Call Detail Records

Telephone exchanges generate so called Call Detail Records (CDRs) which contain detailed information about calls originating from, terminating at or passing through the exchange. Not surprisingly CDRs are used for billing.

CDR Formats

The format in which CDRs are provided varies and is often configurable. Traditionally the generating and handling of CDRs has been known in the US as Automatic Message Accounting or AMA, a system that goes back to the 1940s. Still today, exchanges for use in North America generate CDRs in Bellcore AMA Format or BAF.

Today, PBX and softswitches mostly generate CDRs either in comma delimited formats or they write directly into a database.

CDR Processing (aka Mediation)

In conventional telephone networks, the systems generating CDRs (so called network elements) and the systems processing CDRs (so called operations support systems, or OSS) are separate entities. For this reason, CDRs must first be collected from the network elements and passed on to OSS, a process known as CDR collection or simply call collection.

When the CDRs have been collected (historically by shipping paper tape and later magnetic tape, nowadays by polling and ftp) they must be checked (validation), reformatted (normalisation) and consolidated for further processing.

The combined process of collection, validation, normalisation and consolidation is also referred to as mediation. This can be a very complex process and it is not uncommon that telcos lose a significant amount of revenue due to errors. At the same time the amount of CDRs to be processed at a large telco can be absolutely mindboggling. Since CDRs represent the telco's revenues and the amount of CDRs is tremendous, most larger telcos run their mediation on the most powerful massively parallel and fault tolerant hardware there is known to mankind: The Tandem Himalaya running a proprietary OS called NonStop Kernel (NSK aka Guardian), which in this case is not an overzealous marketing name but represents the true nature of the system. The Himalaya is almost never shut down. All maintenance is done under load, except OS upgrades. The massive parallel architecture is based on CPU pairs which are lockstepped: Any CPU pair that disagrees on the result of a single instruction is instantly removed from the resource pool and shut down. Everything in a Himalaya is hotswappable - Everything! Needless to say, Himalayas cost a fortune. If you have to ask the price, you can't afford one.

The important lesson here is this: CDRs are extremely important. Take very very good care of them!

In the world of VoIP and softswitches where the CDRs are often generated on the same system where they are processed, the chain of mediation is significantly less complex. Even with multiple geographically diverse CDR sources, fast packet switched networks, ubiquitous file sharing facilities and database resources, mediation and billing is still less of a challenge than in the conventional telephone world.

Rating and Billing

Once CDRs have been collected, validated, normalised and consolidated, a charge is calculated for each call identified by the details in one or more CDRs. This process is known as rating and not surprisingly it is carried out by a rating engine. The rating engine can be part of a billing system or it can be a preliminary process outside of the billing system. In any event, the billing system will add up the charges determined during rating for each account, calculate balances and generate invoices. The CDRs then finally end up on the invoice in form of an itemised bill.

See also

• CDR Mediation
• CDRTool

• Asterisk Billing