SGSN delivered Short message Mobile Originated – Charging Data Record

Description

The SGSN delivered Short message Mobile Originated – Charging Data Record (S-SMO-CDR) is a specific type of Charging Data Record (CDR) defined in the 3GPP charging specifications. It is generated by the Serving GPRS Support Node (SGSN) in a GSM/UMTS network when it handles a mobile-originated Short Message Service (SMS) that is delivered via the circuit-switched (CS) core network domain. This CDR is part of the offline charging system, where charging information is collected after the service event for later processing and billing. The S-SMO-CDR contains a standardized set of fields that detail the SMS transaction, providing the necessary data for the billing system to charge the subscriber.

The generation of an S-SMO-CDR is triggered by a specific event: the successful submission of a mobile-originated SMS from the UE to the Short Message Service Centre (SMSC) through the CS domain, where the SGSN is involved in the routing. The procedure typically involves the UE sending the SMS via the CS signaling channels (e.g., over the SDCCH in GSM). The MSC, which handles the CS domain, interacts with the SGSN (if the subscriber is also GPRS-attached) for coordination. Upon successful delivery of the SMS submission to the next network element (towards the MSC), the SGSN's Charging Trigger Function (CTF) will assemble the relevant data into an S-SMO-CDR. This record is then forwarded via the Ga interface to the Charging Gateway Function (CGF) using protocols like GTP' or FTAM.

The content of the S-SMO-CDR is defined in detail in 3GPP TS 32.251 and related specs. Key fields include the IMSI of the originating subscriber, the MSISDN (if available), the SGSN address, the MSC address that handled the SMS, the Service Centre address (SMSC), the time stamps for when the SMS submission was received and when it was forwarded, the cause for termination, and the data volume associated with the SMS (which is effectively the length of the message). It may also include location information (Cell Global Identity or Routing Area) and identifiers for the specific charging session. This structured data allows the billing system to apply the correct tariff for the SMS event, which may depend on factors like destination, time of day, or subscriber plan.

It is important to distinguish the S-SMO-CDR from other SMS-related CDRs. For example, an S-SMT-CDR is generated for a mobile-terminated SMS. Furthermore, if the SMS is delivered via the packet-switched (PS) domain (as in SMS over GPRS or SMS over IP), different CDR types are used, such as the S-SMO-CDR for the PS domain. The S-SMO-CDR (CS domain) is a legacy CDR type primarily relevant for 2G and 3G networks where SMS was a dominant service and often carried over the CS signaling plane. Its generation is a core function of the SGSN's charging subsystem, contributing to the accurate monetization of non-data services in traditional mobile networks.

Purpose & Motivation

The S-SMO-CDR was standardized to address the need for accurate and detailed charging of Short Message Service (SMS) events in GSM and UMTS networks, specifically for messages originating from a mobile device and routed through the circuit-switched core. Prior to detailed standardization, operators needed a consistent way to capture billing information for SMS, which quickly became a major revenue-generating service. The problem was that SMS delivery involves multiple network nodes (UE, BSS, MSC, SGSN, SMSC), and the charging data needed to be collected at the appropriate point of control—the SGSN, which manages the subscriber's mobility and session context in the packet-switched domain, even for this CS-domain service.

The creation of the S-SMO-CDR was motivated by the split architecture of 2G/3G networks, where the SGSN (a PS domain node) could be involved in subscriber activities even when the service itself (like a voice call or an SMS) used the CS domain. This happens because a subscriber can be simultaneously attached to both the CS and PS domains. The SGSN, having knowledge of the subscriber's PS attachment and location, is a logical point to generate a CDR for certain CS services like SMS to ensure charging data is correlated with the subscriber's overall session. It solved the problem of attributing the SMS event to the correct subscriber and collecting consistent data (like location, timestamps, and serving nodes) across all operators for interoperability of charging systems.

Furthermore, it enabled flexible billing models. By providing a standardized record with fields for time, destination (implicit in the SMSC address), and location, operators could implement complex tariff plans, such as charging different rates for international SMS, off-peak discounts, or bundled SMS packages. The S-SMO-CDR, as part of the broader 3GPP offline charging framework, allowed operators to reliably collect usage data from network elements, transmit it to a central billing system, and generate itemized bills for subscribers. Its definition was a critical step in commercializing mobile messaging and ensuring the network infrastructure could support its billing transparently and accurately.