Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 150

During the initial rollout of an LTE/EPS network, an operator may not have the IMS to offer its VoLTE services to subscribers. In such a scenario, the operator will be required to provide voice services to subscribers through the existing legacy GSM/UMTS network. The arrangement of providing voice service to an E‐UTRAN UE and its user is known as the CSFB method, as described earlier in Section 6.2.3, creating a UE context between the MME and the MSC over SGs interface. In this interworking scenario, the E‐UTRAN UE will send the following information during its initial registration, through the ATTACH Request procedure, with the LTE/EPC.

 UE to EPC (ATTACH Request)Attach Type = Combined EPS/IMSI AttachUE Usage Setting = Voice CentricVoice domain preference = CS Voice Only

The above UE Usage Setting and the voice domain preference information may also be sent by a UE if it is not VoLTE capable or fails to register with the IMS. The LTE/EPC will send the following information as part of the ATTACH ACCEPT message to the UE:

 EPC to UE (ATTACH Accept)EPS Attach Result = combined EPS/IMSI attachAdditional Update Result = CS fallback not preferred