The S1AP (S1 Application Protocol) is a signaling protocol used in LTE (Long-Term Evolution) networks. It operates between the eNodeB (Evolved Node B) and the MME (Mobility Management Entity) to establish and maintain the connection between the user equipment (UE) and the core network.
S1AP supports the following functions and procedures:
• Initial UE attachment and detachment
• UE context management
• UE mobility management
• UE paging
• Dedicated bearer activation and deactivation
• NAS (Non-Access Stratum) transport
• Handover preparation and execution
• Error indication and reset
• Load balancing and overload control
• Trace activation and deactivation
• Warning message transmission
S1AP uses the SCTP (Stream Control Transmission Protocol) as the transport layer protocol, and ASN.1 (Abstract Syntax Notation One) as the encoding scheme. S1AP messages are divided into two categories: elementary procedures and criticality diagnostics. Elementary procedures are further classified into three types: initiating, successful outcome, and unsuccessful outcome. Criticality diagnostics are used to report any errors or failures in the S1AP processing.
S1AP is an essential protocol for LTE networks, as it enables the communication and coordination between the eNodeB and the MME, which are the main components of the E-UTRAN (Evolved Universal Terrestrial Radio Access Network) and the EPC (Evolved Packet Core), respectively. S1AP ensures the proper operation and performance of the LTE network, and provides the necessary services and functions for the UE.