Feature Description
The SMF supports the UE performing initial attach on E-UTRAN through MME and S-GW to create the default bearer.
Initial attach on E-UTRAN or EPS follows the procedure defined in 3GPP specification 23.401, Section 5.3.2.1. There are few deviations from the defined procedure to enable connectivity through the 5G core or Legacy GW. The deviations are as follows:
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The Packet Data Network Gateway (PGW-C) in the procedure is replaced by SMF.
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The IP-CAN Session establishment and modification is replaced by SM Policy Association Establishment procedure.
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The online and offline charging functionality using Gy and Gz interfaces is replaced by integrated charging over Nchf interface with Charging Function (CHF).
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The interface with the user-plane node is through N4 interface instead of Sxb interface.
SMF Interworking with Diameter Interfaces
The UE supports E-UTRAN Initial Attach procedures for Diameter interfaces as described in 5.3.2.1, 23.401 with following functions:
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Default EPS bearer establishment.
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Activate predefined PCEF rules with SMF as the Gx reference point is situated between the PCRF and the SMF. It is used for provisioning and removal of PCC rules. The Gx interface functions as a diameter connection. The Gx messages involve installing or removing dynamic rules and activating or deactivating predefined rules, APN-AMBR, and Default Bearer QoS.