Description
Call Request with Number (CRN) is a supplementary service defined in 3GPP TS 27.002 that enables a user to request the network to establish a call to a specific destination number without the user manually dialing the complete number. The service operates within the framework of the Mobile Station Application Execution Environment (MExE) and utilizes the Unstructured Supplementary Service Data (USSD) mechanism for communication between the mobile station and the network. CRN functions by allowing the user to store frequently used numbers in the network or mobile device, then initiate calls by selecting these stored entries rather than entering full telephone numbers.
The technical implementation of CRN involves several key components working together. The mobile station contains the CRN client application that interfaces with the user, while the network side includes the CRN server functionality typically implemented within the Home Location Register (HLR) or a dedicated service node. When a user activates CRN, the mobile station sends a USSD message containing the CRN request to the network. This message includes parameters such as the service code, the index of the stored number (if using stored numbers), or the complete destination number if provided directly. The network processes this request, validates the user's subscription to the CRN service, and then initiates the call setup procedure to the specified destination.
CRN operates through a structured protocol exchange defined in 3GPP specifications. The service supports multiple invocation methods: users can either select from a list of pre-stored numbers in their profile or provide a complete telephone number during the request. The network maintains the stored numbers in the user's service profile, which can be managed through separate provisioning procedures. When processing a CRN request, the network performs subscriber authentication, service authorization, and number validation before proceeding with call establishment. The service integrates with existing call control procedures, ensuring compatibility with standard mobile originating call setup while adding the abstraction layer of number storage and retrieval.
The service architecture supports both mobile-originated and network-initiated scenarios. In the mobile-originated case, the user explicitly requests a call through the CRN interface. The network can also utilize CRN capabilities to implement features like call-back services, where the network automatically establishes a call to a previously attempted but unsuccessful destination. CRN's integration with USSD ensures reliable delivery of service requests even when the user is engaged in other activities like voice calls, as USSD operates on the signaling channel rather than the traffic channel. This makes CRN particularly useful for implementing quick-dial features and automated calling services within the cellular network ecosystem.
Purpose & Motivation
CRN was developed to address the need for simplified calling procedures in mobile networks, particularly as mobile devices became more complex and users needed to manage increasing numbers of contacts. Before CRN, users had to manually dial complete telephone numbers for every call, which was cumbersome for frequently contacted numbers or in situations where quick dialing was necessary. The service aimed to reduce dialing errors, save time, and improve the overall user experience by providing an abstraction layer between the user and the complex dialing procedures.
The creation of CRN was motivated by the growing adoption of supplementary services in GSM and UMTS networks, where operators sought to differentiate their offerings through value-added services. Traditional abbreviated dialing solutions were often implemented solely on the handset, limiting their functionality when users changed devices or needed network-based features. CRN provided a standardized network-based solution that worked consistently across different devices and network conditions. This standardization was particularly important as roaming became more common, ensuring that users could access their stored numbers and quick-dial features regardless of their location or serving network.
CRN also addressed specific use cases that were difficult to implement with existing mechanisms. For emergency services or frequently called numbers, CRN enabled one-touch dialing without requiring the user to remember or locate the complete number. The service supported business applications where employees needed quick access to internal extensions or customer service numbers. By integrating with the network's service control infrastructure, CRN could be combined with other supplementary services like call forwarding or call waiting, creating more sophisticated telephony applications. The service's design considered the limitations of early mobile interfaces, providing a solution that worked effectively even with devices having limited display capabilities or input methods.
Key Features
- Network-based number storage and retrieval
- USSD-based service invocation and control
- Support for both stored numbers and direct number input
- Integration with standard call setup procedures
- Roaming compatibility across different networks
- Simultaneous operation with other supplementary services
Evolution Across Releases
Introduced CRN as a standardized supplementary service within the 3GPP framework. Defined the basic architecture using USSD for service invocation and network-based number storage. Established the protocol procedures for CRN activation, deactivation, invocation, and interrogation. Specified integration with existing call control mechanisms and subscriber profile management.
Defining Specifications
| Specification | Title |
|---|---|
| TS 27.002 | 3GPP TS 27.002 |