This Recommendation outlines minimum requirements for timing devices used in synchronizing network equipment that operate according to the principles governed by the Synchronous Digital Hierarchy (SDH). This 2003 revision of ITU-T Rec. G.813 includes, in addition to a few editorial points, two technical points: - An update in 8.2 b) of the jitter tolerance for Option 2 that has already been published in ITU-T Rec. G.813 Corrigendum 1 in 2001. - The introduction of STM-64 jitter generation values for Options 1 and 2 in clause 7.3.