The value of tSyncIfTimingAdmBITSOutSource specifies the source of the BITS (Building Integrated Timing Supply) output. In an XRS-40 system, the extension chassis is synchronized with the master chassis via the BITS ports. Two modes of operation are permitted: 1) The BITS output ports of the master chassis CPMs are connected to the BITS input ports of the extension chassis CPMs. In this mode, tSyncIfTimingAdmBITSOutSource should be set to 'internalClock(2)', to ensure that a valid clock signal is transmitted on the master chassis's CPMs' BITS output ports. 2) The BITS input ports of all 4 CPMs are connected to external BITS equipment. In this mode, the customer may connect the master chassis's BITS output ports to the external BITS equipment's input ports. This allows for recovered line references to be provided as a frequency source into the external BITS equipment following the inter-site synchronization mode of SDH/SONET. In this case, tSyncIfTimingAdmBITSOutSource should be set to 'lineRef(1)'.