(SONET/SDH) networks nor in SyncE, since they are both only concerned with frequency. 3.4 Timing Distribution Network Until now, this chapter has covered all the components needed to build a timing distribution net- work, SO now it is time to see how the timing signal can be transmitted from its source to where it is needed. This chapter has shown some physical techniques using short cable runs to distribute time signals locally to a node. But of course, it is possible to use digital technologies to distribute fre- quency and phase/time over a network. For simplicity and cost reasons, it is also an advantage to carry the timing information using the same network as that used to carry data. Previous generations of networks were mostly synchronous in nature, for example SDH/SONET (but there are more), and SO they required frequency synchronization. Usually, they derive fre- quency either from the carrier signal itself, or from some time code or other information modulated onto the carrier. But to carry accurate phase and time over the wide area was more difficult. It becomes even more complex because digital networks can be based on a wide array of media: coaxial cable; twisted copper pairs; optical fiber; radio signals; and SO on. And these different meth- ods use different technologies to encode and transport signals. And so, engineers needed to design frequency and phase/time solutions for many different types of transport. But to support the evolution of modern applications, it became increasingly necessary to distribute phase and time as well as frequency on packet-based networks. The time of day (of medium accur- acy) was already carried using a packet mechanism like NTP, but phase timing, especially accurate phase timing, is much more challenging. 3.4.1 Transport of Time and Sync Distributing timing over various transmission media can be accomplished in several ways. There are mainly three categories of solutions available for time synchronization distribution: Physical layer: Frequency distribution in SDH/S