to support a market that generates asymmetric traffic patterns (with ADSL, asymmetry in the name). Even if not the DSL itself, but the network nodes and switches serving the broad- band market tend to have high levels of oversubscription. Like PON, there are better versions of DSL offering methods to alleviate the problem. Microwave: The case of frequency over microwave was covered in the previous section, but a similar issue occurs in microwave systems carrying PTP packets. The modulation of frames onto a radio channel can involve time-based scheduling and time-slots. There may be configura- tion options available to mitigate some of the worst effects, but you should always assume that microwave will need your attention. Thankfully, there are modern solutions to the microwave problem that allow these systems to successfully carry good-quality PTP and SyncE. Each of these categories involves a very large number of details, and it is outside the scope of the book to go into each possible combination of technologies and how they all perform when carrying PTP messages. The point is that one of the most important tasks on a timing design is to discover all the forms of transport needed to carry both PTP messages and frequency signals and decide upon a remediation strategy for each of them. So, what would that remediation look like? In the same way that transport systems can carry the frequency signal using their own native meth- ods, phase/time can use a similar mechanism. Figure 9-36 illustrates a typical approach whereby the phase/time is recovered from the ingress port on the device, carried across the transport, and the PTP is re-created at the egress interface. In this diagram, the reference timing signal, coming from the right, is recovered at the input to the transport system via a PTP slave interface (for example). This phase and time signal is then used by the transport system to transport phase/time to the other components on the system using some other mechanism (which could even be some form of PTP). Ther