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Chapter 7 - Cellular Mode Operation
7-1
Chapter 7 - Cellular Mode Operation
The Omni 288S is equipped with a special cellular communication mode which
enables the modem to perform reliable high speed data transmissions over
cellular phone links. Although all ZyXEL modems can provide the cellular mode,
the ZyXEL U-1496P portable modem is specially designed for mobile use. We
have nick-named the ZyXEL cellular mode as ZyCellular. This chapter not only
explains the ZyCellular mode, but also gives background information on cellular
data communications.
Cellular Phone Systems
We are referring here exclusively to the analog cellular phone system. It is called
AMPS (Advanced Mobile Phone Service) in North America. The cellular phone
system got its name by dividing its covered area into many small cells. Each cell
has a cell site radio station maintaining a radio link with every cellular phone that
is in use in the cell. If a cellular phone moves out of one cell's boundary and goes
into another cell, it will be instructed to switch its radio link to the new cell using
a new frequency channel. This is called cell hand-off.
By limiting the radio transmission power in each cell's radio communication, it
will not interfere with a distant cell's radio communication using the same
frequency. Frequency reuse is the principle of cellular phone systems to increase
the number of radio channels available.
Cellular Impairments
A common problem in cellular communication is cell hand-off. In the process of
cell hand-off, the radio link, hence the modem carrier, will be interrupted for 0.2
second to 1.2 seconds. A normal modem will respond with retrain which takes
about 10 seconds or may even hang up.
A cellular phone may be instructed to change its power depending on its distance
from the cell site station. The radio link will be interrupted for about 0.2 second.
An effect similar to cell hand-off will .
A particularly difficult cellular impairment for data communication is called
multi-path fading. A cellular phone receives the cell site station's radio signal
through many indirectly reflected paths. Due to the phase difference of the arriving
signals caused by the signals taking different paths, the combined signal,
depending on location, may be strong, weak or totally faded. A moving cellular
phone will experience a periodical signal weakness. This is called multi-path
fading. Fading will cause data errors because the carrier is lost due to fading.
The analog cellular phone system was designed for voice communications. It
employs companding (concatenated from compressing and expanding; it makes
small signals better at the sacrifice of large signals) and pre-emphasis which are