6.2 below describes the status displayed by an LTE/EPS registered UE while making a voice call through the legacy GERAN/UTRAN using the CSFB method.
How does a CSFB work?
Indication of CSFB Feature Support
A UE indicates its CSFB feature support capability under the UE Network Capability Mandatory IE in the ATTACH Request message that is sent to the MME. The UE specifies the Attach Type as the combined EPS/IMSI Attach Request under the EPS attach type IE. A combined EPS/IMSI ATTACH Request from the UE enables the MME to update the UE location in the MSC/VLR end also over the SGs interface. Refer to TS 23.272 [38] for further details on the EMM procedures supported over the SGs interface. If the EPC network supports the CSFB feature, the same will be informed to the UE in the ATTACH ACCEPT, TS 24.301 [46], message through the IE Attach Result = combined EPS/IMSI attach and Additional update result optional IE. This IE will carry the value “CS Fallback not preferred” toward the UE, indicating that it can use the CSFB method to initiate a CS call through the legacy 2G/3G network. The same IE indicating the supporting of the CSFB feature can be also sent in a TAU ACCEPT message, TS 24.301 [46], from the MME to UE.
Initiation of CS Call Through the CSFB Method by the UE
An LTE UE initiates a CS call through the CSFB method by sending the EMM/NAS EXTENDED SERVICE REQUEST message to the MME.
This message can be used to establish an MO or MT CS call through CSFB by indicating the same in the Service type mandatory IE; refer to Section 9.9.3.27, TS 24.301 [46]. Any existing Radio Resource Control (RRC) connection will be released by the E‐UTRAN by sending the RRCConnectionRelease, TS36.331 [94], message to the UE with
Example 6.2 CSFB: LTE UE Fall‐back to CS domain for voice call
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.
Example 6.3 LTE Voice Call Through GSM Network and CSFB Feature
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 |