Who is Who in the MODEM world

· Bell 103, V.21

• These modems are the early examples of the modem systems. They are FSK modems. Ones and zeros are sent by changing the frequency of a carrier. Baud rates are 300 bauds/s for both of them which also corresponds to the bitrates for these modems, namely 300 b/s.

V.22, V.22bis

• These modems are more developed modems than the Bell 103 and V.21 series. This is obvious since their baud rate is doubled (600 bauds). V.22 modem uses BPSK (600 b/s) and QPSK (1200 b/s) modulations. V.22 bis modem is a better version for the V.22 series modems. QPSK(1200 b/s) and 16 QAM (2400 b/s) modulations are used for transmission. Channel separation  is established by using separate frequency bands for transmission and reception. The rolloff of the pulse shaping filter is 0.75 for these modems.

V.26, V.26ter

• V.26 modem is a four wire modem. Therefore it uses different lines for transmission and reception (namely leased lines). V.26 and V.26ter modems operate with 1200 bauds. V.26 modem uses QPSK (2400 b/s) modulation for transmission. V.26ter is the first example of the echo cancellation modems. It separates the receiving channel through echo cancellation. V.26ter uses QPSK (2400 b/s) and BPSK (1200b/s) modulations. The rolloff of the pulse shaping filter is 1.

V.32

• V.32 is the first example of the modern modems. It uses echo cancellation for channel separation. It is a powerful modem with a baud rate of 2400 bauds/s. QPSK (4800 b/s), 16 QAM (9600 b/s) and 32 QAM (Viterbi coding)( 9600 b/s) modulations are selected depending on the channel conditions. The rolloff of the pulse shaping filter is not specified in the standards but a value within the 0.125-0.2 range can be considered as a good choice.

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