Cisco Cisco ASR 5000
SGSN-MME Combo Optimization
How It Works ▀
SGSN Administration Guide, StarOS Release 18 ▄
3. The IMSIMgr uses the IMSI received in the request message to identify the SessMgr instance and forwards the
request to the appropriate SessMgr instance.
Figure 92. Selection of SessMgr Instance during Attach
Listed below is the SessMgr instance selection logic during an Attach procedure:
1. During Attach procedure, the LinkMgr/GbMgr forwards the request to the IMSIMgr.
2. The IMSIMgr first verifies if the IMSI is present in the SGSN’s IMSI table. If it is not present, the MME’s IMSI
2. The IMSIMgr first verifies if the IMSI is present in the SGSN’s IMSI table. If it is not present, the MME’s IMSI
table is verified. Once the entry is found the request is forwarded to the appropriate SessMgr.
3. If the entry is not found in either table, then an alternate SessMgr instance is used to process the call.
Limitations
Subscription information is shared between MME and SGSN only when both are connected to an HSS. Combo
Optimization is not be applicable if either the MME or the SGSN is connected to an HLR. Though the subscription
information is shared between the SGSN and MME services, a separate HSS service and diameter endpoint will be
maintained for both the SGSN and the MME. All network-initiated messages are received separately for both the MME
and the SGSN. Subscription data is copied based on time-stamp validation.
Optimization is not be applicable if either the MME or the SGSN is connected to an HLR. Though the subscription
information is shared between the SGSN and MME services, a separate HSS service and diameter endpoint will be
maintained for both the SGSN and the MME. All network-initiated messages are received separately for both the MME
and the SGSN. Subscription data is copied based on time-stamp validation.
A small impact on the performance is observed during Inter-RAT handoffs as subscription data is exchanged between
the SGSN and the MME. This impact is a limited increase in the number of instructions per handoff per UE depending
on the number of APNs configured for the UE in the HSS.
the SGSN and the MME. This impact is a limited increase in the number of instructions per handoff per UE depending
on the number of APNs configured for the UE in the HSS.
It is necessary that the HSS honors the request from the MME/SGSN and not send subscription data when 'Skip-
Subscriber-Data' flag is set in the ULR. However, there are some known and valid cases where the HSS ignores this
flag; for example, if the UE's subscription data changed since the last time the UE attached in 4G. (Typically, UE
subscription data does not change frequently, therefore, HSS overrides are less frequent.)
Subscriber-Data' flag is set in the ULR. However, there are some known and valid cases where the HSS ignores this
flag; for example, if the UE's subscription data changed since the last time the UE attached in 4G. (Typically, UE
subscription data does not change frequently, therefore, HSS overrides are less frequent.)