The central clock architecture previously described applies to each chassis of the 7950 XRS-40. There is a central clock located on each of the CPMs present in the extension chassis. However, there is no configuration for the central clocks on the CPMs of the extension chassis. The central clocks only use the BITS input ports of the extension chassis for their input reference. It is assumed that the quality of the reference provided into the BITS input ports of the extension chassis CPMs is equal to the quality of the Master chassis central clocks. See the Installation Guide for appropriate physical cabling to support this architecture.