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Figure 6.9 illustrates the MO and MT voice CS call scenario made by LTE UEs currently registered in the LTE/EPS network and shows only the important call flows from the CSFB point of view. The dotted line indicates the absence of the associated call flows, which are not shown in this figure.

Assume that the MO UE is engaged in a PS session and the MT UE is currently in the idle condition shown by its current state ECM‐IDLE. Note that notification for an incoming CS call to MT UE depends on its current state. If the MT UE is currently in the

 ECM‐IDLE state, MME notifies an incoming CS call by sending a paging message to the UE.

 ECM‐CONNECTED state, then the MMM notifies an incoming CS call by sending a CS SERVICE NOTIFICATION message to it.

MSC server sends the paging request, SGsAP‐PAGING‐REQUEST, to the MME over the SGs interface. MME further sent the paging message toward the UE through the eNodeB. MO UE completes the MO CS call as per the normal procedures with the MSC server. MT UE sends a paging response to the MSC server and completes the MT CS call as per the normal procedures illustrated in Figure 2.8 earlier. Following this, both the UEs are connected in the legacy GSM network through CSFB arrangement and start voice/CS call conversations with each other.

In this example, illustrated in Figure 6.9, assume that the MT UE disconnected the CS call first, followed by the MO UE. In this case, the GSM BSC instructs both the UEs to release the allocated channels by sending the Channel Release message. This message also contains the LTE EARFCN number, indicating the UEs that they should return to the LTE network. Because of this, both the UEs perform their TAU procedure toward the MME to inform their current location within the LTE/EPS network.

In all of the air interface Layer 3 messages used for making a CS call in the legacy network, the flags CSMO, for the originating side, and CSMT, for the terminating side, are used and set accordingly to indicate to the receiving network element that the message is for CSFB purpose. These flags refer to TS 24.008 [45], are shown in Figure 6.9 illustration. Table 6.1 summarizes the network elements, their logical interfaces, and the related 3GPP TSs to support voice calls for LTE/EPS registered UE through the CSFB and SVRCC features.

Figure 6.9 Illustration: LTE voice call: MO‐MT voice call through CSFB.

Table 6.1 Interworking methods: network elements and their logical interfaces.

Interworking Methods	Network Elements	Logical Interfaces	Related 3GPP TSs
CSFB	MME, MSC Server	SGs	23.272, 29.118, 36.413, 24.301, 23.401
SRVCC	MME, MSC Server SGSN, MSC Server	Sv	29.280, 23.401, 36.413