PSTN in ITU-T standards (www.itu.org)
			n Series D Recommendations - General tariff principles n Series E Recommendations - Overall network operation, telephone service, service operation and human factors n Series G Recommendations - Transmission systems and media, digital systems and networks n Series I Recommendations - Integrated services digital network (ISDN) n Series M Recommendations - Network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile, and leased circuits