n be tolerated when transporting only fre- quency with PTP, it very severely impacts phase accuracy. Vendors and standards organizations are updating many of the transport technologies to support and/or improve the accurate transport of phase and frequency. The mechanisms to achieve this vary widely depending on the type of transport being considered. In summary, if one of these problem- atic systems lies in the path of a timing signal, it needs to contain design features that make it what is known as timing aware. Additionally, if frequency is being carried using a physical technique such as synchronous Ethernet (SyncE) or enhanced SyncE (eSyncE), then you must ensure that the end-to-end network can faith- fully carry that frequency (even if the network is designed with different transport types besides Ethernet). Again, the method to achieve this will depend on the technology of the transport because it normally requires using some native method to carry frequency (a microwave link is a common example). At one end of the link, the device must be able to recover the SyncE frequency, convert it into some native method to carry frequency, recover that frequency at the far end, and apply that same fre- quency signal to the Ethernet port facing the next network element (NE). Figure 11-1 illustrates the interworking with different transport types to carry a timing signal such as frequency. Frequency carried using native transport method Non-Ethernet SyncE network network SyncE SyncE PRC/PRS clock clock Radio Interworking Interworking Figure 11-1 Carrying Frequency Across Different Transport Types Much the same as the transport links, the NEs carrying the packet-based timing signal can fall into the category of being either timing-aware or timing-unaware: The timing-unaware device simply switches packets or messages like any other device, and has no mechanisms to deal with these packets in a way that could be considered timing aware. It has no ability to process time-sensitive packets beyond the normal mechanisms such