Circuit Switched Routing Number

Description

The Circuit Switched Routing Number (CSRN) is a critical network identifier defined in 3GPP specifications that facilitates circuit-switched fallback (CSFB) functionality in LTE/EPC networks. When a mobile device capable of both LTE (packet-switched only) and legacy 2G/3G (circuit-switched) networks registers with an LTE network, the Mobility Management Entity (MME) may assign a CSRN to the subscriber. This number serves as a temporary routing address within the circuit-switched domain, allowing the network to properly route incoming voice calls to the appropriate mobile switching center (MSC) serving the subscriber's current location.

The CSRN operates within the CSFB architecture where the MME interfaces with the MSC Server through the SGs interface. When a mobile terminating call arrives for a subscriber camped on LTE, the MSC uses the CSRN to identify the specific MME serving that subscriber. The CSRN typically follows the E.164 numbering format and is structured to include routing information that directs the call through the correct network elements. The MME stores the CSRN in association with the subscriber's International Mobile Subscriber Identity (IMSI) and other context information, maintaining this mapping for the duration of the CSFB session.

During CSFB procedures, when a voice call needs to be established, the MME uses the CSRN to coordinate with the MSC for paging and handover preparation. The CSRN enables the MSC to correlate the incoming circuit-switched call with the specific LTE context maintained by the MME. This coordination is essential for triggering the fallback procedure where the UE temporarily leaves LTE coverage to establish the voice call in 2G/3G networks. The CSRN remains valid until the subscriber detaches from the network or the CSFB context is cleared, after which it can be reassigned to another subscriber.

The implementation of CSRN supports various CSFB scenarios including mobile-terminated and mobile-originated calls, as well as different fallback methods such as redirection, handover, or cell change order. The CSRN's role extends beyond simple identification—it enables the network to maintain service continuity and proper charging records during the inter-system mobility between packet-switched and circuit-switched domains. This identifier is particularly important in early LTE deployments where voice services relied heavily on fallback mechanisms before VoLTE became widely available.

Purpose & Motivation

The CSRN was created to address the fundamental challenge of providing voice services in early LTE networks that lacked native circuit-switched capabilities. LTE was designed as a pure packet-switched network optimized for data services, leaving a gap for traditional circuit-switched voice calls. The CSRN enables operators to leverage their existing 2G/3G infrastructure for voice services while deploying LTE for enhanced data capabilities, providing a smooth transition path without requiring immediate investment in IMS-based VoLTE solutions.

Before CSFB mechanisms with proper identifiers like CSRN, dual-mode devices would need to periodically scan for 2G/3G networks to receive circuit-switched services, leading to increased battery consumption and potential missed calls. The CSRN solves this by allowing the LTE network to maintain awareness of the subscriber's circuit-switched context and efficiently coordinate fallback procedures only when needed. This approach preserved the user experience of 'always-on' voice availability while maximizing time spent on the more efficient LTE network for data services.

The creation of CSRN addressed specific technical limitations in early inter-system mobility solutions. Previous approaches either required complex dual-radio implementations or suffered from excessive signaling overhead and call setup delays. By providing a standardized temporary identifier that both packet-switched and circuit-switched network elements could reference, CSRN enabled more efficient coordination between the MME and MSC, reducing fallback latency and improving overall system reliability during the critical transition period from circuit-switched to packet-switched voice architectures.

Key Features

• Temporary E.164 formatted identifier for circuit-switched routing
• Enables correlation between LTE/EPC context and circuit-switched domain
• Supports mobile-terminated and mobile-originated CSFB procedures
• Facilitates efficient paging coordination between MME and MSC
• Maintains backward compatibility with legacy 2G/3G voice networks
• Enables proper charging and billing during inter-system mobility

Evolution Across Releases

Defining Specifications