Serving Gateway Call Detail Record

Description

A Serving Gateway Call Detail Record (SGW-CDR) is a specific type of Charging Data Record (CDR) generated by the Serving Gateway (SGW) function within a 3GPP network. Its primary purpose is to log charging-related information for a user's IP Connectivity Access Network (IP-CAN) bearer session. The SGW, acting as a Charging Trigger Function (CTF), collects usage data and formats it into an SGW-CDR according to the rules provided by the Online Charging System (OCS) or Offline Charging System (OFCS). This record is then transferred to a Charging Data Function (CDF) or directly to a billing system for further processing. The creation and content of the SGW-CDR are standardized in 3GPP TS 32.251, which defines the charging architecture and principles.

The SGW-CDR contains a comprehensive set of fields that describe the bearer session. Key data points include the IMSI and MSISDN of the subscriber, the SGW and PGW addresses used for the session, the Access Point Name (APN), and the Radio Access Technology (RAT) type (e.g., LTE, NR). Most importantly, it records quantifiable usage metrics: the total volume of uplink and downlink user data transferred, the session duration (start and stop times), and QoS parameters such as the Allocation and Retention Priority (ARP) and the Guaranteed Bit Rate (GBR) if applicable. The record also includes charging identifiers, cause for session termination, and any applied charging characteristics.

The generation of an SGW-CDR is triggered by specific events during the bearer's lifecycle. These triggers include the initial establishment of the bearer, any modification of the bearer's QoS parameters (which may cause an intermediate CDR to be closed and a new one opened), and the final termination of the bearer session. For long-lived sessions, to prevent data loss and manage record size, the SGW may generate multiple partial SGW-CDRs based on time limits, volume limits, or tariff time changes. These partial records are correlated using a common Record Sequence Number. The SGW-CDR works in tandem with CDRs from other network functions, like the PGW-CDR, to provide a complete picture of a user's data session for accurate billing and network analytics.

Purpose & Motivation

The SGW-CDR exists to enable accurate and reliable charging for mobile data services. In the early days of mobile data, simple billing models were possible, but as services evolved with the introduction of the Evolved Packet System (EPS) in 3GPP Release 8, more granular and flexible charging became a commercial necessity. Operators needed to bill based on data volume, session time, service type (via APN), and quality of service, requiring detailed records from the network nodes that handle user traffic.

The SGW-CDR addresses the problem of attributing resource usage to a specific subscriber and service. The SGW is a pivotal node that sees all user plane traffic for a UE within its serving area, making it an ideal point for measuring per-bearer consumption. Before standardized CDR formats, proprietary logging made interoperability between network equipment and billing systems difficult. The standardization of the SGW-CDR in TS 32.251 ensured multi-vendor compatibility and allowed for the creation of consolidated billing systems that could process records from different network elements. It supports both offline charging (post-paid billing) and, through interaction with the OCS, online charging (pre-paid/real-time credit control). This detailed accounting is fundamental to the business models of mobile network operators.

Key Features

• Records IP-CAN bearer-specific usage data (volume, duration)
• Contains subscriber identifiers (IMSI, MSISDN) and APN
• Logs QoS parameters like ARP and GBR
• Supports partial CDR generation for long sessions
• Includes RAT type and user location information
• Standardized format defined in 3GPP TS 32.251 for interoperability

Evolution Across Releases

Defining Specifications