S1AP (S1 Application Protocol)

Also known as: S1 Application Protocol

S1AP is the 3GPP signalling protocol between the LTE eNodeB and the MME over the S1-MME interface (TS 36.413). It handles UE attach, bearer setup, handover, paging and NAS transport, running over SCTP. Exposure of S1AP outside a trusted transport is a well-known attack surface for LTE core intrusion and DoS.

Categories: Radio Access NetworkCore NetworkSignaling

S1AP in context

The radio access network is where mobile devices attach to the operator's infrastructure. Attacks in this layer include IMSI catching, rogue base stations and downgrade attacks; defenses rest on mutual authentication, integrity-protected signaling and Open RAN supply-chain hygiene.

The mobile core carries subscriber sessions, mobility and policy. In 4G it is the EPC (MME, HSS, S/PGW); in 5G it is the Service-Based Architecture with AMF, SMF, UPF, AUSF, UDM and the NRF.

To place S1AP in the wider telecom-security picture, review 5G SA, AMF, Base Station (BS), Capacity Planning, CAPEX and CSCF — each entry cross-references back to this page so you can walk the topic in either direction.

Related terms

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