 Response E-RAB SETUP / MODIFY E-RAB SETUP / MODIFY REQUEST RESPONSE E-RAB RELEASE E-RAB RELEASE Bearer management COMMAND RESPONSE E-RAB RELEASE INDICATION Function Request Response Paging PAGING Function Request Response HANDOVER REQUIRED HANDOVER COMMAND HANDOVER REQUEST HANDOVER REQUEST ACKNOWLEDGE or HANDOVER FAILURE eNB STATUS TRANSFER Mobility management MME STATUS TRANSFER HANDOVER NOTIFY PATH SWITCH PATH SWITCH ACKNOWLEDGE or REQUEST PATH SWITCH FAILURE VoLTE and ViLTE Function Request Response S1 SETUP RESPONSE S1 SETUP REQUEST S1 SETUP FAILURE ENB CONFIGURATION UPDATE ENB CONFIGURATION ACKNOWLEDGE or UPDATE ENB CONFIGURATION Management of the S1- UPDATE FAILURE MME interface MME CONFIGURATION UPDATE MME CONFIGURATION ACKNOWLEDGE or UPDATE ENB CONFIGURATION UPDATE FAILURE OVERLOAD START OVERLOAD STOP Function Request Response INITIAL UE MESSAGE DOWNLINK NAS Transport of NAS signaling TRANSPORT UPLINK NAS TRANSPORT Table 2.4. S1-AP messages 2.3.1. Context management The context of the mobile has to be established at the level of the eNB and MME entities SO as to transmit the mobile traffic and the NAS signaling as well. The context of the mobile includes the contexts relating to the default bearer, security, capacities of the mobile and roaming restrictions. Context setup for the mobile begins with the INITIAL CONTEXT SETUP REQUEST message transmitted by the MME entity to the eNB entity. This message follows the reception of the INITIAL UE MESSAGE message. Signaling Protocols The MME has to prepare the establishment of the default bearer before receiving the INITIAL CONTEXT SETUP RESPONSE message. This message might contain the cause of the failure to set up the context of the mobile, such as the lack of radio resources. If the eNB is unable to establish the context of the mobile, it responds with the INITIAL CONTEXT SETUP FAILURE message. Release of the context of the mobile is done by way of the UE CONTEXT RELEASE COMMAND message transmitted by the MME entity to the eNB entity, for instance when the mo