.060
for more details about the SGSN congestion control mechanism, and 3GPP TS 23.401 for more details about
the MME congestion control mechanism.
for more details about the SGSN congestion control mechanism, and 3GPP TS 23.401 for more details about
the MME congestion control mechanism.
Overload Reduction by MME on S1−MME Interface
The MME can send an Overload Start message to the E−NodeB (eNB) in order to reduce the signaling load.
This procedure uses non−UE associated signaling. The overload Action Information Element (IE) has an
Overload Response IE within the Overload Start message, which contains information about rejection criteria,
and the eNB takes action appropriately.
This procedure uses non−UE associated signaling. The overload Action Information Element (IE) has an
Overload Response IE within the Overload Start message, which contains information about rejection criteria,
and the eNB takes action appropriately.
Tip: For more information, refer to the 3GPP Technical Specifications (TS) 36.413.