ir 2G and 3G radios. In networks that did not support SyncE, this form of PTP is a possible alternative. The principal enemy of this solution is PDV, and because G.8265.1 does not allow any on-path sup- port, the control of PDV through the intervening network elements is a very critical aspect to meas- ure, understand, and control. Several models of timing test equipment, including those you saw pre- viously, allow for testing and characterization of PDV if needed. One interesting feature is the ability to test for a packet floor in the arriving PTP message stream (see "Packet-Based Frequency Distribution Performance" in Chapter 9 for a discussion of packet floors). The tester can see how many packets pass the criteria for packet selection to be used to cal- culate the timing solution. This metric is known as the floor packet percent (FPP) and is defined as the percentage of packets that fall into the given fixed cluster range starting at the observed floor delay (the theory is covered in G.8260). A network limit for it is defined in G.8261.1 (basically 1% of packets should fall into the cluster around the floor delay). This is a handy test suite when you are having problems with PDV in a network. The second thing commonly tested is much the same as covered in the preceding section, and that is the accuracy of the frequency recovered by the slave clock. Again, the tester device can measure a frequency output signal from the slave clock (as a E1/T1/2048-kHz signal) and measure the wander compared to a reference 10-MHz signal. This is compared to the packet-based equipment clock- slave-frequency (PEC-S-F) MTIE mask from G.8261.1 for a pass/fail determination. One other approach would be where the test device directly negotiates a PTP packet stream with the remote G.8265.1 GM and measures the frequency wander based on timestamps in the arriving PTP messages. This approach tests the network for its ability to carry a packet frequency signal instead of testing the wander in the frequency recovered by a slave clock. So