Options for LHC safety-alarm transmission

The LHC safety-alarm system needs, for reliability reasons, two different media to transmit the alarms to the CERN Safety Control Room. This is a requirement of the French regulatory authority (Direction de Sûreté des Installations Nucléaires de Base). This paper examines the safety systems currently in use at CERN, with particular emphasis on the methods used for transmission of alarm information across the site. The general requirements for transmission of alarms are described. Several options for transmission of LHC safety-alarm information are compared: fieldbuses, telecommunications networks with analogue voice or ISDN lines, the time-division multiplex system, and packet switching 'mesh' networks. The reliability of each of these solutions is estimated, and different topologies for physical links described and compared. It is shown that options are available which give an accepted level of reliability, obviating the need for hard-wired connections.