PPP over Sonet [RFC 1619] uses 'HDLC-like framing' as defined in [RFC 1662]. These documents were developed to address a void in existing SONET standards at that time. T1.X1 is the ANSI accredited T1 Technical Subcommitee responsible for developing SONET standards, including SONET rates, formats and payload mappings. Its output is also brought into the ITU, for example as in ITU-T Recommendation G.707 [G.707]. At the December, 1997 IETF in Washington DC, a BOF was held to discuss a potential weakness in previous mappings. Specifically, long packets (where 'long' can include those carried by PPP) containing a special bit pattern can cause a SONET reciever to declare a 'loss of signal'. The minutes of the BOF are included in Appendix A. T1.X1 has recently defined a mapping for HDLC over SONET [T1.105.02, T1.105] that addresses the potential weakness described above. The work done by T1.X1 on SONET supersedes past efforts that have taken place within the IETF. The recommendation of this applicability statement is that all PPP over SONET implementations use the combination of RFC 1662 plus 'Packet Over Sonet/SDH' [POS]. The first document describes how to encapsulate PPP in HDLC; the second document describes how to encapsulate HDLC in SONET. We also recommend that RFC 1619 be reclassified as Historic.