什么情况下UE应该发起到电路域的联合附着流程?(而非仅附着到EPS域) 根据规范24301的规定,“The combined attach procedure is used by a UE in CS/PS mode 1 or CS/PS mode 2 of operation to attach for both EPS and non-EPS services, or both EPS services and "SMS only". The combined attach procedure is also used by a UE in CS/PS mode 1 or CS/PS mode 2 of operation to attach for EPS services if it is already IMSI attached for non-EPS services. When the UE initiates a combined attach procedure, the UE shall indicate "combined EPS/IMSI attach" in the EPS attach type IE.” 也就是说UE的工作模式在CS/PS mode 1或CS/PS mode 2时,应该发起联合附着。 那UE什么情况下会工作在CS/PS mode 1或CS/PS mode 2呢?规范也做了一些说明(可能还有漏掉的哈): “A UE configured to use CS fallback, shall operate in CS/PS mode 1 or CS/PS mode 2. Such UE may also be configured to use IMS, in which case the voice domain preference as defined in 3GPP TS 24.167 [13B] shall be used for the selection of the domain for originating voice communication services. A UE configured to use SMS over SGs, but not configured to use CS fallback, shall operate in CS/PS mode 1 or CS/PS mode 2.” 也就是说UE如果配置使用了CS fallback这个特性或者没有配置CS fallback但激活了SMS over SGs特性,那就应该工作在CS/PS mode 1或CS/PS mode 2。因此,据我猜想,手机上应该有设置可以激活CS fallback或者SMS over SGs。
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