1x Circuit Switched

Description

1xCS (1x Circuit Switched) refers to the circuit-switched signalling system of the 3GPP2 cdma2000 1xRTT radio access technology. In the context of 3GPP specifications, particularly TS 23.216 and TS 23.272, 1xCS is used to describe the legacy circuit-switched domain of CDMA2000 networks. The term is most commonly encountered in the context of Circuit Switched Fallback (CSFB), where an LTE (E-UTRAN) terminal falls back to a 1xRTT network to handle a mobile-originated or mobile-terminated voice call or SMS. The 1xCS system is defined in 3GPP2 X.S0042-0, which specifies the signalling protocols for the A1/A2 interfaces between the MSC and the BSC, as well as the air interface signalling.

The need for 1xCS in 3GPP arises from the coexistence of LTE and CDMA2000 networks, particularly in North America and parts of Asia. When LTE was introduced, it was designed as a packet-switched-only network, but voice services were still predominantly circuit-switched in CDMA2000. To enable voice calls for LTE subscribers, 3GPP defined CSFB procedures that allow the UE to tunnel signalling messages to the 1xRTT network via the LTE network. The 1xCS signalling system provides the mechanisms for the UE to register with the 1xRTT MSC, to originate and receive calls, and to perform handovers between LTE and 1xRTT. This is accomplished using the S102 interface between the MME and the 1xRTT interworking solution (IWS), which encapsulates 1xCS signalling messages.

Technically, 1xCS encompasses the protocols and procedures for circuit-switched services in cdma2000 1xRTT. The key components include the Mobile Switching Centre (MSC), the Base Station Controller (BSC), and the Base Transceiver Station (BTS). The signalling protocols include the A1 interface for signalling between MSC and BSC, the A2 interface for voice traffic, and the air interface (Um) for radio communication. When a UE is registered with both LTE and 1xRTT (via CSFB), it can receive a paging message for a mobile-terminated call. The MME forwards the page to the UE, which then performs a fallback to 1xRTT by either redirecting to a 1xRTT cell or performing a handover. Once on the 1xRTT network, the UE uses 1xCS signalling to establish the call.

In 3GPP Release 8, the 1xCS term was introduced as part of the CSFB specifications. The definition in TS 23.216 and TS 23.272 refers to the 3GPP2 legacy circuit-switched signalling system. Over subsequent releases, the definition has remained essentially unchanged, reflecting the stable nature of the legacy CDMA2000 signalling. However, as networks evolve towards VoLTE and 5G, the reliance on 1xCS for voice is diminishing, but it remains important for interoperability and roaming in multi-technology environments. The term is also relevant for SRVCC (Single Radio Voice Call Continuity) from LTE to 1xRTT, which allows ongoing voice calls to be handed over from LTE to the circuit-switched domain.

In summary, 1xCS is the circuit-switched signalling system of cdma2000 1xRTT, used in 3GPP to enable fallback and handover procedures for voice and SMS services. It is a critical enabler for multi-mode terminals operating in mixed LTE/CDMA2000 networks.

Purpose & Motivation

The 1xCS term exists to provide a clear reference to the legacy circuit-switched signalling system of CDMA2000 1xRTT within the 3GPP ecosystem. As LTE was introduced as a packet-switched-only network, there was a need to support voice and SMS services for subscribers who were using CDMA2000 networks. The 1xCS system solves this problem by allowing LTE terminals to fall back to the 1xRTT network for circuit-switched services, using procedures defined in 3GPP specifications. This is particularly important for operators who have both LTE and CDMA2000 deployments and need a smooth migration path for voice services.

Before the introduction of 1xCS in 3GPP Release 8, there was no standardised way for an LTE terminal to access CDMA2000 circuit-switched services. The alternative would have been to use dual-radio devices or to rely on VoLTE, which was not yet mature. The 1xCS term and the associated CSFB procedures provide a standardised mechanism that allows a single-radio LTE device to access CDMA2000 CS services by temporarily switching to the 1xRTT radio. This enables operators to continue offering voice services without waiting for full VoLTE deployment and ensures interoperability with existing CDMA2000 infrastructure.

Historically, the 1xCS system was developed by 3GPP2 as part of the cdma2000 family. When 3GPP began work on LTE-Advanced and interworking with other technologies, it adopted the term to describe the legacy CS domain. The definition in 3GPP TS 21.905 and the procedures in TS 23.216 and TS 23.272 are based on 3GPP2 specifications. Although the use of CSFB is declining as VoLTE and 5G become widespread, the 1xCS term remains relevant for legacy support and roaming scenarios.