| Wireless
Data Overview
Circuit
Switched vs. Packet Switched
There are two types of data transmission - Circuit Switched
and Packet switched. Circuit switched actually takes over
a voice channel to transmit and receive data, essentially
keeping a phone line open during the entire session. Cellular
and PCS modems use circuit switched transmission, allowing
you to dial up over your wireless phone just like you would
using a landline connection.
Packet
switched data transmission compresses the data and sends
short data bursts between conversations or during gaps in
conversations on the voice channels. Packet data transmission
is ideal for short messages, including E-mail, news headlines,
and stock quotes.
Sending
Data Over Cellular
Unlike data transmission through a regular telephone line,
transmitting data over a wireless phone has several unique
considerations.
No
Dial Tone
Unlike
landline phones, cellular and PCS phones have no dial tone.
A call is initiated by entering the number and pressing
the send key. As a result, an interface is necessary to
simulate the dial tone.
No
RJ11 jack
Cellular
phones are not equipped with an RJ11 connection like a regular
telephone. In order to connect a laptop computer, fax machine
or other data device to a cellular phone, you'll need an
RJ11 interface. As a result, they require a special
cable or interface to provide a data connection for
a modem.
Noise
and Interference
Because
cellular phones transmit over an FM radio wave, they are
subject to noise, static and interference. While this noise
is generally not a problem with voice communications,
it can disrupt a data transmission. Special error correction
protocols have been developed which optimize the modem for transmission
over cellular.
Call
Hand-offs
During
a cellular call, the system hands off calls from one cell
site to the next while you travel. This handoff actually
interrupts the signal for 250 milliseconds, and a modem
may think the call has been disconnected. Cellular compatible
modems correct this problem, allowing you to maintain a connection
even if your call is handed off.
Other
Wireless Data Devices
Two-way Pagers
Two-way
Pagers allow you to receive E-mail and short messages and
reply directly from your pager. Although Two- way pagers
are bigger than normal pagers, they are much
smaller and more convenient to check short messages than
on a normal laptop computer. Some brokerage firms like Discover
Brokerage and Siebert
Financial even offer real-time stock quotes and online
trading using Two-way pagers. SkyTel
offers nationwide, regional and local Two-way pager service.
Data
Interfaces
A
data interface allows you to connect an external device
(credit card reader, fax, etc.) to a cellular phone. (If
you’re using a laptop, you won’t need a data interface because
it’s built into the PC card modem already). An external
data interface requires a separate power source.
Credit
Card Readers
For
connecting other data devices, you’ll need a separate data
interface that connects to a phone. Instead of using a cellular
modem, a data interface allows you to connect to a credit
card. You might want to use this method if you’re a vendor
an outdoor event and you want to verify credit cards. If
you do this on a regular basis, wireless credit card readers
are available from U.S.
Wireless Data.
Portable Fax Machines
Although you can connect a fax machine to a cellular
phone using a Data Interface, this method can be very cumbersome
and not very efficient. Most cellular modems include fax
software, allowing you to send and receive faxes directly
from your laptop computer. Note: A quick and easy way to
receive faxes while traveling is to use a new service available
from eFax. eFax
gives you a fax mailbox number that lets anyone send faxes
to you while you’re traveling. eFax then converts the fax
to a .pdf file and sends it to your E-mail address, allowing
you to view it at your convenience.
Troubleshooting
Tips for Data Transmission
Slow
Down Your Baud Rate
Even
through your modem may be capable of transmitting at 9,600
or 14,400 baud, the static and interference normally experienced
with a cellular transmission may prevent it from transmitting
that fast. Unless you are using a cellular compatible modem,
you may experience these problems. If you’re having problems,
try slowing down the rate in your modem set-up.
Weak
Cellular Signal
Although
you may be able to carry on a conversation with a marginal
cellular signal, it may disrupt your data transmission.
Check your signal strength indicator to make sure that is
not your problem. If possible, move to an area with stronger
signal strength. Three watt mobile or transportable phones
will usually have fewer problems than .6 watt portables.
Unable
to Connect Calls
Check
the access number and prefix in the modem set-up on your
laptop. If your cellular phone has a different area code
from your calling area, you may need to add "1"
plus the area code to connect.
Fast
Busy Signal
If
your modem can’t connect, try making a voice call to ensure
that you have service available. If you can’t make a voice
call, you won’t be able to make a data connection, either.
If there is a problem with your account, call your carrier’s
customer service number or dial *611.
Unable
to Complete Long Distance Calls
In
some service areas, you may have to enter a credit card
number when making a long distance call. Needless to say,
this simple requirement can wreak havoc on your attempt
to automatically dial through your laptop computer. Some
software programs allow you to enter a telephone credit
card number or allow you to manually dial in order to make
the connection. If your software doesn't allow you to do
this, you will be limited to making calls in the local area.
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