asynchronous V.25bis mode.
Chapter Thirteen - CELLULAR MODE OPERATION
ZyXEL U-1496 series modems are equipped with a special communication mode - cellular mode
- which enables the modem to perform reliable high speed data transmissions over a cellular
phone link. Although all ZyXEL U-1496 series modems provide the cellular mode, the U-1496P
portable modem is specially designed for portable use. It is portable in size and capable of using
the DC12V car battery power. A power cable to plug into the cigarette lighter is provided with the
modem.
This chapter explains cellular mode usage and gives background information on cellular data
communications.
CELLULAR PHONE SYSTEM
Here we talk about 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
talking cellular phone 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 a hand-off.
By limiting the radio transmitting 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
In the process of 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 transmitting 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 hand-off will happen.
A particularly difficult cellular impairment for data communication is called multipath fading. A
cellular phone receives the cell site station's radio signal through many indirectly reflected paths.
Because of the phase difference of each signal path, there is an interference pattern that,
depending on location, means the received signal combination may be strong, weak or totally
vanished. A moving cellular phone will experience a periodical signal drop and this is called
multipath fading. Fading will cause data error because the carrier is lost. It will also interfere with
a modem's adaptive operation and affect the modem's data that is received later.
The analog cellular phone system was designed for voice communications. It employs
companding (concatenated from compressing and expanding; a special data-saving method)
and pre-emphasis which are good for voice, but not suited for a modem signal. A modem signal,
if too strong, may saturate the cellular link and cause distortion. A modem signal power that is
too weak will result in lower signal-to-noise ratio at the receiving end. There is an optimum
transmitting power in cellular data communications but it is different in each case.
CELLULAR MODEMS
