y involve only backhaul, but may additionally entail either fronthaul, or midhaul, or both, under the umbrella of the generic term xHaul. However, the transport network providing these 5G haulings will typically be required to provide additional transport services. Such services will generally incorporate those that bestow essential support for the 5G functionality, such as control and manage- ment of flows and time/frequency synchronization. In addition, the same network will often need to support 4G backhaul, and even previous generations (e.g. 3G voice). While there is a justifiable tendency to disregard such services as negligible in volume compared with that consumed by 5G, they may have specialized requirements, such as non-packet constant bit rate, specific physical interfaces, or stringent delay budgets. Finally, the trans- port network may also furnish services unrelated to mobile communications, such as Inter- net access or critical infrastructure communications. 3GPP standards tend to consider transport as a minor function that effortlessly delivers data over the named interfaces with no availability failures, data rate restrictions, burden- some latency, synchronization glitches, or other degradations. Unfortunately, this is not the case. Even well-engineered transport networks have limitations and occasionally fail to live up to designed requirements. The limitations of transport networks and the best practices to reach the highest levels of performance are the subjects of this section. Now that we understand the different transport segments, we need to appreciate the challenges presented by each such one. There are several types of requirements, the most important of which are network topology (e.g. star, mesh), traffic capacity, traffic character- istics (packet size, burstiness, etc.), delay (E2E propagation latency), reliability (availability and time-to-repair), dynamicity (how quickly services need to be set up and torn down), and synchronization (frequency and time recovery). There may be addi