Digital optical links will be used as part of the CMS Tracker control system to transmit the LHC clock and CMS Level-1 trigger at 80Mbit/s, as well control signals at 40Mbit/s to and from the Tracker front-end.

The links form part of the 'control rings', that start and end at the Front-end Controllers (FECs) in the counting room. The boundary of the optical link system at the front-end in the Tracker is the electrical interface of the digital optohybrid (DOH), where signals are transferred to and from a copper ring of CCU modules which communicate with the various front-end ASICs. At the back-end the boundary of the optical link is at the transceiver (TRx) on the FEC.

There are 8 optical link channels per control ring. Four channels are required to communicate the TTC and control signals. The four other links are part of a redundancy scheme in case of component failures in the control ring.

There will be 320 redundant rings in the Silicon Tracker, which requires 2560 optical links. A similar quantity of links will be required for other CMS sub-detectors including ECAL, Preshower and Pixels.

The philosophy of the digital link project has been to use as many of the same component parts as possible from the much larger analogue optical link system. The resources and effort required for link development have therefore been minimized and we have avoided the need to qualify a new set of front-end components.