rk devices running without any accurate source of frequency may generate multiple slips per second. Closely aligning the frequency on every node in the network is paramount for a quality network. Slips are detrimental to the quality of the user experience, resulting in problems such as clicks in audio streams, poor-quality video, freezes, and the like. Although the use of TDM is increasingly declining, building a source of frequency and distributing it throughout a network is still very relev- ant to many use cases in modern communications. 2.1.1 Legacy Mobile Synchronization-Frequency In earlier generations of mobile telecoms (approximately up to 4G/LTE), the radio equipment at the cell tower only required frequency synchronization. Fortunately, for early 2G systems, this was quite easy to provide because the cell tower was connected to the core mobile network by TDM cir- cuits (such as E1 or T1). Because the back-haul circuits were synchronous, the radio could recover a stable source of frequency using the signal from the incoming line. Figure 2-3 shows how the frequency synchronization of those networks worked. LIIII SDH or SONET TDM circuits E3, T3, or such as E1 or T1 STM-1 Central mobile core Figure 2-3 Frequency Synchronization Using TDM Circuits Because the frequency of the circuit back-hauling the traffic to/from the cell tower was coming from accurate sources in the SDH/SONET network, it was an accurate source of frequency for the equipment in the cell tower. Frequency had one additional important use besides the need to syn- chronize the back-haul circuits. The cell site equipment required frequency synchronization because its inbuilt oscillators were the source for the frequency being transmitted by the radio. This meant that if the oscillator in the radio drifted off its frequency, the radio was transmitting on the wrong frequency and was either interfering with neighboring cells, or transmitting its signal outside its licensed radio bands As a result, frequency synchronization was used to "tune" the