essage is in transit Not introduce asymmetry between forward and reverse transmission Be timing aware, meaning the transport can either carry PTP preferentially or have a technique to carry phase/time natively within the transport The best-in-class deployment for timing over the transport network is to use G.8275.1 in combina- tion with a physical frequency signal because it gives the best result and predictable performance. For this to work, the first prerequisite is that the transport must be able to carry the physical fre- quency (SyncE/eSyncE for Ethernet) between the network elements. The second issue is to understand what the transport does with the PTP message stream. It either can carry the PTP messages as frames or packets just like another other packet, or it can use the PTP to recover the phase/time and carry the signal using another method. This section looks at both these situations in turn. 9.8.1 Carrying Frequency over the Transport System Probably the most common form of interface on a network element is some form of Ethernet. For that interface to carry a traceable frequency signal, the network element, the interface, and the optic all need to support SyncE. Even if there is a specialized transport node between two locations, there is often a short-run patch cable based on the Ethernet connected to it. If the node-to-node connection is Ethernet over a fiber link, the SyncE is an accurate signal that can be recovered at the other end and still be traceable back to a reference source. However, there are many other types of transport that do not allow that frequency to be carried transparently. The sorts of problems that might arise with transport that effectively blocks frequency are as follows: It cannot recover the frequency from the SyncE signal and uses its own internal clock for trans- mission and reception. An example might be a microwave system. An existing synchronous transport can have its own native clocking mechanism and therefore does not allow the transparent transport of an external f