This Recommendation outlines minimum requirements for timing devices used as node clocks in synchronization networks. The requirements relate to frequency deviation, pull-in, hold-in and pull-out range, noise generation, tolerance and transfer, transient response and holdover performances. The node clocks are suitable for use in SDH and PSTN network applications. This Recommendation includes specifications for three types of clocks. The Type I clock is primarily intended for use in networks optimized for the 2048 kbit/s hierarchy. The Types II and III clocks are primarily intended for use in networks optimized for the particular 1544 kbit/s hierarchy that includes the rates 1544 kbit/s, 6312 kbit/s and 44736 kbit/s. Additionally, this Recommendation includes specifications for three other clocks in Annex A. Type IV is typically deployed in existing networks that support the 1544 kbit/s. The Types V and VI clocks were defined for transit and local node applications in the 1988 version of this Recommendation.