required equipment uptime. SSU/BITS systems extract the timing information from an incoming signal, which could be a T1, E1, 2-MHz, or 64-kbps signal from a local or remote dedicated timing source. This timing signal then becomes the master timing reference for all downstream network elements. Figure 6-2 shows a typical BITS unit and a few potential connections using T1 as distribution to different equipment in the building. Central office Media gateways Control systems Core routers Broadband ATM switch aggregator DSLAM PON OLTs Class 5 switch Figure 6-2 Typical BITS Equipment Figure 6-2 shows connections that carry only timing information to different network elements. This is usually referred to as external timing, where the connections carry only timing information and no user data. So, when a piece of digital equipment is timed externally, it means the equipment supports a dedicated interface that is designed to receive a timing signal from the timing distribution network. Figure 6-3 shows how these devices are connected to a master or centralized clock, which could be either a BITS or SSU system. Central clock Equipment Equipment Information signal Timing signal Figure 6-3 Central Clock Distributing Timing to Equipment (Figure 6-2 from ITU-T G.703) This network equipment can then use the timing signal to both regulate the speed of data commu- nications and carry the timing information multiplexed with user/customer data to downstream devices. This mode where user traffic is mixed with a timing signal is referred to as line timing, and because these nodes also transport data, they are considered part of the transport network. Such devices are referred to as SDH or synchronous equipment clocks (SEC) or network elements (NE). Later in this chapter and in the section "Definitions, Architecture, and Requirements" in "Chapter 8. ITU-T Timing Recommendations", you will see that over time, the definition of SEC has evolved to include other types of clocks as well. Note that this equipment is different from the SSU/B