ALICE Data Acquisition (DAQ)

ALICE (A Large Ion Collider Experiment) is the heavy-ion detector studying the physics of strongly interacting matter and the quark-gluon plasma at the CERN LHC.

The detector includes high resolution tracking (silicon detectors, large time-projection chamber), particle identification, and triggering elements. It features two large magnets, a main solenoid and a dipole on the Muon arm. ALICE consists of 19 sub-detectors, being able to take data independently (standalone operation) or in global partitions (set of sub-detectors running together).

 

The LHC Point 2 hosts the ALICE Detector. The ALICE RunControl center, the working rooms, and the DAQ counting room (CR1) are all located at LHC Point 2.

 

The ALICE Data-Acquisition system (DAQ) handles the data flow from the sub-detector electronics to the archiving on tape. A first layer of computers, the Local Data Concentrators (LDCs), reads out the event fragments from the optical Detector Data Links (DDLs). The DDLs are point to point links running at maximum 6Gb/s and up to twelve of them can be connected to the same LDC. Several LDCs may be needed to collect the data from a single sub-detector. A typical data rate from the detectors to the LDCs can go above 13 GB/s.

The event fragments aggregated in sub-events are then transferred to a second layer of computers, the Global Data Collectors (GDCs), in charge of performing the event building. The same GDC receives all the fragments of a given event, and assembles them in a full event, which is then recorded to a Transient Data Storage (TDS) before being migrated to the Permanent Data Storage (PDS) and published via the Grid.

One peculiarity of the ALICE DAQ system is its large bandwidth local data storage located at the experimental area which provides an autonomy of several hours. It is organized around a FibreChannel Storage Area Network (SAN) interconnecting the servers (15 GDCs), the storage (10 storage arrays), and the data movers (10 TDSMs). This SAN has evolved since its initial installation and is now constituted by two 96-port enterprises switches. The servers and the storage equipment are connected to the switches by Fibre Channel links of 16 and 8 Gbit/s respectively.