The present document specifies the relevant parameters and their limiting values that are able to satisfactorily control the amount of jitter and wander present at synchronous digital hierarchy (SDH) and plesiochronous digital hierarchy (PDH) network-network interfaces (NNI). The present document also provides the minimum jitter and wander requirements at SDH and PDH user-network interfaces (UNI). However@ particular terminals or services may have additional jitter and wander requirements and in those cases the relevant standards apply. The jitter and wander requirements specified in the present document are applicable to the interfaces irrespective of the transport mechanism (PDH@ SDH or ATM networks@ for example). The jitter and wander requirements for an interface will be different@ depending on whether the signal at the interface is used to transport data only@ or synchronization as well. The requirements for synchronization interfaces are specified in EN 300 462-3-1 [7] and reference is made to that document where appropriate. The present document also specifies the jitter and wander requirements for interfaces using the generic frame structures at PDH rates as described in ETS 300 337 [3]. The electrical characteristics of the relevant network interfaces for SDH and PDH interfaces are described in specification ETS 300 166 [1] and the characteristics of SDH optical interfaces are described in specification ETS 300 232 [2].