rts SyncE, but also be sure the individual port is also SyncE capable (some ports may have limitations, such as supporting out- put only). There can be issues in some situations involving older interface types. For example, low-speed cop- per interfaces (1 GE or less) might not support SyncE or might have restrictions on the support that it offers (for example, the interface may not support a change in direction of the SyncE flow without a port up/down event). A rule of thumb would be that the slower and older a port is, the less likely it is to implement SyncE or have some caveat. But one issue to be acutely aware of is that SyncE is generally not supported on small form-factor pluggable (SFP) devices for copper cables. These SFPs are devices that plug into a 1-gigabit Ether- net (GE) SFP port on a router and allow the device to be connected to UTP (1000BASE-T) copper cables. This is because pluggable ports compliant with the standards do not have enough pins to connect with the copper SFP to carry SyncE. Note that this is a problem with the SFP port and not a normal copper port, which does not have this limitation. Some proprietary SFPs are available that partially solve this issue, although they have some short- comings. One of the major drawbacks is that they come as two separate devices: one is a SyncE master and the other is a SyncE slave port, and they cannot change state. This means they have lim- ited use in rings because the direction of SyncE in the ring cannot change without swapping the dir- ection of the signal (and maybe the SFPs themselves if their role is hardwired at the factory). The best solution is to be sure that if your solution requires SyncE to run over a copper cable, be sure your network element contains native 1-GE RJ-45 ports for copper 1000BASE-T because cop- per SFPs will almost certainly not work. Frequency Inputs and Outputs (Quality Levels) Receiving a stable and accurate reference frequency signal as an input is important, but an equally important component of that signal is to 