 and improved reliability and accur- acy of the sources and their references. Why all the fuss and bother? Essentially, signals derived from multi- plexing low-speed digital bit streams into higher-order aggregate bit streams, some of which provide private line, and others that provide circuit-, cell-, and packet-switched services must be synchronized and timed exactly the way digital audio and video signals are timed and synchronized, including SMPTE time code, to bit level accuracy within a frame. The only differences are that Telecom digital signals aren't subject to switching transitions such as a split screen or cross-fade, and telecom and digital program content run on PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS entirely different time bases. One other point is that this timing accuracy has absolutely nothing to do with the transmission media-it makes no difference if the transmission is satellite or terrestrial radio, optical transmission, or baseband electrical signal transport. One thing to pay attention to in systems where the bit stream is modulated onto a carrier is the capability of a particular modulator to lock the carrier frequency generator to an external source such as the incoming digital signal or PRC. On the receive end, it may be appropriate to lock the local receiver beat frequency oscillator in the receiver to either a PRC or the incoming digital signal clock. Includ- ing these capabilities in a piece of equipment is not without cost; however, those that do provide that capability will be capable of better bit error performance across the transmission facility because of the absence of asynchronous cross talk causing clock slips. Lastly, be careful not to confuse network primary reference clock with program clock reference in Moving Picture Experts Group (MPEG). The two are completely different animals and have nothing to do with each other. When MPEG2 program streams are trans- ported through networks, the PCR is just another set of bits in the payload. Clock distrib