Signalling Systems No. 4 and No. 5 are in-band signalling systems.It is not planned to specify modified versions of these systems for application to PCM transmission systems.
ITU-T Q.110-1988 Referenced Document
ITU-T G.732 CHARACTERISTICS OF PRIMARY PCM MULTIPLEX EQUIPMENT OPERATING AT 2048 kbit/s*, 1993-01-01 Update
ITU-T G.733 CHARACTERISTICS OF PRIMARY PCM MULTIPLEX EQUIPMENT OPERATING AT 1544 kbit/s*, 1993-01-01 Update
ITU-T Q.110-1988 history
1988ITU-T Q.110-1988 GENERAL ASPECTS OF THE UTILIZATION OF STANDARDIZED CCITT SIGNALLING SYSTEMS ON PCM LINKS