use they are stable sources of frequency (based on fundamental physics of the cesium atom). GNSS receivers normally include a good-quality (stable) oscillator to be able to keep an accurate frequency whenever it temporarily loses the signal from space (a process known as holdover). A typical SSU/BITS device also includes hardware that allows it to maintain a clock signal with higher accuracy for a holdover period should it ever lose its connection to the PRC/PRS source. En- gineers typically implement the SSU/BITS device with a piece of hardware called a digital phase- locked loop (DPLL) driven by a very stable rubidium or high-quality quartz oscillator. So, the abil- ity of the SSU/BITS device to maintain frequency during holdover is better than the normal SDH/SONET transport node, but not as good as a PRC/PRS. "Chapter 6. Physical Frequency Synchronization", and "Chapter 7. Precision Time Protocol", ex- plore sources of frequency and time in more detail. If the frequency on either end of a data link is not accurately aligned with the other, then the receiv- ing device will have a lower or higher frequency than the transmitting device. When this happens, the receiver cannot read and store the data in its buffer at the same rate as the transmitter sends it. Eventually the receiver either will be waiting with an empty buffer for data that has not yet arrived (known as an underflow) or will not be able to store it quickly enough in its buffer and will lose data (known as an overflow). These events lead to loss of quality on the data circuits and show up in what network engineers know as slips, as shown in Figure 2-2. Incoming data Outgoing data Figure 2-2 Slips in a TDM Network The slip rate is proportional to the frequency differences between transmitter and receiver, SO if one measures the differences between the signals, it is possible to calculate the expected number of slips over a time period. Highly stable, closely aligned frequency sources might only result in a slip once every several months, whereas netwo