Answers
What Is a CDR in Telecom Billing?
A CDR is a record of a single chargeable event on a telecom network: a call, an SMS, a data session or part of one. It captures who used the service, when, for how long or how much, and from where. It does not contain the content of the communication. Billing, reconciliation, fraud checks and usage analytics all start from CDRs.
The acronym has two expansions. Most people say call detail record, from the era when voice calls were the main billable event. 3GPP standards use charging data record, which covers data sessions, SMS, IMS services and more. In practice they mean the same thing. You will also see xDR, EDR (event detail record) and UDR (usage detail record) used for the same idea in other contexts.
What a CDR contains
The exact fields depend on the record type and are specified in 3GPP TS 32.298. Typical fields include:
Why the sequence number matters
A long data session does not produce one CDR. The network closes a partial record at set time or volume limits and opens a new one. A day-long IoT session can generate dozens of partial records that must be stitched back together before anyone can price it correctly.
| Field | What it tells you |
|---|---|
| Record type | Voice, SMS, packet data, IMS and so on |
| Subscriber identifiers | IMSI (the SIM identity) and MSISDN (the phone number) |
| Other party | Called or calling number, for voice and SMS |
| Start time and duration | When the event began and how long it lasted |
| Data volume | Bytes uplink and downlink, for data sessions |
| Access point or service identifier | Which APN or service the usage belongs to, used to apply the right tariff |
| Location | Serving cell or network node at the time of use |
| Serving node | Which network element produced the record |
| Cause for record closing | Why the record ended: session end, time limit, volume limit, handover |
| Sequence number | Position of this record within a series, for long sessions |
How CDRs move from the network to billing
In the 3GPP offline charging architecture:
- A network element, such as a packet gateway, detects chargeable usage and sends charging events.
- A Charging Data Function turns those events into CDRs.
- A Charging Gateway Function collects CDRs, validates and consolidates them, and writes them into files.
- The files are transferred to the billing domain, which may be a billing system or a mediation layer in front of it.
CDR file format and offline vs online charging
The file format and transfer mechanism are specified in 3GPP TS 32.297. Each file carries a header with a file sequence number, which lets the receiving side detect missing files. The records themselves are commonly encoded in ASN.1, a compact binary format that needs decoding before it is readable.
CDRs are mainly an offline charging mechanism. Networks that charge usage in real time through an online charging system can also produce CDRs for the same usage, and most operators do, because billing detail, disputes and reconciliation all need a record after the event.
How MVNOs get and use CDRs
An MVNO does not run the radio network, so it does not generate most CDRs itself. The host network operator produces them and delivers them to the MVNO, usually as files over SFTP on a fixed schedule, or through an API feed. What the MVNO receives depends on the wholesale agreement: format, frequency, which record types and how quickly.
MVNOs use those CDRs to:
- Bill subscribers. Especially postpaid, where usage is priced and invoiced after the event.
- Check the wholesale invoice. The host bills the MVNO for usage. The MVNO's own CDR totals are the evidence for agreeing or disputing that invoice.
- Spot problems early. Usage spikes, unexpected roaming, or SIMs that should be inactive but are still generating traffic.
- Answer customer disputes. A CDR is the record of what happened on the account.
CDRs are customer data too
Because CDRs describe individual customers' calls and usage, they are also customer data. In the US, call detail is treated as customer proprietary network information, with rules on who can access it and how.
Related Pages
FAQs
Does a CDR record what was said or sent?
No. A CDR holds metadata: parties, time, duration, volume, location. The content of a call, message or data session is not in it.
How often are CDRs produced?
For short events like SMS, one record per event. For calls and data sessions, at the end of the session, plus partial records at intervals during long sessions. How often files reach an MVNO depends on the host network, anywhere from near real time to daily.
What is the difference between a CDR and a TAP file?
A CDR is produced inside one network. When a subscriber uses another operator's network while roaming, that visited network sends the home operator usage in TAP files, a GSMA-defined format for exchanging roaming records between operators for settlement.