anization (the ITU-T) responsible both for standardization and for timing and synchronization. It also categorizes the large number of recommendations and provides detail about how to read and use them. ITU-T physical and TDM timing recommendations: Provides an overview of the ITU-T re- commendations that concern frequency timing using physical and time-division multiplexing (TDM) techniques such as the synchronous digital hierarchy (SDH). ITU-T recommendations for frequency in packet networks: Provides an overview of the ITU-T recommendations that concern frequency timing using physical techniques in packet- based networks, such as Ethernet. ITU-T packet-based timing recommendations: Provides an overview of the ITU-T recom- mendations that concern frequency, time, and phase timing using packet-based techniques, such as precision time protocol (PTP) in Ethernet and IP networks. Possible future changes in recommendations: Gives a short overview of what is currently be- ing worked on, and therefore what changes might occur over the next few years. The most accurate reference material for timing and mobile comes from numerous standards devel- opment organizations (introduced in "Chapter 4. Standards Development Organizations"). SDOs produce large amounts of written information on each specification as well as the need and usage of timing in numerous deployment scenarios. The problem for those wishing to start educating them- selves is that these standards and recommendation documents are not exactly easy to find, read, or understand. This chapter introduces the most important and relevant of these recommendations from just one organization, the ITU-T. You will see the various specifications placed into context SO that you can decide what material is of value for your application. This chapter does not attempt to be the ulti- mate authority on each recommendation, just an orientation for those unfamiliar with the material in the public domain. For the fine details beyond that, readers who wish to dive down to the nex