"The present document contains design requirements applicable to IMS/PES exchange implementations based on ITU-T Recommendation Q.543 [1]. The definitions of IMS/PES design objectives are based on ""best practice"" performance of legacy PSTN/ISDN signalling. The values contained in the document are ""best practice"" performance values measured on IMS and NGN implementations. The ISDN VGW values are based on carrier grade implementations. The handling of preconditions is out of the scope of the present document."