Description
BSSAP-LE (Base Station System Application Part Location Services Extension) is a critical protocol within the GSM architecture that extends the standard BSSAP protocol to support Location Services (LCS). It operates as an application layer protocol within the Base Station System (BSS) that enables communication between the Serving Mobile Location Center (SMLC) and the Base Station Controller (BSC) for mobile device positioning. The protocol is defined in 3GPP specifications 03.071 and 43.059, which detail the functional requirements and signaling procedures for location services in GSM networks.
The protocol architecture positions BSSAP-LE as an extension to the existing BSSAP protocol stack, utilizing the same underlying transport mechanisms while adding specialized messages and procedures for location services. It operates over the A-interface between the BSC and Mobile Switching Center (MSC), with the MSC acting as a routing point for location-related messages between the SMLC and BSC. The protocol defines specific message types for positioning requests, measurement reporting, and location result delivery, enabling precise coordination between network elements during positioning operations.
BSSAP-LE supports multiple positioning methods including Enhanced Observed Time Difference (E-OTD), Assisted GPS (A-GPS), and Cell-ID based positioning. The protocol handles the complete positioning transaction lifecycle, from initial location request to final location delivery. Key components include the BSSMAP-LE component for non-circuit related messages and the DTAP-LE component for direct transfer application part messages. The protocol manages timing synchronization, measurement collection, and positioning calculation coordination between network elements.
In operation, BSSAP-LE facilitates the exchange of positioning-related information between the SMLC (which manages positioning calculations) and the BSC (which controls radio resources and measurement collection). When a location request is initiated, BSSAP-LE messages coordinate the setup of positioning measurements, collection of timing information from multiple base stations, and delivery of calculated position estimates. The protocol includes error handling mechanisms, quality of service parameters for positioning accuracy, and support for both mobile-terminated and mobile-originated location requests.
The protocol's design ensures backward compatibility with existing BSSAP implementations while providing the specialized functionality needed for location services. It includes security features for authentication and authorization of location requests, particularly important for emergency services and lawful interception scenarios. BSSAP-LE represents a crucial evolution in GSM networks, enabling them to meet regulatory requirements for emergency caller location while supporting commercial location-based services.
Purpose & Motivation
BSSAP-LE was created to address the growing need for accurate mobile device positioning in GSM networks, driven by regulatory requirements for emergency services and commercial demand for location-based services. Prior to its introduction, GSM networks lacked standardized mechanisms for precise location determination, limiting their ability to support emergency caller location (E911/E112 requirements) and commercial location applications. The protocol solved the fundamental problem of coordinating positioning measurements and calculations across different network elements in a standardized way.
The development of BSSAP-LE was motivated by several key factors: regulatory mandates requiring network operators to provide accurate location information for emergency calls, commercial opportunities in location-based services, and the need for standardized interfaces between positioning system components. Before BSSAP-LE, proprietary solutions existed but lacked interoperability and scalability. The protocol provided a standardized way for the SMLC to communicate with the BSC, enabling consistent implementation across different vendor equipment.
Historically, the creation of BSSAP-LE in Release 7 represented a significant advancement in GSM network capabilities, allowing 2G networks to meet evolving location service requirements while maintaining compatibility with existing infrastructure. The protocol addressed limitations in earlier approaches by providing a comprehensive framework for all aspects of location service delivery, from measurement coordination to result delivery. It enabled GSM networks to support the same location service capabilities being developed for 3G networks, ensuring service continuity during network evolution.
Key Features
- Standardized interface between SMLC and BSC for location services
- Support for multiple positioning methods including E-OTD and A-GPS
- End-to-end transaction management for positioning operations
- Quality of Service parameters for positioning accuracy requirements
- Security mechanisms for location request authentication
- Backward compatibility with existing BSSAP implementations
Evolution Across Releases
Initial introduction of BSSAP-LE with comprehensive location service capabilities. Defined the complete protocol architecture including BSSMAP-LE and DTAP-LE components. Established support for E-OTD positioning method and basic cell-ID positioning. Implemented core messaging for location request, measurement coordination, and result delivery between SMLC and BSC.
Defining Specifications
| Specification | Title |
|---|---|
| TS 03.071 | 3GPP TR 03.071 |
| TS 43.059 | 3GPP TR 43.059 |