plex and transport both 2048-kbps based hier- archy (E1, E2, E3, etc.) and 1544-kbps based hierarchy (T1, T2, T3, etc.), and these circuits are also used to carry timing (sometimes without data). Therefore, it is not uncommon for a PRC/PRS to have E1/T1 outputs available or as an option, although it is much more common on SSU/BITS equipment. SSU and BITS Chapter 2 already introduced the basic concept of the dedicated timing distribution network de- ployed solely to synchronize the SDH/SONET network. Figure 3-12 in the earlier section "Fre- quency Clocks" showed the SSU/BITS network as a clock type with a middle level of performance between the PRC/PRS and the "normal" SEC clocks. These SSU/BITS devices use a PRC/PRS as an input source of frequency or may be combined with a PRC/PRS in the same equipment. Besides performance, the other place where they differ from a PRC/PRS is that SSU/BITS devices are designed to distribute frequency signals to a very large number of downstream clocks. Fre- quently, they will have several extension shelves with output ports to distribute their signal. Beyond the 2-MHz option covered in the previous section, these output signals will be normal E1 or T1 signals. The format and range of connector types for SDH/SONET networks is beyond the scope of this book, but modern systems use 100-Ohm (T1) or 120-Ohm (E1) twisted-pair cable with an RJ-45 style connector (technically, the plug is named RJ-48c). Obviously, the future for this type of equipment is limited with physical distribution of frequency going to packet systems (PTP), SyncE, or optical systems like OTN; however, many service pro- viders still maintain a large and widespread network of SSU/BITS equipment. 3.3.1 Source of Frequency, Phase, and Time: PRTC The PRTC differs from the PRC/PRS in that it is also concerned about providing phase and time of day as well as frequency. Like the PRC/PRS, the PRTC provides reference timing signals to other clocks in the network hierarchy. Unlike most PRC/PRS systems, especially those based 