The MME decides on which APN to select as a default APN based on information provided by the UE during the initial attach, and the subscriber profile received from the HSS in the Update Location Answer. The APN identifies the Packet Data Network Gateway (P-GW) that will be selected for connectivity to the PDN.

The question is: What if the UE selects an APN that is not indicated in the HSS as the default APN? Which one should the MME use as the default APN?

Let us state few facts:

- As stated in TS 29.272, the Subscription-Data AVP includes the APN-Configuration-Profile in sent as part of the Update Location Answer to the MME, which includes a Context-Identifier field which will be used in selecting the default APN.

From TS 29.272, the APN-Configuration-Profile has:

“APN-Configuration-Profile ::= <AVP header: 1429 10415>

{ Context-Identifier }

{ All-APN-Configurations-Included-Indicator }

1*{APN-Configuration}

*[AVP]

“

- The APN-Configuration-Profile may include a list of allowed APNs for a subscriber, (the APN-Configuration field).

From TS 29.272, the APN-Configuration field:

“APN-Configuration ::= <AVP header: 1430 10415>

{ Context-Identifier }

* 2 [ Served-Party-IP-Address ]

{ PDN-Type }

{ Service-Selection}

[ EPS-Subscribed-QoS Profile ]

[ VPLMN-Dynamic-Address-Allowed ]

[MIP6-Agent-Info ]

[ Visited-Network-Identifier ]

[ PDN-GW-Allocation-Type ]

[ 3GPP-Charging-Characteristics ]

[ AMBR ]

*[ Specific-APN-Info ]

[ APN-OI-Replacement ]

[ SIPTO-Permission ]

[ LIPA-Permission ]

*[ AVP ]

“

- The default APN is identified by the MME when a matching Context-Identifier field value in the APN-Configuration of a specific APN matches the Context-Identifier field value that is indicated in the APN-Configuration-Profile. There should be one match.

- The initial attach can have several scenarios, some of which:

      • The UE does not specify an APN value, and the MME selects the default one from the list provided by the HSS in the subscriber profile, (5.3.2.1 of TS 23.401). This would be based on the Context-identifier value match as stated above.
      • The UE specifies an APN value; the MME will select it as the initial APN. According to (5.3.2.1 of TS 23.401) with regards to the initial attach, it states: “For Request Type indicating "Initial request", if the UE does not provide an APN, the MME shall use the PDN GW corresponding to the default APN for default bearer activation. If the UE provides an APN, this APN shall be employed for default bearer activation”.

The initially connected APN for a UE does not have to be the marked default APN in the subscriber’s profile. The default APN is used by the MME in cases where the UE does not provide an APN during the initial attach process. But if the UE provides an APN during the initial attach process, this APN must be part of the APN list that is returned by the HSS to the MME. Therefore, it is a valid scenario to have the UE provided APN during the initial attach process provide an APN that is different than its default APN as indicated in the subscription profile in the HSS.