This paper presents the measurements and analyses of transmission qualities of the existing urban and toll public switching telephone network (PSTN) for the purllose of opening it to data transmission. In the tests, random noise and impulsive noise occuring on the telephone network are investigated and bit error rate and block error rate representing the transmission qualities on the existing telephone network are measured. We recommend the fastest practical data transmission speed on the network and investigate a possibility of opening the public switching telephone network (PSIN) to the data transmission based on the above measurements.