uirements for LTE and LTE-A". The ITU-T G.823/G.824 recommendations for transport protocols (such as E1/T1 or SDH/SONET) specify an accuracy of 16-ppb frequency delivered by the back-haul network to meet the overall 50- ppb budget at the radio interface. Hence, in 2G or 3G FDD-based networks, T1/E1 links were widely used to provide frequency accuracy to the base stations. Table 10-2 summarizes the require- ments for 2G/3G FDD- and TDD-based networks. Table 10-2 Synchronization Requirements for 2G and 3G Networks Application Frequency Accur- Frequency Accuracy Phase Accuracy References acy (Network) (Air Interface) 16 ppb 50 ppb TS25.411 TS25.431 CDMA, 3GPP2 CDMA2000 16 ppb 50 ppb C.S0010-B +10 us (for less than 8 hours 3GPP2 during holdover) C.S0010-C UMTS-FDD 16 ppb 50 ppb TS25.104 UMTS-TDD 16 ppb 50 ppb +2.5 us TS25.105 TS25.402 TD-SCDMA 16 ppb 50 ppb TS25.123 10.3 Timing Requirements for LTE and LTE-A "Chapter 4. Standards Development Organizations", outlines the role of 3GPP in the evolution in generations of mobile technologies. Technical specifications are still continuously evolving from the early days of 2G GSM, through to the 5G of the present day and onto future releases to meet new demands for services, performance, and features. 3GPP documents are structured as releases, whereby each release has a set of new features added into it. The first set of 3GPP releases were termed Phase 1 and Phase 2. However, later, the release was named based on the year of the anticipated release. For example, Release 96 was planned for approval in 1996 and released in Q1 1997. After Release 99 of the UMTS/WCDMA standard, 3GPP reverted to using specific release numbers. For example, Release 2000, which defines the UMTS all-IP core standard, was renumbered as Release 4. LTE was first introduced in 3GPP Release 8; however, the work continued to add many use cases and enhancements to LTE specifications beyond Release 8. Release 10, also known as LTE-Ad- vanced (LTE-A), specifies the more advanced features of LTE. And in Rele