Prototype Results of an Optical Communication Network for the CBM DAQ-System

For the CBM Data Acquisition (DAQ) System a specific protocol has been developed considering the special demands of limited area for hardware, radiation tolerance, support for different types of network traffic and the need of synchronization mechanisms. Within the CBM network only a single bidirectional fiber link is used for the three network traffic classes: Data Transport Messages (DTM), Detector Control Messages (DCM), and Deterministic Latency Messages (DLM). DTMs are optimized for data transfer from the detector frontend to the backend computing cluster. The DCM class provides a higher fault tolerance level to guarantee the delivery of control messages. DLMs are used for precise time synchronization. The CBM DAQ protocol fulfills all requirements mentioned before and was implemented for the usage within the Field Programmable Gate Arrays (FPGA) of all developed readout chain boards. The chain, developed by several groups, consists of Read-Out-Controller (ROC) boards interfacing and initializing the front-end electronics and building link-packets, Data-Combiner-Boards (DCB) combining data streams from multiple ROCs, and finally ActiveBuffer-Boards (ABB) for receiving, buffering and reorganizing the data. The ABB also writes the data via PCI express to the memory of the back-end node.