Modems - Computer Busses - page 256

15.9.3 M -ary modulation
With M -ary modulation a change in amplitude, phase or frequency represents one of M pos-
sible signals. It is possible to have M -ary FSK, M -ary PSK and M -ary ASK modulation
schemes. This is where the baud rate differs from the bit rate. The bit rate is the true measure
of the rate of the line, whereas the baud rate only indicates the signalling element rate, which
might be a half or a quarter of the bit rate.
For four-phase differential phase-shift keying (DPSK) the bits are grouped into two and
each group is assigned a certain phase shift. For two bits there are four combinations: a 00 is
coded as 0 ° , 01 coded as 90 ° , and so on:

00 ⇒ 0 ° 01 ⇒ 90 °

11 ⇒ 180 ° 10 ⇒ 270 °

It is also possible to change a mixture of amplitude, phase or frequency. M -ary amplitude-
phase keying (APK) varies both the amplitude and phase of a carrier to represent M possible
bit patterns.
M -ary quadrature amplitude modulation (QAM) changes the amplitude and phase of the
carrier. 16-QAM uses four amplitudes and four phase shifts, allowing it to code four bits at a
time. In this case, the baud rate will be a quarter of the bit rate.

Typical technologies for modems are:

- used up to 1200 bps

FSK

- used at 2400 bps

Four-phase DPSK

- used at 4800 bps

Eight-phase DPSK

- used at 9600 bps 16-QAM

15.10 Typical modems

Most modern modems operate with V.22bis (2400 bps), V.32 (9600 bps) or V.32bis
(14 400 bps); some standards are outlined in Table 15.4. The V.32 and V.32bis modems can
be enhanced with echo cancellation. They also typically have built-in compression using
either the V.42bis standard or MNP level 5.

Table 15.4 Example AT modem commands

ITU recommendation Bit rate

(bps)

Modulation

V.21 300 FSK

V.22 1 200 PSK

V.22bis 2 400 ASK/PSK

V.27ter 4 800 PSK

V.29 9 600 PSK

V.32 9 600 ASK/PSK

V.32bis 14 400 ASK/PSK

V.34 28 800 ASK/PSK