Specifications of signalling systems R1 and R2 : recommendations Q.310 - Q.490 / 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
Specifications of signalling systems No. 7 : recommendations Q.700 - Q.716 / International Telecommunication Union
Specifications of signalling system No. 7 : recommendations Q.721 - Q.766 / International Telecommunication Union