h clock synchronizes with its peers to determine the best quality. For example, in a mutual synchronization approach, there is no master clock concept, and every clock contributes to maintaining the network timing. a) Master-Slave Synchronization b) Mutual Synchronization Figure 3-2 Synchronous Clocks As covered in Chapter 2, in a synchronous network, frequency synchronization is what allows two connected systems to run at the common synchronized frequency (or bit rate). Legacy time-division multiplexing (TDM) networks use frequency synchronization to ensure that the rate of transmission of the bits is aligned to avoid frame loss or slips between nodes. Building an efficient yet affordable transport network is based on distributing frequency from a few expensive devices (with high-qual- ity components such as oscillators) to masses of devices made using more affordable components. The section "Timing Distribution Network" later in this chapter provides further details. 3.1.1 Defining Frequency Anything that rotates, swings, or ticks with a regular cycle time can be used to define a time inter- val. If the motion is regular and the interval of repetition remains stable, it forms the basis for build- ing a clock. There are device movements or other phenomena that occur naturally at a specific fre- quency, and a device (or electrical part) that does this is called a resonator. In electronics, when an energy supply circuit is added to a resonator, it continues its cyclical behavior indefinitely, and that device becomes an oscillator. If the length of time that one oscillation takes (the period) is known and remains stable, a device can accurately measure the length of a time interval by simply counting the ticks. This is the reason that timing engineers say that a clock is simply an oscillator plus a counter. By simply attaching a circuit to count and record the number of rotations, swings, or oscillations, one can build a clock and use it to measure time. It follows that for measurements of very small time intervals