Information Technology Reference
In-Depth Information
15.10.1 V.42bis and MNP compression
There are two main standards used in modems for compression. The V.42bis standard is de-
fined by the ITU and the MNP (Microcom networking protocol) has been developed by a
company named Microcom. Most modems will try to compress using V.42bis but if this fails
they try MNP level 5. V.42bis uses the Lempel-Ziv algorithm, which builds dictionaries of
code words for recurring characters in the data stream. These code words normally take up
fewer bits than the uncoded bits. V.42bis is associated with the V.42 standard which covers
error correction.
15.10.2 V.22bis modems
V.22bis modems allow transmission at up to 2400 bps. It uses four amplitudes and four
phases. Figure 15.12 shows the 16 combinations of phase and amplitude for a V.22bis mo-
dem. It can be seen that there are 12 different phase shifts and four different amplitudes.
Each transmission is known as a symbol, thus each transmitted symbol contains four bits.
The transmission rate for a symbol is 600 symbols per second (or 600 b aud), thus the bit rate
will be 2400bps.
Trellis coding tries to ensure that consecutive symbols differ as much as possible.
90°
Amplitude 4
Phase 3
Phase 2
Amplitude 3
Phase 1
180°
0°
Amplitude 2
Amplitude 1
270°
Figure 15.12
Phase and amplitude coding for V.32
15.10.3 V.32 modems
V.32 modems include echo cancellation which allows signals to be transmitted in both direc-
tions at the same time. Previous modems used different frequencies to transmit on different
channels. Echo cancellation uses DSP (digital signal processing) to subtract the sending sig-
nal from the received signal.
V.32 modems use trellis encoding to enhance error detection and correction. They encode
32 signalling combinations of amplitude and phase. Each of the symbols contains four data
bits and a single trellis bit. The basic symbol rate is 2400 bps; thus the actual data rate will be
9600 bps. A V.32bis modem uses seven bits per symbol; thus the data rate will be 14 400 bps
(2400
×
6).
