o account for the varia- tions of each individual stream (a process called bit stuffing) and combined with other bits (framing bits) to form a DS2 stream. Bit stuffing is used again to multiplex up to DS3. The DS1s are neither visible nor accessible within a DS3 frame. DS3s are multiplexed up to higher rates in the same manner. At the higher asynchronous rate, they cannot be accessed without demultiplexing.6 Chapter 3 In a synchronous system, such as SONET/SDH, the average frequency of all clocks in the system will be the same (synchronous) or nearly the same (plesiochronous). Every clock can be traced back to a highly stable reference supply (see Figure 3-16). Hence, the STS-1 rate remains at a nominal 51.84 Mbps, enabling many synchronous STS-1 signals to be stacked together when multiplexed without any bit stuffing. Pointers accommodate differ- ences in the reference source frequencies and phase wander, and prevent frequency differences during synchronization failures. This implies that the STS-1s are easily accessed at higher STS-n rates. Low-speed synchronous VT signals are also simple to interleave and transport at higher rates; sin- gle-step multiplexing up to STS-1 requires no bit stuffing and VTs are eas- ily accessed. A SONET network must be able to derive its timing from a Stratum 3 or higher clock. The internal clock of a SONET network element may derive its timing signal from a Building-Integrated Timing Supply (BITS) used by voice-switching systems and other equipment at the CO or service node. A SONET NE taking input from BITS may act as a master clock for other SONET nodes, providing timing on its outgoing OC-n signal. Other SONET nodes will operate in a slave mode called loop timing and have their inter- nal clocks timed by the incoming OC-n signal. Switching Figure 3-16 Clock distribution. External Reference Timing (Master Mode) Internal Clock Switching SONET Internal Clock Timing for SONET Outgoing OC-n Incoming OC-n provides timing for internal clock and outgoing OC-n. SONET SONET Internal Clo