k breaks the chain of frequency between the trans- mitter and receiver. Therefore, we need another mechanism to ensure that the AC at each end is run- ning at the same frequency. There are two solutions (see Figure 2-10) to introduce frequency synchronization into the IP/MPLS packet-switched network. One uses SyncE, and the other uses a profile of PTP specifically designed to replace the TDM frequency timing with packet-based frequency timing. These two solutions have been widely deployed, either separately or in combination. IP/MPLS SyncE or PTP E1/T1 E1/T1 Figure 2-10 Timing Solutions for Circuit Emulation Not only business, government, and enterprise customers have already taken steps to replace all SDH/SONET infrastructure with packet-based CEM routers, but also data service providers. Their data circuit customers never even realize that the T1/E1 circuit they have purchased has been mi- grated to packet transport inside the core network of the service provider (SP). Their TDM link only reaches to the nearest point of presence (POP) or local exchange, where it is converted to use a packet transport for delivery to the remote location over the SP's IP/MPLS network. We have not yet mentioned another two techniques to carry clocking over the packet network, known as adaptive clock recovery (ACR) and differential clock recovery (DCR). Chapter 9 cov- ers these techniques in more detail within the context of packet-based distribution of frequency. 2.2.1 Audiovisual One example of synchronization that is commonly overlooked is that used in audiovisual produc- tion and distribution, at least until it fails. The most important aspect is the synchronization between the visual stream and the audio channels. Human beings are very quick to notice when the speech being heard does not align with the moving lips or sounds are not aligned with actions. Figure 2-11 illustrates the importance of aligning the arrival time of audio and video and how much divergence the viewer can detect. As the two signals begin to diverge, an in