4.3.25 Dedicated Core Networks (DCNs)

23.4013GPPGeneral Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) accessRelease 18TS

4.3.25.1 General

This feature enables an operator to deploy multiple DCNs within a PLMN with each DCN consisting of one or multiple CN nodes. Each DCN may be dedicated to serve specific type(s) of subscriber. This is an optional feature and enables DCNs to be deployed for one or multiple RATs (e.g. GERAN, UTRAN, E-UTRAN, WB-E-UTRAN and NB-IoT). There can be several motivations for deploying DCNs, e.g. to provide DCNs with specific characteristics/functions or scaling, to isolate specific UEs or subscribers (e.g. M2M subscribers, subscribers belonging to a specific enterprise or separate administrative domain, etc.).

A DCN comprises of one or more MME/SGSN and it may comprise of one or more SGW/PDN GW/PCRF. This feature enables subscribers to be allocated to and served by a DCN based on subscription information ("UE Usage Type"). The feature in this clause handles both DCN selections without any specific UE functionality, i.e. it works also with UEs of earlier releases and UE assisted DCN selection.

The main specific functions are for routing and maintaining UEs in their respective DCN. The following deployment scenarios are supported for DCN:

– DCNs may be deployed to support one RAT only, (e.g. only dedicated MMEs are deployed to support E-UTRAN and dedicated SGSNs are not deployed), to support multiple RATs, or to support all RATs.

– Networks deploying DCNs may have a default DCN, which is managing UEs for which a DCN is not available or if sufficient information is not available to assign a UE to a DCN. One or multiple DCNs may be deployed together with a default DCN that all share the same RAN.

– The architecture supports scenarios where the DCN is only deployed in a part of the PLMN e.g. only for one RAT or only in a part of the PLMN area. Such heterogeneous or partial deployment of DCNs may, depending on operator deployment and configuration, result in service with different characteristics or functionality, depending on whether the UE is inside or outside the service area or RAT that supports the DCN.

NOTE 1: Heterogeneous or partial deployment of DCNs may result in increased occurrence of UEs first being served by a CN node in the default DCN and then being redirected to a CN node in the DCN that serves the UE when the UE moves from areas outside of DCN coverage to an area of DCN coverage. It may also result in an increased re-attach rate in the network. As this has impacts on the required capacity of the default CN nodes deployed at edge of DCN coverage, it is not recommended to deploy DCNs heterogeneously or partially.

– Even if the DCN is not deployed to serve a particular RAT or service area of PLMN, the UE in that RAT or service area may still be served by a PDN GW from the DCN.

High level overview for supporting DCNs is provided below. Details are captured in appropriate clauses of this specification, TS 23.060 [7] and TS 23.236 [30].

– An optional subscription information parameter ("UE Usage Type") is used in the selection of a DCN. An operator configures which of his DCN(s) serves which UE Usage Type(s). The HSS provides the "UE Usage Type" value in the subscription information of the UE to the MME/SGSN. Both standardized and operator specific values for UE Usage Type are possible.

– The serving network selects the DCN based on the operator configured (UE Usage Type to DCN) mapping, other locally configured operator’s policies and the UE related context information available at the serving network, e.g. information about roaming. UEs with different UE Usage Type values may be served by the same DCN. Moreover, UEs that share the same UE Usage Type value may be served by different DCNs.

– If the configuration shows no DCN for the specific "UE Usage Type" value in the subscription information, then the serving MME/SGSN serves the UE by the default DCN or selects a DCN using serving operator specific policies.

– Some subscribers may be configured without "UE Usage Type" value. In this case, the MME/SGSN may select the DCN that serves the UE using locally configured operator’s policies and the UE related context information available at the serving network (other than UE provided DCN-ID). The MME/SGSN performs procedures described in clauses 5.19.1 and 5.19.2.

– The "UE Usage Type" is associated with the UE (describing its usage characteristic), i.e. there is only one UE Usage Type" per UE subscription.

– For each DCN, one or more CN nodes may be configured as part of a pool.

– For MME, the MMEGI(s) identifies a DCN within the PLMN. For SGSNs, a group identifier(s) identifies a DCN within the PLMN. That is, the group of SGSNs that belong to a DCN within a PLMN. This identifier may have the same format as NRI (e.g. an NRI value that does not identify a specific SGSN node in the serving area) in which case it is called "Null-NRI" or it may have a format independent of NRI, in which case it is called "SGSN Group ID". The "Null-NRI" or "SGSN Group ID" is provided by an SGSN to RAN which triggers the NNSF procedure to select an SGSN from the group of SGSNs corresponding to the Null-NRI/SGSN Group ID (see clause 5.19.1).

NOTE 2: SGSN Group IDs enable to handle deployment scenarios where in a service area all NRI values are allocated to SGSNs and hence no NRI value remains that can be used as Null-NRI.

– The dedicated MME/SGSN that serves the UE selects a dedicated S-GW and P-GW based on UE Usage Type.

– At initial access to the network if sufficient information is not available for RAN to select a specific DCN, the RAN may selects a CN node from the default DCN. A redirection to another DCN may then be required.

– To redirect a UE from one DCN to a different DCN, the redirection procedure via RAN, described in clause 5.19.1, is used to forward the NAS message of the UE to the target DCN.

– All selection functions are aware of DCN(s), including the network node selection function (NNSF) of RAN nodes, for selecting and maintaining the appropriate DCN for the UEs.

4.3.25.1a UE assisted Dedicated Core Network selection

This feature is to reduce the need for DECOR reroute by using an indication (DCN-ID) sent from the UE and used by RAN to select the correct DCN. The DCN-ID shall be assigned to the UE by the serving PLMN and is stored in the UE per PLMN ID. Both standardized and operator specific values for DCN-ID are possible. The UE shall use the PLMN specific DCN-ID whenever a PLMN specific DCN-ID is stored for the target PLMN.

The HPLMN may provision the UE with a single default standardized DCN-ID which shall be used by the UE only if the UE has no PLMN specific DCN-ID of the target PLMN. When a UE configuration is changed with a new default standardized DCN-ID, the UE shall delete all stored PLMN specific DCN-IDs.

The UE provides the DCN-ID to RAN at registration to a new location in the network, i.e. in Attach, TAU and RAU. RAN selects serving node (MME or SGSN) based on the DCN-ID provided by the UE and configuration in RAN. For E-UTRAN the eNodeB is configured with DCNs supported by the connected MMEs at the setup of the S1 connection. For UTRAN and GERAN the BSS/RNC is configured with the DCNs supported in the connected SGSN via O&M. Both standardized DCN-IDs and PLMN specific DCN-IDs can in the RAN configuration be assigned to the same network. If information provided by the UE (e.g. GUTI, NRI, etc.) indicates a node (MME or SGSN) for attach/TAU/RAU and a serving node (MME or SGSN) corresponding to the UE information can be found by the RAN node, the normal node selection shall take precedence over the selection based on DCN-ID. At registration the MME/SGSN may check if the correct DCN is selected. The check is performed as specified in clause 4.3.25.1. If the MME/SGSN concludes that the selected DCN is not the correct DCN, a DECOR reroute is performed and the SGSN/MME in the new DCN assigns a new DCN-ID to the UE. The serving MME/SGSN can also assign a new DCN-ID to the UE if e.g. the DCN-ID in the UE has become obsolete or when the UE Usage Type has been updated in the subscription information leading to a change of DCN. This is performed as part of the GUTI Reallocation procedure.

4.3.25.2 Considerations for Roaming

In the case of roaming, if the HPLMN of the visiting UE does not support DCNs, i.e. doesn’t provide the UE Usage Type, the serving MME/SGSN may select the DCN that serves the UE using operator specific policies based on other subscription or UE provided information.

In the case of roaming, if the HPLMN provides the UE Usage Type parameter to the VPLMN, this parameter is provided irrespective of its value (standardized or operator specific). The handling of the UE Usage Type parameter in the VPLMN is based on operator policies, e.g. roaming agreements.

If the UE assisted DCN selection feature is supported:

– If the UE has a DCN-ID for the VPLMN the UE shall send that PLMN specific DCN-ID to the RAN, and

– If the UE has no PLMN specific DCN-ID for this VPLMN and if the UE has a pre-provisioned default standardized DCN-ID it shall send the pre-provisioned default standardized DCN-ID to the RAN.

4.3.25.3 Considerations for Network Sharing

If the network supports the MOCN configuration for network sharing (see TS 23.251 [24]), each network sharing operator has separate CN(s). Mechanisms for selection of serving operator for supporting and non-supporting UEs are defined in TS 23.251 [24]. Each of the sharing operators may deploy one or more DCNs.

If Selected PLMN information is provided by the UE, the RAN selects the CN operator based on this provided information and then DECOR rerouting may, if needed, be initiated within the CN of the selected operator. If the UE assisted DCN selection feature is supported and both the Selected PLMN information and DCN-ID is provided by the UE, the RAN first selects the CN operator followed by selection of a DCN supported by the selected CN operator.

If Selected PLMN information is not provided by the UE (may only happen in GERAN and UTRAN), the network initiates MOCN redirection, including CS/PS coordination, to select a CN operator that can serve the UE. After this, DECOR rerouting is initiated if needed. The serving node in the selected DCN ends the MOCN redirection. If the UE assisted DCN selection feature is supported and Selected PLMN information is not provided by the UE, the network initiates CN operator selection and after the CN operator selection is concluded the DCN is selected based on the UE provided DCN-ID. As the PLMN information included in the RAI is the Common PLMN (refer to TS 23.251 [24]), which does not reflect the selected CN operator, the network may also return the PLMN ID of the selected CN operator. When the UE receives the NAS Accept message, the UE associates the DCN-ID with both the PLMN ID of the selected CN operator and the Common PLMN IDs.

The functions for redirecting or maintaining UEs in specific DCNs are configured to work within the CNs of the same operator.