12. Introduction to
GSM / GPRS Wireless Modems
is a GSM Modem?
GSM modem is a wireless modem that works with a GSM wireless network.
A wireless modem behaves like a dial-up modem. The main difference
between them is that a dial-up modem sends and receives data through
a fixed telephone line while a wireless modem sends and receives data
through radio waves.
GSM modem can be an external device or a PC Card / PCMCIA Card.
Typically, an external GSM modem is connected to a computer through a
serial cable or a USB cable. A GSM modem in the form of a PC Card /
PCMCIA Card is designed for use with a laptop computer. It should be
inserted into one of the PC Card / PCMCIA Card slots of a laptop
a GSM mobile phone, a GSM modem requires a SIM card from a wireless
carrier in order to operate.
mentioned in earlier sections of this SMS tutorial, computers use AT
commands to control modems. Both GSM modems and dial-up modems
support a common set of standard AT commands. You can use a GSM modem
just like a dial-up modem.
addition to the standard AT commands, GSM modems support an extended
set of AT commands. These extended AT commands are defined in the GSM
standards. With the extended AT commands, you can do things like:
writing and deleting SMS messages.
the signal strength.
the charging status and charge level of the battery.
writing and searching phone book entries.
number of SMS messages that can be processed by a GSM modem per
minute is very low -- only about six to ten SMS messages per minute.
is a GPRS Modem?
GPRS modem is a GSM modem that additionally supports the GPRS
technology for data transmission. GPRS stands for General Packet
Radio Service. It is a packet-switched technology that is an
extension of GSM. (GSM is a circuit-switched technology.) A key
advantage of GPRS over GSM is that GPRS has a higher data
can be used as the bearer of SMS. If SMS over GPRS is used, an SMS
transmission speed of about 30 SMS messages per minute may be
achieved. This is much faster than using the ordinary SMS over GSM,
whose SMS transmission speed is about 6 to 10 SMS messages per
minute. A GPRS modem is needed to send and receive SMS over GPRS.
Note that some wireless carriers do not support the sending and
receiving of SMS over GPRS.
you need to send or receive MMS messages, a GPRS modem is typically
12.3. Which is
Better: Mobile Phone or GSM / GPRS Modem?
general, a GSM/GPRS modem is recommended for use with a computer to
send and receive messages. This is because some mobile phones have
certain limitations comparing to GSM/GPRS modems. Some of the
limitations are described below:
mobile phone models (example: Ericsson R380) cannot be used with a
computer to receive concatenated SMS messages.
What is a
concatenated SMS message?
A concatenated SMS message is a message
that contains more than 140 bytes. (A normal SMS message can only
contain at most 140 bytes.) Concatenated SMS works like this: the
sender's mobile device breaks a message longer than 140 bytes into
smaller parts. Each of these parts are then fitted in a single SMS
message and sent to the recipient. When these SMS messages reach the
destination, the recipient's mobile device will combine them back to
What is the cause of the problem?
mobile phone receives the SMS messages that are parts of a
concatenated SMS message, it combines them to one message
automatically. The correct behavior should be: when the mobile phone
receives the SMS messages that are parts of a concatenated SMS
message, it forwards them to the computer without combining them.
mobile phone models cannot be used with a computer to receive MMS
messages. Because when they receive a MMS notification, they handle
it automatically instead of forwarding it to the computer.
mobile phone may not support some AT commands, command parameters
and parameter values. For example, some mobile phones do not support
the sending and receiving of SMS messages in text mode. So, the AT
command "AT+CMGF=1" (it instructs the mobile phone to use
text mode) will cause an error message to be returned. Usually
GSM/GPRS modems support a more complete set of AT commands than
SMS messaging applications have to be available 24 hours a day. (For
example, an SMS messaging application that provides ringtone
downloading service should be running all the time so that a user
can download ringtones any time he/she wants.) If such SMS messaging
applications use mobile phones to send and receive SMS messages, the
mobile phones have to be switched on all the time. However, some
mobile phone models cannot operate with the battery removed even
when an AC adaptor is connected, which means the battery will be
charged 24 hours a day.
the above issues, mobile phones and GSM/GPRS modems are more or less
the same for sending and receiving SMS messages from a computer.
Actually, you can consider an AT-command-enabled mobile phone as
"GSM/GPRS modem + keypad + display + ...".
is not much difference between mobile phones and GSM/GPRS modems in
terms of SMS transmission rate, since the determining factor for the
SMS transmission rate is the wireless network.
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