e precision time pro- tocol (PTP) can address the problem of frequency synchronization across the IP/MPLS back-haul network instead of using SyncE. This approach makes sense where the operator is not able to de- ploy SyncE-enabled equipment but needs another method. Today, many Long Term Evolution (LTE) networks require only frequency synchronization, al- though it depends on the services and radio techniques that the operator decides to deploy. Many operators decided not to switch on these advanced services because their networks were unprepared to provide the phase synchronization those services need. With each subsequent updated release of the mobile standards, that option is increasingly unavailable, and phase synchronization is becom- ing mandatory. 2.1.1 Legacy Mobile Synchronization-Phase In the late 1980s, regulators, manufacturers, and operators were working on the design and deploy- ment of 2G networks to replace existing analogue systems. In Europe at this time, the European Telecommunications Standards Institute (ETSI) produced the Global System for Mobile communic- ations (GSM) mobile standard based on time-division multiple access (TDMA) radio techniques. Most of the world eventually adopted GSM as the standardized digital mobile system, but some countries, most notably the United States, selected a different system more compatible with their existing analogue systems. The U.S. operators adopted several competing systems, but one was based on code-division multiple access (CDMA) radio technology and marketed as cdmaOne. This early split in the mobile world over 2G technologies persisted into later decisions concerning 3G standards. While most of the world evolved their GSM systems to a 3G system developed by the 3GPP standards organization, called the Universal Mobile Telecommunications System (UMTS), operators in the United States chose another system based on CDMA. This CDMA system was much more compatible with the U.S. operators' existing 2G networks at the signaling layer and SO made sense for i