The modem's speed using a 90 degree phase shift is 2 x 400 which is equal to 8 kbps. The signal shifted by 45 degrees increases the speed to 3 x 400 which is equal to 12 kbps.
V. 90 (56 Kbps) Modem. ITU - T developed many version of modem named modem standards. The standards ranges from V. 22 with speed of 1200 bits/sec, full duplex, PSK encoding to V. 90 with 56 kbps download speed and 33.6 kbps upload speed and V. 92 which is the upgraded version of V. 90 standard with upload speed of 47 kbps. In this section, the version V. 90 modem standard created in 1998 by ITU is considered for discussion.
The 56 kbps modem was designed for a one end digital connection from the server to the PSTN. V. 90 have become a word wide standard with 56 kbps modem technology, the transmission rate is 33.6 kbps and receiving rate is 56 kbps. This is due to the fact that the upstream channel (user to server) requires ADC at local exhchange.
Fig. 3.28 explains the 56 kbps modem connection. This conversion produces noise and reduces the speed of the modem to 33.6 kbps.
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