 (known in CEM specifications as attachment circuits). Customer CEM packets Customer premises premises Frequency Frequency clock clock Ethernet/IP TDM attachment TDM attachment circuit SyncE circuit PRC/PRS Figure 9-5 TDM Circuits Emulated over an Ethernet Network, Timed with SyncE All the CEM routers at every POP in the SP network receive a frequency signal via SyncE from a reference PRC/PRS. Alongside SyncE, the CEM router also receives a QL signal, via the Ethernet synchronization messaging channel (ESMC), that indicates this signal is traceable to a high-quality source. The CEM uses these signals to both clock the TDM attachment circuit and set the SSM bits to signal the QL to the customer device. At each of the customer premises, the TDM device sees that the circuit is traceable to a high-quality source. So, it recovers the frequency from the line and uses it as the device clock and applies that frequency to receive and transmit its signal to the POP. Mobile operators first encountered this problem in the 3G era when many mobile radio devices moved from E1/T1 transport to Ethernet, and yet the radio still required frequency synchronization. At that time, large segments of the operators' back-haul network did not support SyncE (the pre- ferred solution) with the network elements they had installed. Given these constraints, many adop- ted a packet-based approach to frequency distribution, most commonly using the PTP telecom pro- file for frequency synchronization, G.8265.1. For more details on the mobile use cases for fre- quency, please see Chapter 11. One of the constraints of the preceding model is the following question: What if the SP's customer has its own source of frequency that it wants the SP to carry transparently for the customer-mean- ing, the customer wants its own frequency on both ACs and not the frequency from the SP? See Figure 9-6 in the upcoming section "Adaptive Clock Recovery" for an illustration of this problem. Differences Between Physical and Packet Methods This section provides a quic