he details such as selecting which PTP master to use on the network. The local side output from the PTP stack is a series of t1-t4 timestamps extracted through the PTP message flow as outlined previously. Network Hardware Software Clock and XXXXX Stack Buffer Application Framer Recovery Timestamping points: [1] Media (impossible) [2] PHY Progressively worse [3] MAC performance [4] Driver [5] Application FIGURE 8-30 Timestamping points. Chapter 8 Synchronization in open radio access networks 301 Time recover servo-Yet another software function, the importance of which is often overlooked. The function of the servo is to filter the time information contained within the flow of timestamps from the stack and use this to control the alignment of the local physical time counter. How this is done, and how complex the servo needs to be, depends on the amount of PDV on the network that must be filtered out. For FTS networks with very low PDV, the servo is little more than a low-pass filter-it can be thought of as a software PLL, implementing a low-pass filtering function for data in much the same way a PLL device does for a physical clock. However, for PTS networks, which will, in general, have much higher PDV, a much more sophisticated servo implementation is needed. This class of servo often employs advanced proprietary (and closely guarded by their vendors) packet-selection algorithms to pick and choose dynamically which timestamps to process and which to ignore. A good servo of this type can often achieve better than two orders of magnitude filtering. For example, if 100 us of PDV was present (which is a realistic figure for some PTS networks), the servo would still be able to align the time within the local device to within 1 us. Fig. 8-31 shows the vastly different performance between a simple servo (in this case from the Linux ptp41 package) and a commercial proprietary servo, when used in a PTS network. Digitally or numerically controlled oscillator (DCO or NCO)-A hardware component that converts the output from t