1.What is a GSM Modem
A 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
A 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 computer.
Like a GSM mobile phone, a GSM modem requires a SIM card from a wireless carrier
in order to operate.
As 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.
In 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:
Reading, writing and deleting SMS messages.
Sending SMS messages.
Monitoring the signal strength.
Monitoring the charging status and charge level of the battery.
Reading, writing and searching phone book entries.
The 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.
2. What is a GPRS
A 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 transmission speed.
GPRS 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.
If you need to send or receive MMS messages, a GPRS modem is typically needed.
3. Which is Better: Mobile Phone or GSM / GPRS Modem?
In 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:
Some 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 one message.
What is the cause of the problem?
When the 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
Many 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.
A 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 mobile phones.
Most 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.
Besides 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 + ...".
There 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.