hat do not need to work in a contiguous carrier aggregation environment, with its more stringent sync requirements, O-RAN defines two classes of O-RU with regards to synchronization capability: A standard RU-This allows for the O-RU itself to introduce up to 80 ns of additional error between the network connection and the antenna connector. This roughly corresponds to what ITU-T refers to as a Class B slave clock. An enhanced eRU-This reduces the O-RU error budget down to 35 ns. This performance level is more comparable to an ITU-T Class B slave clock. Fig. 8-27 shows how these numbers work into the overall budget for the different use categories. Note that no Category B (noncontiguous CA) operation, either an RU or and eRU can be used. However, to support Category A (contiguous CA) operation, only an eRU has the performance needed to meet the requirements. Therefore when designing an O-RU con- sideration needs to be given as to what type of application it will be used for. Sync solution implementation So, within an O-DU or O-DU, what does the actual sync subsystem implementation look like? Frequency-only systems In the days when only frequency synchronization was a requirement, the key component to achieve this was the phase-locked loop (PLL), as shown in Fig. 8-28. Fronthaul Link Fronthaul Link Fronthaul Link (PtP LLS C1 (PtP LLS C1 -80ns (PtP LLS C1 through FHS LLS C2) through FHS LLS C2) through FHS LLS C2) Category A (Contiguous CA) Category B (Non-Contiguous CA) Option 1 Category B (Non-Contiguous CA) Option 2 FIGURE 8-27 Overall fronthaul budgets. Reference Phase Error Voltage Controlled Output Input Fin Frequency Detector Signal Filter Oscillator Nx Fin Feedback Input F Divide-by-N Counter FIGURE 8-28 A PLL. PLL, Phase-locked loop. Chapter 8 Synchronization in open radio access networks 299 Input Output Low frequency noise Noise Frequency High frequency noise passes un-filtered is filtered FIGURE 8-29 The filtering effect of a PLL. PLL, Phase-locked loop. Conceptually, a PLL is relatively simple. It takes