receive information about the QL and traceability to the signal source. For further information, see the section "Clock Quality Traceability" in Chapter 3. Not all types of frequency transport support the ability to carry information about the clock quality. For example, the pure frequency signals such as 10 MHz and 2 MHz do not have any method of signaling clock quality or traceability. To solve this shortcoming, the engineer can configure the net- work elements with a QL to assume for the input signal whenever it is available. That way, if the engineer configures a 10-MHz input to have a QL of QL-PRC (primary reference clock), the router will assume that the signal is PRC quality while the link is up. Note that in North America, one would configure it to be QL-PRS (primary reference source) for an Option II network because QL-PRC is for an Option I network (see following point on options). When the PRC/PRS frequency source loses its input reference (such as the GNSS signal from space), it must signal this loss to the downstream node, but again, for signals like 10 MHz, there is no QL available. So to signal the loss of quality, the PRC/PRS takes down (squelches) the fre- quency signal it is transmitting. The downstream network element relying on that reference sees the signal disappear and therefore rules that frequency source (and the QL configured for it) out of con- tention as a source. It then selects a new source of frequency out of the signals that it can see, based on the QL levels. Some other signal types support QL information only when configured to use a specific mode of transmission. This means that it is critical that the engineer configure both the PRC/PRS and the router to use a form of transmission that supports carrying the QL. By doing that, the router will be able to receive the incoming QL signals and use that information to select the best frequency source available. Of course, the received QL on any interface can be overridden by configuration on the router. A good example of where this is