dem link over the PSTN. maximum capacity of the voice channel and many higher data rate access technologies have become available. If an analog subscriber line is replaced by an ISDN line, the full capacity of the allocated channel in the network can be utilized and end-to-end data rate will then be 64 Kbps (B-channel) or 128 Kbps (two B-channels). 6.4.1.1 V Series Recommendations of ITU-T The ITU-T (CCITT) has defined many standards for voice-band modems with a variety of speeds and these recommendations are identified by the let- ter V and a number attached to it. Modems of different manufacturers work together if they support the compatible V standard. Many modern modems support previous lower speed standards as well and they are able to adapt their speed to the level supported by the other end. To illustrate development of voice-band modems and modulation methods, some examples of modem standards are described briefly next. V.21: 300-bps full-duplex (bidirectional transmission). One of the first modems that used carrier frequencies at different transmission directions (1,080 and 1,750 Hz). FSK is used SO that binary 1 corre- sponds to the carrier frequency of the direction in question (1,080 or 1,750 Hz) minus 100 Hz and binary 0 corresponds to the carrier fre- quency plus 100 Hz. Introduction to Telecommunications Network Engineering V.22: 600/1,200-bps full-duplex. This standard provided an accept- able dial-up data connection for the transfer of text messages in both directions. The transmission directions use different carrier frequen- cies. One example of user applications is a remote text mode termi- nal. The modulation scheme is PSK with two or four carrier phases and a modulation rate of 600 bauds. V.22bis: 2,400-bps full-duplex. This modem was designed to update the V.22 modem at the end of the 1980s. The data rate was doubled with 16 amplitude-phase combinations phases (16-QAM) of the carrier. The modulation rate is 600 bauds. V.23: 1,200/600-bps modem that transmits 1,200 or 600 bps and 75 Kbps in 