 a clock from the wider network, for example, a clock recovered from a SyncE or SONET link and low-pass filters it to produce a cleaner output signal. The low-pass filtering function essentially removes any noise in the input signal above a certain frequency-the PLL bandwidth. In this way, any noise intro- duced to the signal by the network is removed. In other words, the recovered clock is cleaned up. How much noise to remove depends on the bandwidth of the PLL-the lower the bandwidth, the more noise is removed. An example of this can be seen in Fig. 8-29. However, this filtering does not come without a price. The PLL is not really removing the input noise, but rather replacing it with noise from the local clock, which will typically be some form of quartz-based oscillator. Therefore the lower the bandwidth of the PLL, the higher the stability of the local oscillator needs to be, otherwise the noise of the oscillator itself will have a significant impact on the filtered clock. For this reason, PLLs that are used for filtering network noise, as opposed to, for example, noise arising between components within a piece of equipment, are typically used with a temperature compensated or stabilized oscillator, such as a TCXO or OCXO. These oscillators not only have much better stability (and hence lower noise) than a simple crystal oscillator but also have a price many times higher-several tens of dollars in the case of an OCXO. Time-synchronization systems As we move toward time synchronization using PTP, potentially in conjunction with SyncE, the architecture of the sync subsystem becomes considerably more complex. This complexity arises from the fact that PTP uses packets transported over the data-plane for synchroniza- tion, as opposed to physical signals, meaning that there is now the requirement to add data 300 Open Radio Access Network (O-RAN) Systems Architecture and Design processing capability to the sync subsystem-something that typically requires a mixture of both hardware and software. Xoxo shows an exampl