Recommendation I.231.2 (11/1988): Circuit-mode bearer service categories : Circuit-mode 64 kbit/s, 8 kHz structured bearer service usable for speech information transfer
recommendation/international standard from the International Telecommunication Union
ITU Telecommunication Standardization Sector
Circuit-mode bearer service categories : Circuit-mode 64 kbit/s, 8 kHz structured bearer service usable for speech information transfer