acket timing signal fail (PTSF) is defined, a condition where either the Announce messages from the master are lost (PTSF-lossAnnounce); or the timing messages are lost (PTSF- lossSync); or the messages are outside the input tolerance of the slave (PTSF-unusable). PTSF- unusable usually comes about because of excessive PDV of the Sync messages. These signal failure events trigger the algorithm to select another reference with the highest quality level that is not experiencing PTSF. Quality (clockClass) values with a range of 80-110 (quite a different range from most other profiles) are supported. These values are used to carry clock quality level information from the master to the slave. See the following bullet point. Allows interoperability with other synchronization networks (such as SyncE and SDH). The profile does this by mapping QL values (SSM or ESMC) to the clockClass field of the An- nounce message. Doing this allows full traceability of the clock back to the PRC/PRS, even though the timing signal may have crossed other networks such as SDH and SyncE. These last two points are the primary benefit of deploying G.8265.1 for frequency. Table 1 of G.8265.1, reproduced in this book in Table 9-2, shows the mapping between the various methods of transporting QL information: the synchronization status message (SSM), G.781 (Option I, II and III) and PTP G.8265.1 telecom profile. G.8265.1 is deployed as PTPoIP over an Ethernet, IP, and MPLS network with the nodes between the master and slave simply switching the frames like any other. Of course, the network should be conditioned with quality of service (QoS) to try and reduce the PDV of these time-sensitive messages. The slave is provisioned with the IP address of any potential master clocks and the slave sets up the unicast negotiation, asking for a stream of Announce messages and then timing messages. This is all statically defined and configured; there is no discovery mechanism. In the case where the slave is provisioned with multiple master IP addresses, the f