ITU V.90:1998 A Digital Modem And Analogue Modem Pair For Use On The Public Switched Telephone Network (pstn) At Data Signalling Rates Of Up To 56 000 Bit/s Downstream And Up To 33 600 Bit/s Upstream Defines the operation of a digital modem and analogue modem pair to be used on the Public Switched Telephone Network (PSTN) at data signalling rates not in excess of 56 000 bit/s in the downstream dir