n happen from various effects such as temperature changes and atmospheric variations in the case of wireless media. Truly synchronous transmission media became a reality with SONET/ SDH network transmission standards and along with that came a significant improvement in network stability. How to deal with clock distribution and stability is an overall, end-to- end network issue. Dealing with it from a design level perspective PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS requires obtaining a set of facts about the capabilities and limitations of the common network equipment and the network service provider's clocking and distribution infrastructure, and the specific part of that infrastructure providing use of facilities and services to all the sites included in the overall network. More specifically, decisions about network interface equipment should not be made without a clear definition and understanding of the performance characteristics of the equipment with and without external clock reference. The second most important piece or set of information is a definition and under- standing of the communications network-timing source and its rela- tionship to the network channel interface at each site. Once a picture as outlined above has been obtained, it can be assessed in light of the requirements for content transport. The broad perspective includes a situation on the one hand where it may be acceptable to rely totally on service provider network timing sources as the master timing and clocking reference, while the oppos- ite extreme is to equip each site with a set of stratum 1 clock refer- ences. Obviously, somewhere in between is likely the practical approach for the enterprise. Don't forget that network clocking and synchronization is an entirely different matter than station clocking and synchronization. As long as the communications network runs on an accurate enough clock and assuming it is configured to carry live or real-time program content, the content will be clocked from the ori