Timing and synchronization in the LHC experiments

Data synchronization is an important aspect in the operation of the trigger and readout systems of the LHC experiments. In this paper we discuss the current understanding of the problem as developed by the LHC collaborations. Some of the main synchronization issues, namely, the assignment of bunch crossing to data, the overall alignment of the trigger system and the synchronization of the front-end readout pipelines, are covered in some detail. We discuss the tools required for distribution of timing and control signals and for the fast collection of front-end status, as well as the functions performed by the central trigger control unit. Methods to determine and monitor the timing parameters in the experiments, the sources of synchronization losses and the recovery procedures are briefly surveyed.