SGSN (IP-CAN bearer) generated – Charging Data Record

Description

The S-CDR (SGSN generated CDR for IP-CAN bearer) is a standardized data record defined by 3GPP for offline charging in the packet-switched core network. It is generated by the Serving GPRS Support Node (SGSN) during a user's data session. The S-CDR specifically accounts for the usage of an IP Connectivity Access Network (IP-CAN) bearer, which is the logical pipe providing IP connectivity between the User Equipment (UE) and the Packet Data Network (PDN). The creation and closure of an S-CDR are triggered by specific events in the bearer's lifecycle, such as bearer establishment, modification (e.g., QoS change), and termination. During the session, the SGSN collects charging information in real-time, which is then formatted into a CDR and transferred to the Charging Gateway Function (CGF) via the Ga interface.

The structure of the S-CDR is highly detailed and composed of numerous fields as per 3GPP TS 32.251 and TS 32.298. Key data fields include the Record Sequence Number for ordering, the IMSI and IMEI for subscriber and device identification, the SGSN addresses, the Access Point Name (APN) used, and the PDP context identifiers. Crucially, it contains usage metrics: the data volume transferred (separated into uplink and downlink), the session duration, and the QoS parameters (such as QoS Class Identifier - QCI, Allocation and Retention Priority - ARP, and Guaranteed/ Maximum Bit Rate) that were active during the bearer's lifetime. The record also timestamps all significant events and may include location information (like Routing Area and Cell Global Identity) and cause codes for session termination.

In the broader charging architecture, the S-CDR works in conjunction with other CDR types. For a typical data session, an S-CDR from the SGSN and a G-CDR from the GGSN/PGW are generated. The S-CDR focuses on the access network and radio resource usage managed by the SGSN, while the G-CDR focuses on external data network connectivity and volume. The Charging Data Function (CDF) within the SGSN assembles these records. They are then sent via the Charging Gateway Function (CGF) to the Billing Domain (BD) for mediation, rating, and ultimately, invoice generation. The S-CDR is therefore an atomic unit of billing information, providing the granular detail necessary for complex tariff models, such as tiered data plans, time-of-day charging, or QoS-differentiated billing.

Purpose & Motivation

The S-CDR was created to enable accurate and standardized offline (post-paid) charging for packet-switched mobile data services in 2.5G (GPRS) and 3G (UMTS) networks. Prior to its standardization, billing for data services was fragmented and proprietary. The S-CDR solves the critical business problem of measuring and recording resource consumption in the packet core in a vendor-interoperable way. It provides operators with a reliable audit trail for usage that occurs under the purview of the SGSN, which manages mobility and session control within the access network.

Its introduction in 3GPP Release 6 was part of a broader effort to mature the packet core charging framework, aligning with the growth of mobile data beyond simple WAP browsing. The S-CDR addresses the need to charge not just for raw data volume, but also for the quality of service delivered. By including detailed QoS parameters, it enables new business models where premium services (like video streaming) could be billed differently from best-effort web browsing. Furthermore, its separation from the G-CDR allows for more granular fault isolation and cost allocation between the radio access/core network functions and the gateway functions connecting to the internet. This detailed accounting was essential for the commercial rollout of flat-rate and bundled data plans, as it gave operators the tools to understand network usage patterns and cost structures.