Technical data

68 Theoretical and general applications www.westermo.com
Is bit/s the same as baud?
The transfer rate of a telecom modem is described both in terms of bit/s (Bit rate) and
in Baud (Baud rate). This has resulted in some confusion, which is why an explanation
is called for.
Bit rate = The number of bits sent via the serial interface per second;
measured in bit/s
Baud rate = The number of signal combinations sent over the line interface
per second; measured in Baud
To increase the transfer rate on a telecom modem, more bits are modulated together
and transferred via the telephone network. In the adjoining example, the technique of
phase modulation is shown where two bits are described by the phase variation of the
line signal (V.22).
In the example opposite the bit rate is 1200 bit/s and the baud rate 600 baud.
When additional signals are modulated together a higher transfer rate is achieved.
In some standards, for example in V.22bis, amplitude and phase modulation are
combined (also called QAM Quadrature Amplitude Modulation), which results in
4 bits being transferred on each modulation.
In standards such as V.32, the line is modulated using a technique called TCM (Trellis
Code Modulation), which corresponds to QAM but where a supplement of an extra
bit is made for error correction. This is necessary as the border between the trans-
ferred bit combinations decreases, which results in a higher error correction require-
ment.
In the example opposite the bit rate is 2400 bit/s and the baud rate 600 baud.
In the example opposite the bit rate is 9600 bit/s and the baud rate 2400 baud.
0° = 00
+270° = 11
+180° = 10
+90° = 01
B A C K