purpose NMS. So, although the probes are useful in measuring and monitoring synchronization aspects of a net- work, understand that there is additional cost and effort needed to integrate these devices into the overall network management. Perhaps this is the reason, at the time of writing, such probes are not yet as prevalent as you might think. 12.5.1 Field Testing One aspect often overlooked in the deployment of a timing network is the issue of field testing. The need for field testing arises when a segment of the network seems to be exhibiting clocking issues, or the application using the clocking appears to have problems. An example might be if the mobile radio people are noticing substandard radio performance or unexpected interference in some loca- tion. To eliminate clocking issues as a potential cause, the operator might decide to undertake some on-site field testing. Obviously, this step should be taken only after all other likely causes have been eliminated. By this stage, the operations staff should understand that the timing nodes are operating correctly: all nodes are traceable to sources of frequency and phase, and the clocks are aligned and stable. See the pre- ceding section on troubleshooting for details of what to check on the clocks. If all that seems nor- mal, and any probe deployed in that location does not detect any issues, then it is time to conduct field testing. The idea of field testing is to use a reference clock to confirm that the suspected nodes (such as a cell site router) connected to the application (such as a mobile radio) are frequency locked and phase aligned. For this reason, the DUT should have measurement ports that allow a quick check of the frequency and phase of the node. There are two measurement ports commonly used by operators: BITS port that transmits a frequency signal such as 2048 kHz or E1/T1. An alternative might be a 10-MHz timing port that has been configured for output. 1PPS signal, either from a standalone connector (such as BNC or DIN 1.0/2.3) or via a signa