 all the installed equipment, including digital switching exchanges, digital cross-connects, and terminal equipment. Such a clock for the complete building (referred to as a central office or CO) is known as the building timing in- tegrated supply (BITS) in the ANSI standards and synchronization supply units (SSU) or standalone synchronization equipment (SASE) in the ITU-T and ETSI standards. With recent advances, the ITU-T has incorporated both ANSI and ETSI considerations, SO the rest of this chapter uses the BITS and SSU terms interchangeably. Figure 6-1 shows the distribution of a BITS or SSU timing circuit into a building through either a GPS receiver or a signal from an atomic clock, such as a primary reference clock (PRC). Without a centralized source such as SSU/BITS, you can potentially have many different timing sources within the same building because of the number of synchronous links that might exist throughout a building. GPS timing from satelite Timing Internal facility circuit to timing circuits atomic clock External timing circuit to different facility Figure 6-1 BITS or SSU Timing Distribution The SSU/BITS system operates by serving as a clock synchronization receiver that extracts a stable timing signal from a reference clock and then distributes it internally within the office or building to all equipment requiring a traceable frequency reference. Thus, the BITS node effectively becomes the master timing supply for all deployed equipment requiring frequency synchronization within a network location. It can also provide external timing links to structures within the same campus, such as adjacent buildings. Most of these connections are deployed as T1 or E1 circuits, and many vendors carry these signals by using either coaxial or unshielded twisted-pair (UTP) cabling. Typically, the BITS system has multiple timing reference inputs, which act as redundant timing sources in case the primary timing source is lost. During such failures, the BITS system continues to supply timing to help achieve the 