Apr 1, 2000
An implementation of a reliable message broadcast
for the CMS event builder system
The CMS DAQ system utilizes a dedicated network to distribute
commands to all the DAQ units responsible for reading out the detector
Front-Ends. This network must have low latency ($\sim ms$) since it must
carry the signals indicating the presence of a new event (maximum
rate of 100 kHz) in the Front-Ends which in turn do not have deep
buffering capability. A reliable broadcast functionality
is necessary for this.
Although many implementations of reliable broadcast/multicast
protocols exist and are available, the requirements of total reliability
and short latency have kept us from using any of these.
A NACK-based reliable broadcast protocol with Forward-Error
correction was implemented on a system of PCs connected
FastEthernet along with software for the CMS event builder system development.
The implementation was tested in various system configurations
and proved its technical feasibility. Several modifications
to increase the system performance and stability are also introduced
and are described here.