s that are frequency synchronized (syntonized) to the nominal fre- quency (small tolerance allowed). Time budget: Phase error delivered across the network within the timing budget required both for the application use cases and time division duplex (TDD) radios. Convergence: Requires a short time interval to "lock on" and align frequency and phase to the sources of time. Holdover time: Provides a reasonable buffer period for the network to maintain time accuracy SO that the radio units can maintain their frequency and phase within specification if they lose their connection to the reference clocks or sources of time. Resilience: The ability of the network to adapt to failures and network rearrangements-for ex- ample, an outage of the GPS signal. Cost: The design is cost-effective to build, monitor, and operate. Adaptability and flexibility: The architecture can be implemented across different topologies and transmission technologies. It is very important to understand radio synchronization requirements, the use cases, and the trade- offs involved to architect a good synchronization network design. For that reason, this chapter dis- cusses the evolution of mobility networks, radio access network architectures, and their synchroniz- ation requirements. 10.1 Evolution of Cellular Networks The first generation of mobile network was launched by Nippon Telegraph and Telephone (NTT) in Tokyo in 1979. The version of mobile was analog in nature, implemented on simple frequency-divi- sion multiple access (FDMA) technology. In 1983, the USA launched its first 1G cellular networks, and Motorola produced one of the first mobile phones to be used across various states. Following the first generation, the mobile networks have been evolving to a new generation roughly every 10 years. The second generation (2G) of cellular network was launched in Finland in 1991. The starting point for the 2G mobile technologies was the Global System for Mobile communications (GSM) and In- terim Standard 95 (IS-95), also known as cdmaOne. Both we