 the re- covery mechanism usually needs to select only a subset of the entire packet population by some form of filtering. It is a similar problem with measurement methods when trying to estimate the likely performance of a packet stream. Despite those good reasons, SyncE still might not be the most appropriate solution for an operator. Before talking about the deployment of PTP as a packet method to carry frequency, you should un- derstand some of the other popular methods that can solve some of these problems. There is addi- tional reference information on these techniques in G.8261, specifically clause 8. Adaptive Clock Recovery One way in which it is possible to carry a different clock over a piece of shared infrastructure is to use a technique called adaptive clock recovery (ACR). This is used in the case where the clock coming from the customer must be carried transparently and regenerated at the AC at the other end of the service. One common name for this external clock is called the service clock-as opposed to the network clock, the name given to the clock used by the SP network. An example of the use of ACR might be where the customer has its own private telephone network in headquarters but requires a link to a small business telephone exchange (PABX) at a remote of- fice. This circuit is now being emulated with packets, SO the service clock needs to be transparently carried over the CEM circuit between the two locations. Before discussing the basics of how this system works, you will have to understand a little bit about CEM itself. Obviously, this book cannot include a full treatment of CEM, SO consult the references (mainly from the IETF) at the end of the chapter for a source of further reading. The basic idea of CEM is that the interworking router terminates the TDM circuit and an electronic circuit regularly samples the incoming signal (for an E1, it would be 2048 kHz) and recovers the data. It then bundles that data into an IP packet (for example, once every millisecond) and forwards it to the ot