SGSN delivered Short message Mobile Terminated – Charging Data Record

Description

The S-SMT-CDR (SGSN delivered Short message Mobile Terminated – Charging Data Record) is a standardized data record defined within the 3GPP charging architecture. It is generated by the Serving GPRS Support Node (SGSN) in a 2G/3G packet-switched network when it handles the delivery of a mobile-terminated short message (SMS-MT) to a subscriber. The record is a specific type of Charging Data Record (CDR) that forms part of the Offline Charging System (OFCS). Its creation is triggered by specific events in the SMS delivery process, such as the successful submission of the message to the mobile station or a delivery failure. The SGSN, acting as the Charging Trigger Function (CTF), collects relevant information about the event, formats it according to the specifications in 3GPP TS 32.251 and TS 32.272, and forwards the S-SMT-CDR to the Charging Data Function (CDF) via the Ga reference point.

The architecture for S-SMT-CDR generation is integrated into the SGSN's charging functions. When an SMS-MT arrives for a subscriber, the SGSN interacts with the Mobile Switching Centre (MSC) and the SMS Gateway MSC (SMS-GMSC) via the Gd interface for SMS delivery over the GPRS network. During this procedure, the SGSN identifies charging-relevant parameters. These include the serving node identity (SGSN address), the subscriber's identifier (e.g., IMSI, MSISDN), the originating address of the SMS, the time of the delivery attempt, the cause for termination of the record (e.g., successful delivery, failure reason), and data volume indicators related to the signaling message transfer. This data is compiled into a CDR with a specific record type identifier.

The S-SMT-CDR plays a crucial role in the operator's billing ecosystem. After generation, it is transmitted to the network's billing domain. Here, it is used to apply the appropriate tariff for the SMS-MT service, which may differ based on the subscriber's plan, the message's origin (e.g., domestic, international), or the network's policy. The record provides an auditable trail for each billing event, enabling customer billing, inter-operator settlement (if the message originated from another network), and revenue assurance by correlating service delivery with generated charges. Its standardized format ensures interoperability between network equipment from different vendors and the operator's billing systems.

Purpose & Motivation

The S-SMT-CDR was introduced to provide a standardized and reliable mechanism for charging mobile-terminated SMS messages delivered via the GPRS packet-switched core network (via the SGSN). Prior to standardized charging records, operators faced challenges in accurately billing for data services, including SMS over GPRS, which could lead to revenue leakage or billing disputes. The creation of specific CDR types for different service scenarios was a key part of 3GPP's effort to define a comprehensive and flexible charging framework (Charging and Billing) for all network services.

Its purpose is to solve the problem of service-specific accounting within a packet-switched domain. While voice calls in circuit-switched networks had well-established charging models, the rise of packet-based services like SMS over GPRS required new data structures to capture the unique attributes of these transactions. The S-SMT-CDR addresses this by precisely logging the delivery attempt of an SMS-MT, including success/failure status, which is essential for fair and accurate billing. It enables operators to implement complex tariff models, support prepaid billing via integration with online charging systems, and generate detailed customer invoices. The motivation was driven by the commercial necessity to monetize the rapidly growing SMS service and ensure a consistent charging approach across multi-vendor network deployments.