CMS data to surface transportation architecture

The front-end electronics of the CMS experiment will be read out in parallel into approximately 650 modules which will be located in the underground counting room. The data read out will then be transported over a distance of ~200 m to the surface counting room where they will be received into deep buffers, the Readout Units. The latter also provide the first step in the CMS event building process, by combining the data from multiple detector data sources into larger-size (~16 kB) data fragments. The second and final event-building step merges 64 such super-fragments into a full event. The first stage of the Event Builder, referred to as the Data to Surface (D2S) system is structured in a way to allow for a modular and scalable DAQ system whose performance can grow with the increasing luminosity of the LHC