33
6179-2201
Data Rate
In modems this is often different to the baud rate. For instance the Data Rate of V.32bis is 14 400bit/s
and the baud rate is 2 400 symbols/second.
Direct Mode
The Data to be transmitted is sent directly to the data pump (the modem circuit). No compression,
error correction or buffering occurs, allowing the data to be transmitted across the link unaltered by
the modem.
Duplex
Means that the communication is bi-directional. In half duplex, the devices take turns sending and
receiving. In full duplex, sending and receiving can take place simultaneously.
Hayes commands
A set of commands for controlling PTT modems. Often referred to as the AT command set, since all
commands are started with AT. Most modems support these commands, but there are variations and
commands that are specific for a certain modem.
ISP
Internet Service Provider.
LED
Light-Emitting Diode. A semi-conductor which emits light when it receives an electrical current.
In modems they are used as indicators for data and status signals.
Modem
Acronym of the words modulator and demodulator. Modulates or transforms the signal from
computer equipment into electrical or acoustic signals for transmission. The receiver has a
similar modem which retransforms the signal, demodulation.
Modem Modulation Standards
V.21
300 bit/s, similar to Bell 103
V.23
1 200/75 bit/s Split speed line
V.22
1 200 bit/s full duplex.
V.22bis
2 400 bit/s full duplex.
V.32
9 600 bit/s full duplex.
V.32bis
14 400 bit/s full duplex.
V.34
33 600 bit/s full duplex.
V.90/K56flex
Asymmetrical transmission standards 56kbit/s downstream data, 33.6kbit/s
upstream data.
Modulation Techniques
DPSK
Differential Phase Shift Keying. Employed in data rates up to 4 800 bit/s.
FSK
Frequency Shift Keying. Used in the lowest data rate standards.
QAM
Quadrature Amplitude Modulation. A technique used for data rates up to 9 600 bit/s.
TCM
Trellis Coded Modulation. Used in the high speed modulations.
PCM
Pulse Coded Modulation. Used in the V.90 standard for downstreaming data from a digital end.
