requency signal. This can be an issue with SDH/SONET optical, as one example. The section "Adaptive Clock Recovery" earlier in the chapter covered the case of carrying a service clock over a transport with a different clock. The path includes at least one link from a third-party service provider that will not allow cus- tomer frequency signals on its network or cannot transparently carry it. The transport features a hardware limitation whereby the frequency cannot pass a certain point because of a lack of some physical connection to complete the circuit. A good example is the case of using a 1000BASE-T copper SFP in a 1-GE interface port. Some feature is not implemented or some optional part of it is missing. An example would be an Ethernet switch without SyncE support because having one node without the feature breaks the chain of traceability. The industries responsible for many different types of transport equipment are moving to reduce these problems and refresh their product portfolios to introduce systems allowing the transport of frequency (and phase/time). Of course, replacing the devices already deployed in service provider networks is a much slower process. A good example of this migration are the microwave systems used for mobile back-haul networks. When mobile systems were connected to their core systems with TDM networks, the TDM fre- quency was used to synchronize the cellular radio as well as the radio in the microwave systems. The microwave would transmit its signal using this reference frequency, and this signal could be re- covered at the far end. When operators adopted mobile base stations based on packet technology, then hardware designers developed microwave systems supporting the carriage of packets, and the carriage of frequency on the Ethernet links was not necessary. A TDM source was still available to provide frequency to the base station (and the microwave). As those TDM links were decommissioned and SyncE increased its reach, the microwaves developed the ability to recover SyncE at the