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In the previous sections, we have described the interworking scenarios that allow the coexistence of the legacy GSM, GPRS, UMTS with the LTE/EPS, and IMS networks. Interworking and interoperations provide communications services within a PLMN or between PLMNs for roaming users. The ability to provide a particular communications service to a subscriber through interworking and interoperations of networks depends on the capability, e.g. multiple radio access technologies (RAT), of a particular UE/MS and its network configurations. Based on its capabilities, a UE/MS may operate in a particular mode or more than one mode at a time.

 Mode of operations

Table 6.2 summarizes the mode of operations that an MS/UE can operate within the GPRS, UMTS, and LTE/EPS systems. For more details on the individual mode of operation, refer to TS 24.301 [46] and TS 23.060 [31]. A mode of operation determines the availability of types of services, i.e. CS or PS, to a UE.

The capabilities of an MS/UE are specified in terms of its ability to provide either the CS service or PS service at a time only or both the types of services at the same time to a subscriber. For these purposes, an MS/UE should be able to register itself with the CS domain and PS domain core networks simultaneously. In addition to this, for an E‐UTRAN UE, it should also support multiple RATs to provide communication services through the legacy networks in case the IMS is not available/interfaced with LTE/EPC. UE mode of operations in the case of the 5G system has been described briefly in Section 6.4.

 How an MS/UE provides its mode of operation to the CN: Domain Preference

To facilitate voice or data services to a subscriber in a GPRS/UMTS and LTE/EPS network, an MS/UE provides its mode of operation preference under the Attach Type IE in the ATTACH Request message, TS 24.301 [46], TS 24.008 [45], that is sent to the core network. In a GPRS/UMTS and LTE/EPS network, the MS/UE sets the Attach Type IE to GPRS Attach and EPS Attach only to enable the PS services only. Similarly, in a GPRS/UMTS and LTE/EPS network, the MS/UE sets the Attach Type IE to Combined GPRS/IMSI attach and Combined EPS/IMSI attach to enable both the voice and data services to a user.

Table 6.2 GPRS, UMTS, and LTE UE mode of operation.

System	MS/UE Mode of Operation	Purposes
GPRS	Class A	MS registered with CS and PS domains, for voice and data services at the same time.
Class B	MS registered with CS and PS domains. Either voice or data service is possible at a time.
Class C	MS registered with PS domain only, for data service is possible.
UMTS	CS/PS Mode	Similar to GPRS Class‐A mode.
PS Mode	Similar to GPRS Class‐C mode.
CS Mode	UE registered with CS domain only, for voice service.
LTE	PS Mode 1	UE registered with the EPS network only. UE’s usage is a voice‐centric service only.
PS Mode 2	UE registered with the EPS network only. UE’s usage is a data‐centric service only.
CS/PS Mode 1	UE registered with EPS and CS domain legacy networks for EPS and non‐EPS services. UE’s usage is voice‐centric service only.
CS/PS Mode 2	UE registered with EPS and CS domain legacy networks for EPS and non‐EPS services. UE’s usage is data‐centric service only.

A UE ATTACH Request contains an optional IE: Voice domain preference and UE’s usage setting, refer to TS 24.301 [46], TS 24.008 [45], using which a UE can provide additional information on its corresponding mode of operation to the core network. Using the optional IE: Voice domain preference and UE’s usage setting, an E‐UTRAN UE indicates its CSFB (to the legacy 2G or 3G network for a voice call) capability in case the network or the UE does not support VoLTE call or the UE fails to registers with the IMS.

Note: Till the TS 24.008 [45] 9.1.0 version, the CSFB capability of a UTRAN UE was indicated within the MS Network Capability mandatory IE in the UE ATTACH Request message to the GPRS core network. However, from TS 24.008 9.2.0 version onwards, the CSFB capability information is conveyed to the GPRS core network through the Voice domain preference and UE’s usage setting optional IE only. This IE is also used by an E‐UTRAN UE to convey its CSFB capability information in the ATTACH Request message to the LTE/EPC. CSFB capability information is no longer part of the MS Network Capability IE.

 E‐UTRAN UE Mode of Operations and Their Transitions

In this paragraph, the mode of operation of an E‐UTRAN UE, as mentioned in Table 6.2, is further described. An E‐UTRAN UE registered with an LTE/EPS network for EPS only services is said to be operating in PS mode. An E‐UTRAN UE registered with LTE/EPS and legacy GSM/UMTS network for EPS and non‐EPS, i.e. voice, SMS, services are said to be operating in CS/PS mode. In addition to this, an E‐UTRAN UE may be required to switch between the PS and CS/PS modes of operation, and vice versa, because of various reasons that take place at the network level at runtime without the user’s notice. Some of the typical reasons are as follows:

 Unavailability of CSFB feature,

 Unavailability of IMS,

 Changes in preferences, network operations,

 Roaming, and

 Failure to register with an IMS.

An E‐UTRAN UE may be also required to switch between the PS and CS/PS mode of operations, and vice versa, because of a user action. For example, consider that a VoLTE‐capable UE is registered with the LTE/EPC and IMS. The user has turned off the VoLTE preference in the handset, making the UE unable to facilitate voice‐over IMS to the user. In such a case, the UE is required to register with the CS domain core network so that it can fall back to the legacy network to facilitate voice calls to the user. As a result of the transition from the PS to CS/PS mode of operations, and vice versa, a UE requires to perform protocol‐related procedures so that it can stay updated with the core network with its current location information and continue enabling seamless voice and data services to a user.

Examples 6.4 and 6.5 below highlight the information passed by a UE as part of its signaling messages to the LTE/EPC network based on the UE mode of operations described above.