the reverse direction. This asymmetrical transmission provides enough capacity to send keystrokes from the terminal while transmitting larger amounts of data in the other direction. FSK is used in both directions and 1,300 Hz corresponds to 1 and 2,100 Hz corresponds to 0 in the 1,200-bps direction. In the 75-bps direc- tion frequencies are 390 Hz as 1 and 450 Hz as 0. V.32: 9,600-bps full-duplex. The modulation method is QAM, a combination of amplitude and phase modulation. The modulation rate is 2400 bauds and 16 combinations of carrier amplitudes and phases are used. V.32bis: This modem is an enhancement of V.32 with a new modu- lation scheme. It transmits data at 14.4 Kbps. The modulation method is QAM with 128 different combinations of amplitude and phase of the carrier. The modulation rate is 2,400 bauds. V.34: This standard supports data rates up to 28.8 Kbps full duplex over dial-up telephone lines and uses QAM with a modulation rate of 3,200 bauds. Error-free operation at this high data rate requires a very clean speech channel. If errors occur too frequently, this modem falls back to lower speed in steps of 2,400 bps to reduce the number of errors. V.34+: Enhancement to V.34 with a data rate of 33.6 Kbps. The modulation method is QAM and the modulation rate is 3,200 bauds as in V.34. V.90: This standard supports 33.6-Kbps upstream and 56-Kbps downstream data rates. Note that the downstream 56-Kbps rate requires that the source computer have digital access to PSTN and A/D conversion is not performed in the transmitting end. Data Communications The highest data rate modems use SO many constellation points that errors occur frequently. To reduce the error rate, they add error control bits to correct most errors and this method where modulation and error control coding are combined is known as trellis-coded modulation (TCM). It is likely that essentially higher data rate voice-band modems will not be standardized. Essentially higher data rate service requires end-to-end digi- tal connections provided by, fo