Specifications of signalling systems Nos. 4 and 5 : recommendations Q.120 - Q.180 / International Telecommunication Union
Specifications of signalling systems No. 6 : recommendations Q.251 - Q.300 / International Telecommunication Union
Digital local, transit, combined and international exchanges in integrated digital networks and mixed analogue-digital networks : supplements: recommendations Q.500-Q.554 / International Telecommunication Union
Interworking of signalling systems : recommendations Q.601-Q.699 / International Telecommunication Union