. In the EMM-DEREGISTERED state, the mobile location is not known to the MME entity, and therefore, it cannot be contacted. The switch to the registered state takes place when the mobile attaches, which comprises the following four procedures: - mutual authentication of the mobile and the MME entity; - registration of the mobile location with the MME entity; VoLTE and ViLTE: Voice and Conversational Video Services over the 4G Mobile Network, First Edition. André Perez. C ISTE Ltd 2016. Published by ISTE Ltd and John Wiley & Sons, Inc. VoLTE and ViLTE - assignment of the globally unique temporary identity (GUTI) to the mobile; - establishment of the default bearer. The switch to the deregistered state takes place when the mobile detaches or when the attachment of the mobile, the update of its location or the service request are rejected by the MME entity. 2.1.1. EMM messages 2.1.1.1. Attachment and detachment The procedure of attachment is initiated by the mobile in the deregistered state, by sending the message EMM ATTACH REQUEST to the MME entity. This message contains the mobile identity, GUTI or international mobile subscriber identity (IMSI) and the tracking area identity (TAI). The mobile attaches the message ESM PDN CONNECTIVITY REQUEST to establish the default bearer. Upon receiving this message, the MME entity begins the authentication and security procedures for the NAS protocol. If they are successfully completed, the MME entity responds with the message EMM ATTACH ACCEPT, containing a new GUTI, and the message ESM ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST, to establish the default bearer. The mobile responds with the message EMM ATTACH COMPLETE containing the message ESM ACTIVATE DEFAULT EPS BEARER CONTEXT ACCEPT, to acknowledge the previous message. If the procedures are not successful, the MME entity responds with the message EMM ATTACH REJECT, containing the message ESM PDN CONNECTIVITY REJECT, which causes the mobile to detach. Detachment may be initiated by the mobile or the MME entity by sendin