e topic. Go to https://www.itu.int/pub/T-TUT-HOME-2020 to download the report. 9.1.1 Frequency Distribution in a Packet Network "Chapter 6. Physical Frequency Synchronization", covered many critical aspects of carrying a fre- quency signal across a transport network using a physical method (the signal is carried in a wire or fiber). But there are many cases where the operator wishes to decommission those types of net- works, or the equipment to transport those signals is no longer cost effective. Some service providers (SP) who provide legacy transport services (such as E1/T1 and lower speed optical) are similarly moving away from TDM-based networks and adopting packet-based transport. Many times, the SP leaves the customer side of the circuit as TDM-based but migrates the core of the network delivering that service to a packet-based transport. At the point where the customer TDM circuit is terminated in the nearest point of presence (POP), the TDM signal is sampled, turned into packets, and forwarded to the other end. At the other end, the TDM circuit is regener- ated for the terminating link to the customer. The industry generally refers to this approach as cir- cuit emulation (CEM), which is illustrated in Figure 9-5. Replacing a portion of an end-to-end synchronous network link obviously means that although each end of the link might be running a synchronous protocol, there is a "gap" in the middle where that synchronous scheme is not present. Therefore, the SP must somehow build a timing network to re- place the one that it removed with the decommissioned TDM core. For simple financial reasons, most SPs decide that this new timing signal should be transferred using the transport network carry- ing the services. As described in Chapter 6, for Ethernet-type networks, the obvious choice in a packet network for frequency transport is to use SyncE (a slight diversion from packet-based frequency for a moment). Figure 9-5 illustrates the approach using SyncE to provide a timing signal to the customer TDM cir- cuits