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.