Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 142

Assume that a mobile user currently subscribes to a subscription plan consisting of LTE (4G) data along with voice call services provided by an operator. The UE will perform a combined attach, EPS as well as IMSI, procedure with the LTE/EPS network. Also, consider that the operator’s LTE/EPS network does not have the IMS facility currently to provide voice‐over LTE services to subscribers. The UE may display the type of network, i.e. 4G, which is currently registered with, at the top corner of the handset. Whenever the user attempts to make a voice call or an incoming call arrives for the user, the UE will fall back to either 2G or 3G network which will be shown at the top corner of the handset. Once the voice call completes, the UE will return to and access the LTE/EPS network and registers with it again, showing the 4G status at the top corner of the handset.

 “releaseCause = cs‐FallbackHighPriority” for CS fallback to UTRAN or

 “releaseCause = other” for CS fallback to GERAN.

CSFB method works on the redirection concept where LTE UE is redirected to the target ARFCN of the legacy network. For redirection purposes, the RRCConnectionRelease message will also include the redirectedCarrierInfo IE containing the following information for CSFB:

 RAT/RAN, i.e. GERAN, UTRAN, and E‐UTRAN, to be redirected.

 Corresponding Carrier Frequency, also called ARFCN, to be used by the UE.

For the various options of RATs that are allowed for CSFB purposes, refer to TS 36.331 [94]. The UE tunes to the indicated carrier frequency/ARFCN of the legacy network. To make a CS voice call, UE will start exchanging normal air interface Layer 3 signaling messages with the network as illustrated earlier in Section 2.2.5.

Example 6.3 below describes and illustrates the typical signaling messages flows during a mobile originated voice call made by an LTE/EPS registered UE through the legacy GERAN/UTRAN using the CSFB method.