How does AT commands work?Connecting to hyperterminal with
ME.
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Answer Posted / rajendra prasad reddy renumall
AT commands are instructions used to control a modem. AT is
the abbreviation of ATtention. Every command line starts
with "AT" or "at". That's why modem commands are called AT
commands. Many of the commands that are used to control
wired dial-up modems, such as ATD (Dial), ATA (Answer), ATH
(Hook control) and ATO (Return to online data state), are
also supported by GSM/GPRS modems and mobile phones.
Besides this common AT command set, GSM/GPRS modems and
mobile phones support an AT command set that is specific to
the GSM technology, which includes SMS-related commands
like AT+CMGS (Send SMS message), AT+CMSS (Send SMS message
from storage), AT+CMGL (List SMS messages) and AT+CMGR
(Read SMS messages).
Note that the starting "AT" is the prefix that informs the
modem about the start of a command line. It is not part of
the AT command name. For example, D is the actual AT
command name in ATD and +CMGS is the actual AT command name
in AT+CMGS. However, some books and web sites use them
interchangeably as the name of an AT command.
Here are some of the tasks that can be done using AT
commands with a GSM/GPRS modem or mobile phone:
Get basic information about the mobile phone or GSM/GPRS
modem. For example, name of manufacturer (AT+CGMI), model
number (AT+CGMM), IMEI number (International Mobile
Equipment Identity) (AT+CGSN) and software version
(AT+CGMR).
Get basic information about the subscriber. For example,
MSISDN (AT+CNUM) and IMSI number (International Mobile
Subscriber Identity) (AT+CIMI).
Get the current status of the mobile phone or GSM/GPRS
modem. For example, mobile phone activity status (AT+CPAS),
mobile network registration status (AT+CREG), radio signal
strength (AT+CSQ), battery charge level and battery
charging status (AT+CBC).
Establish a data connection or voice connection to a remote
modem (ATD, ATA, etc).
Send and receive fax (ATD, ATA, AT+F*).
Send (AT+CMGS, AT+CMSS), read (AT+CMGR, AT+CMGL), write
(AT+CMGW) or delete (AT+CMGD) SMS messages and obtain
notifications of newly received SMS messages (AT+CNMI).
Read (AT+CPBR), write (AT+CPBW) or search (AT+CPBF)
phonebook entries.
Perform security-related tasks, such as opening or closing
facility locks (AT+CLCK), checking whether a facility is
locked (AT+CLCK) and changing passwords (AT+CPWD).
(Facility lock examples: SIM lock [a password must be given
to the SIM card every time the mobile phone is switched on]
and PH-SIM lock [a certain SIM card is associated with the
mobile phone. To use other SIM cards with the mobile phone,
a password must be entered.])
Control the presentation of result codes / error messages
of AT commands. For example, you can control whether to
enable certain error messages (AT+CMEE) and whether error
messages should be displayed in numeric format or verbose
format (AT+CMEE=1 or AT+CMEE=2).
Get or change the configurations of the mobile phone or
GSM/GPRS modem. For example, change the GSM network
(AT+COPS), bearer service type (AT+CBST), radio link
protocol parameters (AT+CRLP), SMS center address (AT+CSCA)
and storage of SMS messages (AT+CPMS).
Save and restore configurations of the mobile phone or
GSM/GPRS modem. For example, save (AT+CSAS) and restore
(AT+CRES) settings related to SMS messaging such as the SMS
center address.
Note that mobile phone manufacturers usually do not
implement all AT commands, command parameters and parameter
values in their mobile phones. Also, the behavior of the
implemented AT commands may be different from that defined
in the standard. In general, GSM/GPRS modems designed for
wireless applications have better support of AT commands
than ordinary mobile phones.
In addition, some AT commands require the support of mobile
network operators. For example, SMS over GPRS can be
enabled on some GPRS mobile phones and GPRS modems with the
+CGSMS command (command name in text: Select Service for MO
SMS Messages). But if the mobile network operator does not
support the transmission of SMS over GPRS, you cannot use
this feature.
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