derlying transport network between the nodes, which relates to how accurately and faithfully it carries timing signals. The topology is important in two fundamental ways: The shape of the network (for example, whether it is a ring or a hub-and-spoke layout) The size or length of the network (number of nodes in a ring, or the length of a chain) The basic issue is to determine how the timing signal can be distributed (with redundancy) to end application and meet the network timing budget with the lowest cost. If the GNSS receivers are located at the radio site, then the characteristics of the transport and network are not important. But the more the source of timing is centralized, the more important the network design becomes to carry the timing signal. Although the design must consider how the nodes are geographically dispersed, distance itself is not a huge factor because PTP compensates for distance. Carrying timing over very long distances might, however, require features on the transport system that affect the timing design (for example, requiring a long-reach optical transport method that introduces asymmetry). In this book the trans- port system refers to the technical method used to carry the frequency signal and PTP messages from node to node. It follows then that the design must always consider the technology of the underlying transport if it is to be used to carry a timing signal. The simplest transport to deal with is essentially dark fiber- pulses of light go into one end of the fiber-optic cable and they appear out the other end, with no ad- ditional processing. As a rule, however, the more complex the transport, the more likely it is to introduce impairment to the accurate transport of time. When the transport has active components, or intelligent devices in the link between nodes, the engineer must assume that those components are likely to introduce either asymmetry, packet delay variation (PDV), or both. To transport timing with PTP, both these characteristics are a problem. Although asymmetry ca