What is SBA and why 5G uses it

From Point-to-Point to Service-Based

The 4G Evolved Packet Core relied on point-to-point interfaces where each pair of network elements communicated over a dedicated protocol. The MME talked to the HSS on S6a, to the SGW on S11, and to the PGW on S5. That approach produced a mesh of twelve or more distinct interface specifications, each with its own protocol stack and message set. Adding a new network function required defining new interfaces to every element it needed to reach, which slowed innovation and complicated multi-vendor integration in real deployments.