can also playback a recorded digital voice message either locally
or to the line as an announcement.
Digital voice messages need speech compression to reduce the storage requirement. This
speech compression is done in real time by the modem and it does the decompression during
playback. A simple voice capability using an ADPCM (Advanced Digital Pulse Coded
Modulation) algorithm can reduce the speech data rate to 1/2 or 1/3 of the original rate. We call
it advanced voice capability when a compression scheme can reduce the speech data rate even
further with advanced algorithms like CELP (Code Excited Linear Prediction).
CELLULAR MODEM
A cellular modem is a data modem that can be used on the analog
cellular mobile phone system. With the rising popularity of
notebook computers this has an increasing importance for people
on the move.
There are two kinds of phenomenons in the cellular mobile phone
system that are particularly bad for modem data communications.
One is called cell hand-off, meaning that a cellular phone is
instructed to change the radio link from one cell site to another
when travelling across a site boundary. The radio link will be
temporarily interrupted and most modems will retrain or even hang
up. Instructing a cellular phone to change transmitting power
will cause similar problems. The other particularly annoying
phenomenon is multipath fading. Here, a moving cellular phone
will experience periodical fading or drop-out of the radio
signal. This causes high data error rates and low throughput.
Communication protocols like MNP10 do not do anything to improve
the physical layer of data communication robustness and cannot
really help in cellular data communication.
ZyXEL developed cellular modes for its modems using its
proprietary data pump that conquers the cell hand-off and
multipath problems. The V.42 link layer protocol with selective
reject greatly improves data file transfer throughput.
AUTOMATIC REDIAL / CALL HUNTING
Most modems will automatically redial a telephone number several
times, if a connection could not be established on the first try.
The modems of the U-1496 series also offer the ability to do
cyclic redials of a set of up to ten different numbers.
MODEM AND LEGAL MATTERS
In some countries the PTT authorities require modems to conform
to a set of specific rules, e.g., restricting the behavior of
modems during automatic redial. Modems must receive an official
approval within those countries to be used legally. ZyXEL already
has provided specifically designed models which have received
such an approval. Check with your PTT authority to find out about
existing restrictions.
