The load on S6d/S6a interfaces towards an HSS is reduced effectively by utilizing the resources in a co-located SGSN-MME node scenario. Requests for subscription data in Update Location Request (ULR) are skipped by setting the 'skip-subscriber-data' bit in the ULR flags this, in turn, reduces the load on the HSS. The Skip Subscriber Data AVP is used and the subscriber data is shared across the SGSN and the MME services.

As per 3GPP TS 29.272, setting the 'skip-subscriber-data' bit in the ULR indicates that the HSS may skip sending subscription data in Update Location Answer (ULA) to reduce signaling. If the subscription data has changed in the HSS after the last successful update of the MME/SGSN, the HSS ignores this bit and sends the updated subscription data. If the HSS skips sending the subscription data, then the GPRS-Subscription-Data-Indicator flag can be ignored.

Important

The SGSN supported the Skip-Subscription-Data bit prior to Release 18.0. Support for this functionality was added to the MME in Release 18.0.

Ensuring that packets are routed internally reduces network latency for S3/Gn interface messages. This is achieved by configuring the SGTP and EGTP services in the same context for the SGSN and the MME configurations.

For outbound Inter-RAT SRNS Relocations, the MME gives preference to the co-located SGSN, irrespective of the order/priority or preference/weight configured for the SGSN entry in DNS Server. When Inter-RAT handovers take place between the co-located MME and the SGSN, the new call arrives at the same Session Manager that hosted the call in the previous RAT. If the subscription data is available for a given UE at the co-located SGSN, then the MME does not need to request this data from the HSS and provides UE subscription data obtained from the SGSN. This optional function can be turned on or off through the MME Service configuration.

Combo Optimization is available for subscribers with an EPC-enabled UE and an EPC subscription configured at the HSS. During handoff from 4G to 3G or 4G to 2G, the EPC subscription will be copied from the MME. Combo Optimization is also applicable for Non-EPC subscribers if core-network-interface is selected as S4 for the EPS-subscription.

The above diagram displays the interworking of various modules when the Combo Optimization feature is enabled in a co-located SGSN-MME setup.

When the subscriber does RAU from MME to SGSN, or vice versa, a DNS query is initiated to fetch the address of the peer node. Based on the IP address obtained, the peer MME or SGSN is selected. When a DNS response is received with a list of peer SGSN addresses, the MME matches the configured EGTP/SGTP SGSN service address in the system and uses it for the S3/Gn UE Context Transfer procedures. If a DNS response is not received and a locally configured EGTP/SGTP SGSN service is present as a peer-SGSN, the peer-SGSN will be selected. Context transfer and copying of subscription information happens internally between the SGSN and the MME nodes. The SGSN maintains the s6d interface towards the HSS and the MME maintains the S6a interface towards the HSS. All network-initiated messages are sent separately towards the SGSN and the MME nodes respectively.

This section includes various diagrams that illustrate the session manager (SessMgr) selection logic during RAU, SRNS, and Attach procedures:

Listed below is the SessMgr instance selection logic during a RAU procedure from the MME to SGSN:

Listed below is the SessMgr instance selection logic during an SRNS procedure:

Listed below is the SessMgr instance selection logic during an Attach procedure:

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.

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.

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.)

Execute the following command to verify the configuration of this feature.

This section provides information regarding show commands and/or their outputs in support of the Combo Optimization feature on the SGSN-MME Combo Node:

This section provides information regarding show commands and/or their outputs in support of the Combo Optimization feature on the SGSN:

This section provides information regarding show commands and/or their outputs in support of the Combo Optimization feature on the MME:

The following bulk statistics in the MME schema facilitate tracking MME optimization functionality for the SGSN-MME nodes when co-located in the same chassis with the Combo Optimization functionality enabled: