A modular data processing system (200) for processing communication session data from a cellular mobile communications system (210) comprises at least two autonomous data processing units (340). The autonomous data processing units (340) are arranged in parallel, and are operable simultaneously. Each data processing unit (340) receives and processes raw communication session data from an associated set of controllers of the cellular mobile communications system (210). At least one data collection subsystem (215) supplies the raw communication session data to the at least two autonomous data processing units (340), for calls made within the coverage area of the set of controllers associated with the data processing unit. Processed data may be available more rapidly with the invention. The invention is scalable, and the system may expand as a cellular mobile communications system grows.