Sending SMS Messages from a Computer / PC Using AT Commands (AT+CMGS, AT+CMSS) (Tutorial 25)

Either of the AT commands +CMGS (command name in text: Send Message) and +CMSS (command name in text: Send Message from Storage) can be used to send SMS messages from a computer / PC. The key difference between them is that the +CMGS AT command takes the SMS message to be sent as a parameter, while the +CMSS AT command takes the index number that specifies the location of the SMS message in the message storage area as a parameter. Following is an example for illustrating the difference. Suppose you want to send the text message "Sending text messages is easy." from a computer / PC to the mobile phone number 91234567 using the +CMGS AT command in SMS text mode. Here is the command line to be used:

AT+CMGS="91234567"Sending text messages is easy.

To send the same text message using the +CMSS AT command, first you have to use the AT command +CMGW (command name in text: Write Message to Memory) to write the text message to the message storage area. In SMS text mode, the command line should be:

AT+CMGW="91234567"Sending text messages is easy.

Let's say the SMS text message is now located at index 3 of the message storage area. You can use the +CMSS AT command to send the text message to the message center by the following command line:

AT+CMSS=3

As storage space is limited, if every SMS text message sent is left in the message storage area, there will come a time when no more SMS text messages can be written. To free some storage space, you can use the AT command +CMGD (command name in text: Delete Message) to delete the SMS text message from the message storage area, like this:

AT+CMGD=3

As you can see above, sending an SMS message by the +CMSS AT command is a bit cumbersome, since it involves more steps and AT commands. However, a copy of the sent SMS message can be saved in the message storage area. This is not achievable with the +CMGS AT command.

In some situations, it is more convenient to send SMS messages by the +CMSS AT command than the +CMGS AT command. For example, if you have to send the same SMS message to multiple recipients, using the +CMSS AT command is more convenient:

AT+CMSS=3,"91234567"

AT+CMSS=3,"97777777"

AT+CMSS=3,"96666666"

...

Note: To keep things simple in the above examples, we assume that the same message storage area is used by the AT commands +CMSS, +CMGW, +CMGD and for receiving SMS messages. But this may not be true in real situations. To learn how to set the message storage areas to be used by different SMS operations, please refer to the earlier section "Preferred Message Storage (AT+CPMS)" of this SMS tutorial.

25.1. Comparison Table of the AT Commands +CMGS and +CMSS

The comparison table below summarizes the differences between the AT commands +CMGS and +CMSS that are discussed in the previous section.

	+CMGS AT command	+CMSS AT command
Parameters	+CMGS takes the SMS message to be sent as a parameter.	+CMSS takes the index number that specifies the location of the SMS message in the message storage area as a parameter.
Sending new SMS messages	Using +CMGS is more convenient. +CMGS can be used to send new SMS messages without the help of other AT commands. However, no copy of the SMS message sent is saved in the message storage area.	+CMSS has to be used together with +CMGW and +CMGD for sending new SMS messages. As a result, the process involves more steps and is a bit cumbersome. However, a copy of the SMS message sent can be saved in the message storage area.
Sending the same SMS message to multiple recipients	Using +CMGS is less convenient.	Using +CMSS is more convenient.

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Deleting SMS Messages from Message Storage (AT+CMGD) (Tutorial 24)

The AT command +CMGD (command name in text: Delete Message) is used to delete SMS message(s) from message storage. The message storage area from which SMS messages are deleted is specified by the +CPMS AT command (command name in text: Preferred Message Storage). Details about the +CPMS AT command can be found in the earlier section "Preferred Message Storage (AT+CPMS)" of this SMS tutorial.

1. Syntax of the +CMGD AT Command

The syntax of the +CMGD AT command is: (Optional parameters are enclosed in square brackets.)

+CMGD=index[,flag]

In the above line, index is an integer specifying the location of the SMS message to be deleted from the message storage area by the +CMGD AT command, and flag is an integer specifying whether to delete SMS messages according to their message status. The SMS specification has defined these flag values: 0, 1, 2, 3 and 4.

• 0. Meaning: Delete only the SMS message stored at the location index from the message storage area. This is the default value.

• 1. Meaning: Ignore the value of index and delete all SMS messages whose status is "received read" from the message storage area.

• 2. Meaning: Ignore the value of index and delete all SMS messages whose status is "received read" or "stored sent" from the message storage area.

• 3. Meaning: Ignore the value of index and delete all SMS messages whose status is "received read", "stored unsent" or "stored sent" from the message storage area.

• 4. Meaning: Ignore the value of index and delete all SMS messages from the message storage area.

To find the flag values supported by your GSM/GPRS modem or mobile phone, you can perform a test operation using the +CMGD AT command. See the next section for the details.

2. Finding the Supported Indexes and Flag Values

Performing a test operation with the +CMGD AT command allows you to find all indexes that are supported by the index parameter. The test operation may also return all supported values of the flag parameter. Here is the format of the information response returned after the execution of the test command "+CMGD=?": (Optional fields are enclosed in square brackets.)

+CMGD: (list_of_indexes)[,(list_of_flag_values)]

• The values inside the first parentheses are the values that are supported by the index parameter of the +CMGD AT command.

• The values inside the second parentheses are the values that are supported by the flag parameter of the +CMGD AT command. Note: A GSM/GPRS modem or mobile phone might not provide these values since the SMS specification defines them as optional.

As an example, here is the response returned from my Nokia 6021 mobile phone to Windows' HyperTerminal program after the execution of the test command "+CMGD=?":

AT+CMGD=?
+CMGD: (1-10),(0-4)

OK

From above, we know that the Nokia 6021 mobile phone allows any integer between 1 and 10 to be assigned to the index parameter of the +CMGD AT command. Also, the Nokia 6021 mobile phone supports all of the five flag values defined in the SMS specification.

Do you remember that we described the format of values inside parentheses in the section "Test Command -- Checks Whether a Certain AT Command is Supported"? If not, you may want to go back and have a look. Here are a few examples showing some of the possibilities: "(0-4)", "(0,1,2,3,4)", "(0,2-4)".

3. Example Demonstrating How to Use the +CMGD AT Command to Delete SMS Text Messages

Now let's see an example that demonstrates how to use the +CMGD AT command to delete an SMS text message from the message storage area. Suppose you have written an SMS text message to the message storage area by the +CMGW AT command (details about the +CMGW AT command can be found in the earlier section "Writing SMS Messages to Memory / Message Storage (AT+CMGW)" of this SMS tutorial), like this:

AT+CMGW="+85291234567"
> A simple demo of SMS text messaging.
+CMGW: 1

OK

The information response of the +CMGW AT command tells us that the SMS text message is stored in the memory location at index 1. To delete the SMS text message, specify 1 to the index parameter of the +CMGD AT command, like this:

(Important note: Make sure you are writing text messages to and deleting text messages from the same message storage area. If not, you may delete the wrong text message. Details about message storage areas can be found in the earlier section "Preferred Message Storage (AT+CPMS)" of this SMS tutorial.)

AT+CMGD=1
OK

The final result code OK indicates the SMS text message "A simple demo of SMS text messaging." was deleted successfully. If the operation fails, the final result code returned will either be ERROR or +CMS ERROR.

Here is another example. Suppose you want to delete all SMS messages stored in the message storage area. To do this, assign 4 to the flags parameter of the +CMGD AT command. The value assigned to the index parameter does not matter since it will be ignored by the GSM/GPRS modem or mobile phone.

AT+CMGD=1,4
OK

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Writing SMS Messages to Memory / Message Storage (AT+CMGW) (Tutorial 23)

The AT command +CMGW (command name in text: Write Message to Memory) is used to write an SMS message to memory (i.e. message storage). The memory/message storage area to which SMS messages are written is specified by the +CPMS AT command (command line in text: Preferred Message Storage). Details about the +CPMS AT command can be found in the earlier section "Preferred Message Storage (AT+CPMS)" of this SMS tutorial.

Syntax of the +CMGW AT Command in SMS Text Mode

In SMS text mode, the syntax of the +CMGW AT command is: (Optional parameters are enclosed in square brackets.)

+CMGW[=address[,address_type[,message_status]]]sms_message_body

Before we discuss each of the parameters, let's see an example that gives you some idea of how an actual command line should look like:

AT+CMGW="+85291234567",145,"STO UNSENT"This is an example for illustrating the syntax of the +CMGW AT command in SMS text mode.

The address Parameter

The first parameter of the +CMGW AT command, address, specifies the destination address to send the SMS message to. Usually it is a mobile number formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163). For example, "+85291234567" and "91234567". Note that the value passed to the address parameter should be a string, i.e. it should be enclosed in double quotes.

The address parameter is optional and so it can be omitted. Later when you want to send the SMS message out, you can specify the destination address by the +CMSS AT command (command name in text: Send Message from Storage).

The address_type Parameter

The second parameter of the +CMGW AT command, address_type, specifies the type of the address assigned to the address parameter. Two values are commonly used. They are 129 and 145:

• 129. Meaning: The value of address is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) but it is not sure whether the value of address is an international number, a national number or a number of other types. Example addresses: "85291234567", "91234567".

• 145. Meaning: The value of address is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) and it is an international number. Example address: "+85291234567".

As address_type is an optional parameter, it can be omitted. If you do so, the GSM/GPRS modem or mobile phone will use the default value of the address_type parameter, which is:

• 129 if the value of address does not start with a "+" character. For example, "85291234567".

• 145 if the value of address starts with a "+" character. For example, "+85291234567".

The message_status Parameter

The third parameter of the +CMGW AT command, message_status, specifies the status of the SMS message to be written. The SMS specification has defined four status values:

• REC UNREAD. It refers to the message status "received unread".

• REC READ. It refers to the message status "received read".

• STO UNSENT. It refers to the message status "stored unsent". This is the default value.

• STO SENT. It refers to the message status "stored sent".

Note that the value assigned to the message_status parameter should be a string. Thus, it should be enclosed in double quotes.

As message_status is an optional parameter, it can be omitted. If you do so, the GSM/GPRS modem or mobile phone will use the default value of the message_status parameter, which is "STO UNSENT".

The Character

, which represents the carriage return character, follows the message_status parameter. When the GSM/GPRS modem or mobile phone receives the carriage return character, it will send back a prompt formed by these four characters: the carriage return character, the linefeed character, the ">" character and the space character. If you don't understand what this means, don't worry. This should be clear to you when you see the example in the section "Example Demonstrating How to Use the +CMGW AT Command to Write SMS Text Messages to Message Storage in SMS Text Mode".

The sms_message_body Parameter

The fourth parameter of the +CMGW AT command, sms_message_body, specifies the SMS message body to be written to the memory/message storage area. Entering the character will cancel the +CMGW AT command. If you don't understand what this means, see the example in the section "Example Demonstrating How to Use the +CMGW AT Command to Write SMS Text Messages to Message Storage in SMS Text Mode".

The Character

When you finish entering the SMS message body, you have to enter the character to mark the end of the SMS message body. The GSM/GPRS modem or mobile phone will then attempt to write the SMS message to the memory/message storage area.

SMSC Number Stored with the SMS Message (SMS Text Mode)

In SMS text mode, the +CMGW AT command does not have a parameter that allows you to specify an SMSC number. (Note that the +CMGW AT command does have such parameter when the GSM/GPRS modem or mobile phone is operating in SMS PDU mode.) However, when an SMS message is written to message storage, an SMSC number is actually stored with it. The SMSC number stored is the one specified by the +CSCA AT command (command name in text: Service Centre Address). Later if you send the SMS message, it will be transmitted through this SMSC. Once an SMS message has been written to message storage, it is not possible to change the SMSC number stored with the SMS message by using AT commands.

Note that it is possible that the command behavior on your mobile device is slightly different from what was described above.

More information about this issue is available in the "SMSC Number to be Used by the +CMSS AT Command to Send SMS Messages" section of this SMS tutorial.

Format of the Information Response of the +CMGW AT Command in SMS Text Mode

If the GSM/GPRS modem or mobile phone writes the SMS message to the message storage area successfully, it will return an information response to the computer / PC. In SMS text mode, the information response of the +CMGW AT command has the following format:

+CMGW: index

index is an integer that tells us the memory location to which the SMS message was written in the message storage area.

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Preferred Message Storage (AT+CPMS) (Tutorial 22)

The AT command +CPMS (command name in text: Preferred Message Storage) serves several purposes. It can be used to:

1. Select the message storage area that will be used when sending, receiving, reading, writing or deleting SMS messages.

2. Find the number of messages that are currently stored in the message storage area.

3. Find the maximum number of messages that can be stored in the message storage area.

Selecting the Message Storage Areas to be Used for SMS Reading, Writing, Deleting, Sending or Receiving

To select the message storage areas to be used for SMS reading, writing, deleting, sending or receiving, you should perform a set operation with the +CPMS AT command. The syntax is: (Optional parameters are enclosed in square brackets.)

+CPMS=message_storage1[,message_storage2[,message_storage3]]

Before we discuss each of the parameters, let's see an example that gives you some idea of how an actual command line should look like:

AT+CPMS="ME","SM","MT"

The message_storage1 Parameter

The first parameter of the +CPMS AT command, message_storage1, specifies the message storage area that will be used when reading or deleting SMS messages. (For details about reading SMS messages, see AT commands +CMGR and +CMGL. For details about deleting SMS messages, see the AT command +CMGD.)

The message_storage2 Parameter

The second parameter of the +CPMS AT command, message_storage2, specifies the message storage area that will be used when sending SMS messages from message storage or writing SMS messages. (For details about sending SMS messages from message storage, see the AT command +CMSS. For details about writing SMS messages, see the AT command +CMGW.)

The message_storage3 Parameter

The third parameter of the +CPMS AT command, message_storage3, specifies the preferred message storage area for storing newly received SMS messages. If you use the +CNMI AT command (command name in text: New Message Indications to TE) to tell the GSM/GPRS modem or mobile phone to forward newly received SMS messages directly to the PC instead of storing them in the message storage area, you do not need to care about the message_storage3 parameter.

Values that May be Assigned to the Parameters of the +CPMS AT Command

Here are the values defined in the SMS specification that may be assigned to the parameters message_storage1, message_storage2 and message_storage3:

• SM. It refers to the message storage area on the SIM card.

• ME. It refers to the message storage area on the GSM/GPRS modem or mobile phone. Usually its storage space is larger than that of the message storage area on the SIM card.

• MT. It refers to all message storage areas associated with the GSM/GPRS modem or mobile phone. For example, suppose a mobile phone can access two message storage areas: "SM" and "ME". The "MT" message storage area refers to the "SM" message storage area and the "ME" message storage area combined together.

• BM. It refers to the broadcast message storage area. It is used to store cell broadcast messages.

• SR. It refers to the status report message storage area. It is used to store status reports.

• TA. It refers to the terminal adaptor message storage area.

Note that a GSM/GPRS modem or mobile phone may not support all the above values and it may support other manufacturer specific values. To learn how to find the values supported by your GSM/GPRS modem or mobile phone, please refer to the section "Finding the Message Storage Areas Supported by a GSM/GPRS Modem or Mobile Phone".

Format of the Information Response of the +CPMS AT Command

If the GSM/GPRS modem or mobile phone executes the +CPMS set command successfully, it will return an information response to the computer / PC. The information response of the +CPMS AT command has the following format:

+CPMS: used_space1,max_space1,used_space2,max_space2,used_space3,max_space3

• used_space1 is the number of messages currently stored in the message storage area specified by the message_storage1 parameter.

• max_space1 is the maximum number of messages that can be stored in the message storage area specified by the message_storage1 parameter.

• used_space2 is the number of messages currently stored in the message storage area specified by the message_storage2 parameter.

• max_space2 is the maximum number of messages that can be stored in the message storage area specified by the message_storage2 parameter.

• used_space3 is the number of messages currently stored in the message storage area specified by the message_storage3 parameter.

• max_space3 is the maximum number of messages that can be stored in the message storage area specified by the message_storage3 parameter.

Finding the Message Storage Areas Supported by a GSM/GPRS Modem or Mobile Phone

Before we can use the +CPMS AT command to select the message storage areas to be used for various SMS operations, we have to know what message storage areas are supported by the GSM/GPRS modem or mobile phone. Among the six types of message storage area mentioned in the last section, "SM", "ME" and "MT" are commonly supported by GSM/GPRS modems and mobile phones. To find out the message storage areas that are supported by your GSM/GPRS modem or mobile phone, you can perform a test operation with the +CPMS AT command using the command line below:

AT+CPMS=?

If the GSM/GPRS modem or mobile phone executes the above command line successfully, it will return an information response to the computer / PC. The information response of the +CPMS AT command has the following format:

+CPMS: (storage1a,storage1b,...),(storage2a,storage2b,...),(storage3a,storage3b,...)

The first, second and third parentheses contain the values supported by the message_storage1 parameter, the message_storage2 parameter and the message_storage3 parameter respectively.

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Setting or Reading the Service Center Address / SMSC Address (AT+CSCA) (Tutorial 21)

Before you send SMS messages, one important thing that should be done is to make sure the correct service center address (SMSC address) has been set properly. Typically a service center address/SMSC address is a phone number formatted using the ISDN / telephony numbering plan (ITU E.164/E.163). For example, +85291234567. The AT command +CSCA (command name in text: Service Centre Address) can be used to set or read the service center address through which SMS messages are sent. The setting will be used by the AT commands +CMGW (command name in text: Write Message to Memory) and +CMGS (command name in text: Send Message).

In SMS text mode, the only way to specify the service center address to be used by +CMGW and +CMGS is through +CSCA. However, in SMS PDU mode, it is possible to specify the service center address to the AT commands +CMGW and +CMGS directly as a parameter value.

The mobile network operator usually pre-sets the correct service center number in the default profile of settings stored in the SIM card. Most mobile phones provide some ways for the user to view or change the service center number saved in the profiles of settings. For example, on my Nokia 6021 mobile phone, the message center number saved in the default profile of settings can be viewed and changed by going to Menu -> Messages -> Message settings -> Text messages -> Sending profile -> Default profile -> Message centre number.

Some GSM/GPRS modems and mobile phones (for example, Nokia 6021, Sony Ericsson T68i, Philips 598, etc) use the service center number in the default profile of settings as the default value of the +CSCA AT command. As a result, if the service center number in the default profile of settings is correct, you do not need to use the +CSCA AT command to set the service center number.

Note

To some mobile devices such as Nokia 6021, the +CSCA AT command does not save the service center number in non-volatile memory. Hence, if you switch off the GSM/GPRS modem or mobile phone, the setting will be gone. To save the service center number set with the AT command +CSCA, you have to make use of the AT command +CSAS (command name in text: Save Settings). To restore the saved service center number, the AT command +CRES (command name in text: Restore Settings) is required.

The command behavior is different on some mobile devices such as Sony Ericsson T68i and Philips 598. Every time the AT command +CSCA is used to change the SMSC address, the new SMSC address is saved to non-volatile memory.

21.1. Syntax of the +CSCA AT Command

21.1.1. Setting the Service Center Address

To set the service center address, perform a set operation with the +CSCA AT command using the following syntax: (Optional parameters are enclosed in square brackets.)

+CSCA=address[,address_type]

The address Parameter

The first parameter of the +CSCA AT command, address, specifies the SMSC address to be set. Usually it is a phone number formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163). For example, "+85291234567". Note that the value assigned to the address parameter should be a string, which means you should use double quotes to enclose the phone number.

The address_type Parameter

The second parameter of the +CSCA AT command, address_type, specifies the type of the SMSC address assigned to the address parameter. Two values, 129 and 145, are commonly used.

• 129. Meaning: The SMSC address is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) but it is not sure whether the SMSC address is an international number, a national number or a number of other types. Example addresses: "85291234567" and "91234567".

• 145. Meaning: The SMSC address is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) and it is an international number. Example address: "+85291234567".

As address_type is an optional parameter, it can be omitted. If you do so, the GSM/GPRS modem or mobile phone will use the default value of the address_type parameter, which is:

• 129 if the SMSC address does not start with a "+" character. For example, "85291234567".

• 145 if the SMSC address starts with a "+" character. For example, "+85291234567".

21.1.2. Reading the Service Center Address

To read the service center address, you can perform a read operation using the +CSCA read command "+CSCA?". The information response returned has the following format:

+CSCA: address,address_type

The definition of address and address_type is the same as above.

21.2. Example Demonstrating How to Use the +CSCA AT Command to Set and Read the Service Center Address

Now let's see an example that demonstrates how to use the +CSCA AT command to set and read the service center address. As said earlier, sometimes a proper service center address is in use by default and there is no need to make any changes. So first let's type the read command "+CSCA?" in a terminal program (for example, HyperTerminal in Microsoft Windows) to check if a proper service center address is in use currently. The response returned from a GSM/GPRS modem or mobile phone should be something like this:

AT+CSCA?
+CSCA: "+85291111111",145

OK

If you find that the service center address is not correct, use the +CSCA AT command to change it, like this:

AT+CSCA="+85290000000",145
OK

The final result code OK indicates the service center address was set successfully. If the execution of the command line fails, the final result code will be ERROR.

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Operating Mode: SMS Text Mode and SMS PDU Mode (Tutorial 20)

The SMS specification has defined two modes in which a GSM/GPRS modem or mobile phone can operate. They are called SMS text mode and SMS PDU mode. (PDU stands for Protocol Data Unit.) The mode that a GSM/GPRS modem or mobile phone is operating in determines the syntax of some SMS AT commands and the format of the responses returned after execution. Below are the SMS AT commands affected:

• +CMGS (Send Message)

• +CMSS (Send Message from Storage)

• +CMGR (Read Message)

• +CMGL (List Messages)

• +CMGW (Write Message to Memory)

• +CNMA (New Message Acknowledgement to ME/TA)

• +CMGC (Send Command)

The syntax of the unsolicited result codes below also depends on the mode in which the GSM/GPRS modem or mobile phone is operating:

• +CMT (Used to forward received SMS messages to the computer / PC.)

• +CBM (Used to forward received cell broadcast messages to the computer / PC.)

• +CDS (Used to forward received status reports to the computer / PC.)

These two AT commands are useful to you only if SMS text mode is used:

• +CSMP (Set Text Mode Parameters)

• +CSDH (Show Text Mode Parameters)

Comparison of SMS Text Mode and SMS PDU Mode

Below we compare SMS text mode and SMS PDU mode from various aspects. The comparison should help you learn the differences between these two modes and decide which mode should be used by your SMS messaging application.

Syntax of SMS AT Commands and Responses

When the GSM/GPRS modem or mobile phone is operating in different modes, the syntax of certain SMS AT commands and the responses returned after command execution is different. Here's an example for illustration. Let's say you would like to send the SMS message "It is easy to send text messages." to the mobile phone number +85291234567. In SMS text mode, this is the command line that you should enter:

AT+CMGS="+85291234567"It is easy to send text messages.

However, if the GSM/GPRS modem or mobile phone is operating in SMS PDU mode, executing the above command line will cause an error to occur. This is because the syntax of the +CMGS AT command is different in SMS PDU mode. To do the same task, the following command line should be used instead:

AT+CMGS=4207915892000000F001000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

Defined Values for Certain Parameters

When the GSM/GPRS modem or mobile phone is operating in different modes, the defined values for certain parameters are different. Usually string values are defined for text mode while numeric values are defined for PDU mode. For example, the +CMGL AT command is used to list SMS messages stored in message storage. It takes one parameter that specifies the status of the SMS messages to be retrieved. The following table lists the defined values for the parameter in text mode and PDU mode:

Message status	Defined values in text mode	Defined values in PDU mode
Received unread	"REC UNREAD"	0
Received read	"REC READ"	1
Stored unsent	"STO UNSENT"	2
Stored sent	"STO SENT"	3
All messages	"ALL"	4

Suppose you would like to list all SMS messages from message storage. If the GSM/GPRS modem or mobile phone is operating in SMS text mode, you should assign the string value "ALL" to the +CMGL AT command, like this:

AT+CMGL="ALL"

In SMS PDU mode, the numeric value 4 should be assigned to the +CMGL AT command instead:

AT+CMGL=4

Input/Output Format of SMS Messages Used by SMS AT Commands

When the GSM/GPRS modem or mobile phone is operating in different modes, the input/output format of SMS messages used by SMS AT commands is different. In SMS text mode, headers and body of SMS messages are inputted/outputted as separate parameters/fields. In SMS PDU mode, TPDUs (Transport Protocol Data Units) in hexadecimal format are inputted and outputted. Headers and body of SMS messages are encoded in the TPDUs.

Here is an example for illustration. To send the SMS message "It is easy to send text messages." to the mobile phone number +85291234567, the following command line should be used in SMS text mode. As you can see below, the destination phone number header and message body are provided to the +CMGS AT command as separate parameters.

AT+CMGS="+85291234567"It is easy to send text messages.

To send the same SMS text message in SMS PDU mode, the following command line should be used instead. The message body, destination phone number header and some other headers are encoded in the hexadecimal sequence.

AT+CMGS=4207915892000000F001000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

Ease of Use

As you can see in the previous example, it is easier to use AT commands in SMS text mode. You do not have to learn about the structure of different types of TPDUs in the bit level and the encoding and decoding of the hexadecimal sequence.

Supported Features of SMS Messaging

Although it is easier to use AT commands in SMS text mode, it supports fewer features of SMS messaging than SMS PDU mode. This is because you do not have complete control over the header values and message body in SMS text mode. Some tasks, although can be done in text mode, require the programmer to have knowledge about PDU mode and TPDU. For example, to request a status report from SMSC in SMS text mode, you have to set bit 5 of the first octet of the SMS-SUBMIT TPDU to 1 by the AT command +CSMP (command name in text: Set Text Mode Parameters). Similar tasks include setting the message validity period and sending a flash SMS message that immediately pops up on the phone screen when it arrives at the destination.

Level of Support

SMS PDU mode is more commonly supported by GSM/GPRS modems and mobile phones than SMS text mode.

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Checking if the GSM/GPRS Modem or Mobile Phone Supports the Use of AT Commands to Send, Receive and Read SMS Messages (Tutorial 19)

After testing the communication between the PC and GSM/GPRS modem/mobile phone, the next thing that you may want to do is to check if the GSM/GPRS modem or mobile phone supports the use of AT commands to send, receive and read SMS messages. Most GSM/GPRS modems support all three functions, while only some mobile phones do.

Sending SMS Messages

To find out whether a GSM/GPRS modem or mobile phone supports the sending of SMS messages through AT commands, you have to:

1. Use the AT command +CSMS (command name in text: Select Message Service) to check whether mobile-originated SMS messages are supported.

2. Perform test operations to check whether +CMGS (command name in text: Send Message) and/or +CMSS (command name in text: Send Message from Storage) are supported.
   (You may want to check the AT commands +CMGW [command name in text: Write Message to Memory] and +CMGD [command name in text: Delete Message] in addition as they are sometimes used together with +CMSS.)

Receiving SMS Messages and Reading SMS Messages from Message Storage

To find out whether a GSM/GPRS modem or mobile phone supports the receiving and reading of SMS messages through AT commands, you have to:

1. Use the AT command +CSMS (command name in text: Select Message Service) to check whether mobile-terminated SMS messages are supported.

2. Perform test operations to check whether +CNMI (command name in text: New Message Indications to TE), +CMGL (command name in text: List Messages) and/or +CMGR (command name in text: Read Message) are supported.

If the GSM/GPRS modem or mobile phone supports the +CNMI AT command, it can send a notification or directly forward the message to the PC whenever a new SMS message arrives.

If the GSM/GPRS modem or mobile phone does not support +CNMI but supports +CMGL and/or +CMGR, the PC has to poll the GSM/GPRS modem or mobile phone repeatedly in order to know if any new SMS messages have arrived.

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Testing the Communication between the PC and GSM/GPRS Modem or Mobile Phone (Tutorial 18)

Suppose you have connected your GSM/GPRS modem or mobile phone to your PC / computer and started a terminal program (such as HyperTerminal on Microsoft Windows). Now you are ready to enter your first command. The first thing that is usually done is to test the communication between the PC and GSM/GPRS modem/mobile phone to confirm that everything is working properly so far. Simply enter "AT" in the terminal program to perform the test. When the GSM/GPRS modem or mobile phone receives "AT", it will send back the final result code "OK" to indicate that it has received your command successfully, like this:

AT
OK

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AT Command Operations: Test, Set, Read and Execution (Tutorial 17)

There are four types of AT command operations:

• Test operation. A test operation is used to check whether a certain AT command is supported by the GSM/GPRS modem or mobile phone.

• Set operation. A set operation is used to change the settings used by the GSM/GPRS modem or mobile phone for certain tasks.

• Read operation. A read operation is used to retrieve the current settings used by the GSM/GPRS modem or mobile phone for certain tasks.

• Execution operation. An execution operation is used to perform an action or retrieve information/status about the GSM/GPRS modem or mobile phone.

The command syntax for performing an operation will be described in detail in the following sections.

Test Command -- Checks Whether a Certain AT Command is Supported

A test operation is used to check whether a certain AT command is supported by the GSM/GPRS modem or mobile phone. All extended AT commands support the test operation. The syntax is:

command=?

where command is an AT command. When an AT command is used in the above syntax to perform a test operation, it is called a test command.

Here is an example. The AT command +CGMI (command name in text: Request Manufacturer Identification) is used to get the manufacturer name of the GSM/GPRS modem or mobile phone. To test whether +CGMI is supported, you can make use of the test command "+CGMI=?". The complete command line that should be entered is:

AT+CGMI=?

If the GSM/GPRS modem or mobile phone supports the AT command +CGMI, the result code "OK" will be returned, like this:

AT+CGMI=?
OK

If the GSM/GPRS modem or mobile phone does not support the AT command +CGMI, the result code "ERROR" will be returned, like this:

AT+CGMI=?
ERROR

In the above example, the AT command +CGMI does not have any parameters. If the AT command to be tested has parameter(s), the parameter value(s) supported by the GSM/GPRS modem or mobile phone may be printed additionally. Below is an example that illustrates the format of the response. +COMMAND1 is a fictitious AT command that has four parameters.

AT+COMMAND1=?
+COMMAND1: (0,1),(0-10),(0,1,5-10),("GSM","UCS2")

OK

The supported values of each of the four parameters are enclosed in parentheses. Commas are used to delimit the parentheses and the values inside parentheses. A hyphen is used to indicate a range of values. The values inside parentheses can be of the string type.

In the above example, the response of the test command "+COMMAND1=?" provides us the following information:

• (0,1). The first parameter accepts either 0 or 1.

• (0-10). The second parameter accepts any integer between 0 and 10.

• (0,1,5-10). The third parameter accepts 0, 1 or any integer between 5 and 10.

• ("GSM","UCS2"). The fourth parameter accepts either the string "GSM" or "UCS2".

To a few AT commands, the test operation does not return the parameter values supported. Instead, it returns the values that are allowed to appear in the information response of the AT command. An example is the +CBC AT command (command name in text: Battery Charge). The +CBC command is used to retrieve the connection status and charge level of the battery of the mobile device. Two values are returned in the information response of the +CBC AT command. The format is:

+CBC: connection_status,charge_level

For example, if the battery is placed in the mobile device with no charger connected and the charge level is 80%, the result of the execution of the +CBC AT command will be:

AT+CBC
+CBC: 0,80

OK

If you run the test command "+CBC=?", all the supported values that are allowed to appear in the connection status field and charge level field will be provided. With my Nokia 6021, the result is:

AT+CBC=?
+CBC: (0,1),(0-100)

OK

"(0,1)" means the connection status field in the information response of the +CBC AT command can contain either 0 or 1, while "(0-100)" means the charge level field can contain any integer between 0 and 100.

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Result Codes of AT Commands (Tutorial 16)

Result codes are messages sent from the GSM/GPRS modem or mobile phone to provide you information about the execution of an AT command and the occurrence of an event. Two types of result codes are useful to you when dealing with AT commands for SMS messaging:

• Final result codes

• Unsolicited result codes

Final Result Codes of AT Commands

A final result code marks the end of an AT command response. It is an indication that the GSM/GPRS modem or mobile phone has finished the execution of a command line. Two frequently used final result codes are OK and ERROR. Only one final result code will be returned for each command line. Thus, you will not see both OK and ERROR in the response of a command line.

1. The OK Final Result Code

The OK final result code indicates that a command line has been executed successfully by the GSM/GPRS modem or mobile phone. It always starts and ends with a carriage return character and a linefeed character.

Here is an example for illustration. Let's say you send the command line "AT+CMGL;+CGMI" to your GSM/GPRS modem. The AT command "+CMGL" is used to list SMS messages stored in the message storage area and the AT command "+CGMI" is used to get the manufacturer name of the GSM/GPRS modem. If everything works properly without any errors, the command line, together with the response returned, should be something similar to this:

AT+CMGL;+CGMI
+CMGL: 1,"REC UNREAD","+85291234567",,"06/11/11,00:30:29+32"
Welcome to our SMS tutorial.
Nokia
OK

As mentioned earlier, when a terminal program such as HyperTerminal of Microsoft Windows sees a carriage return character, it moves the cursor to the beginning of the current line. When it sees a linefeed character, it moves the cursor to the same position on the next line. Hence, the command line you entered, together with the response returned, will be displayed like this in a terminal program such as HyperTerminal of Microsoft Windows:

AT+CMGL;+CGMI
+CMGL: 1,"REC UNREAD","+85291234567",,"06/11/11,00:30:29+32"
Welcome to our SMS tutorial.

Nokia

OK

2. The ERROR Final Result Code

The ERROR final result code indicates that an error occurs when the GSM/GPRS modem or mobile phone tries to execute a command line. After the occurrence of an error, the GSM/GPRS modem or mobile phone will not process the remaining AT commands in the command-line string.

Below are some common causes of error:

• The syntax of the command line is incorrect.

• The value specified to a certain parameter is invalid.

• The name of the AT command is spelt incorrectly.

• The GSM/GPRS modem or mobile phone does not support one or more of the AT commands, command parameters or parameter values in the command-line string.

Like the OK final result code, the ERROR final result code always starts and ends with a carriage return character and a linefeed character.

Here is an example for illustration. Suppose you want to instruct your GSM/GPRS modem to list SMS messages from the message storage area and get the manufacturer name of the GSM/GPRS modem. You intend to type the command line "AT+CMGL;+CGMI" but make a careless mistake by typing "+CMFL" instead of "+CMGL". The GSM/GPRS modem will return the ERROR final result code, as shown below:

AT+CMFL;+CGMI
ERROR

As an error occurs when the GSM/GPRS modem processes "+CMFL", the GSM/GPRS modem stops the execution of the command line and so the second AT command "+CGMI" is not processed.

If you type the second AT command "+CGMI" incorrectly instead of the first AT command "+CMGL", the GSM/GPRS modem will output the result of the execution of the AT command "+CMGL" before outputting the ERROR final result code, like this:

AT+CMGL;+CGMU
+CMGL: 1,"REC UNREAD","+85291234567",,"06/11/11,00:30:29+32"
Welcome to our SMS tutorial.
ERROR

As mentioned earlier, when a terminal program such as HyperTerminal of Microsoft Windows sees a carriage return character, it moves the cursor to the beginning of the current line. When it sees a linefeed character, it moves the cursor to the same position on the next line. Hence, the command line you entered, together with the response returned, will be displayed like this in a terminal program such as HyperTerminal of Microsoft Windows:

AT+CMGL;+CGMU
+CMGL: 1,"REC UNREAD","+85291234567",,"06/11/11,00:30:29+32"
Welcome to our SMS tutorial.

ERROR

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General Syntax of Extended AT Commands (Tutorial 15)

The general syntax of extended AT commands is straightforward. The syntax rules are provided below. The syntax of basic AT commands is slightly different. We will not cover the syntax of basic AT commands in this SMS tutorial since all SMS messaging commands are extended AT commands.

Syntax rule 1. All command lines must start with "AT" and end with a carriage return character. (We will use to represent a carriage return character in this SMS tutorial.) In a terminal program like HyperTerminal of Microsoft Windows, you can press the Enter key on the keyboard to output a carriage return character.

Example: To list all unread inbound SMS messages stored in the message storage area, type "AT", then the extended AT command "+CMGL", and finally a carriage return character, like this:

AT+CMGL

Syntax rule 2. A command line can contain more than one AT command. Only the first AT command should be prefixed with "AT". AT commands in the same command-line string should be separated with semicolons.

Example: To list all unread inbound SMS messages stored in the message storage area and obtain the manufacturer name of the mobile device, type "AT", then the extended AT command "+CMGL", followed by a semicolon and the next extended AT command "+CGMI":

AT+CMGL;+CGMI

An error will occur if both AT commands are prefixed with "AT", like this:

AT+CMGL;AT+CGMI

Syntax rule 3. A string is enclosed between double quotes.

Example: To read all SMS messages from message storage in SMS text mode (at this time you do not need to know what SMS text mode is. More information will be provided later in this SMS tutorial), you need to assign the string "ALL" to the extended AT command +CMGL, like this:

AT+CMGL="ALL"

Syntax rule 4. Information responses and result codes (including both final result codes and unsolicited result codes) always start and end with a carriage return character and a linefeed character.

Example: After sending the command line "AT+CGMI" to the mobile device, the mobile device should return a response similar to this:

Nokia
OK

The first line is the information response of the AT command +CGMI and the second line is the final result code. and represent a carriage return character and a linefeed character respectively. The final result code "OK" marks the end of the response. It indicates no more data will be sent from the mobile device to the computer / PC.

When a terminal program such as HyperTerminal of Microsoft Windows sees a carriage return character, it moves the cursor to the beginning of the current line. When it sees a linefeed character, it moves the cursor to the same position on the next line. Hence, the command line "AT+CGMI" that you entered and the corresponding response will be displayed like this in a terminal program such as HyperTerminal of Microsoft Windows:

AT+CGMI
Nokia

OK

Information Response and Final Result Code

Don't forget the meanings of information response and final result code stated above, since you will see these two terms frequently as you go through this SMS tutorial.

AT+CGMI <-- Command line entered
Nokia <-- Information response

OK <-- Final result code

Case Sensitivity of AT Commands

In the SMS specification, all AT commands are in uppercase letters. However, many GSM/GPRS modems and mobile phones allow you to type AT commands in either uppercase or lowercase letters. For example, on Nokia 6021, AT commands are case-insensitive and the following two command lines are equivalent:

AT+CMGL

at+cmgl

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Introduction to AT Commands (Tutorial 14)

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.

Basic Commands and Extended Commands

There are two types of AT commands: basic commands and extended commands.

Basic commands are AT commands that do not start with "+". For example, D (Dial), A (Answer), H (Hook control) and O (Return to online data state) are basic commands.

Extended commands are AT commands that start with "+". All GSM AT commands are extended commands. For example, +CMGS (Send SMS message), +CMSS (Send SMS message from storage), +CMGL (List SMS messages) and +CMGR (Read SMS messages) are extended commands.

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How to Use Microsoft HyperTerminal to Send AT Commands to a Mobile Phone or GSM/GPRS Modem? (Tutorial 13)

What is Microsoft HyperTerminal?

Microsoft HyperTerminal is a small program that comes with Microsoft Windows. You can use it to send AT commands to your mobile phone or GSM/GPRS modem. It can be found at Start -> Programs -> Accessories -> Communications -> HyperTerminal. If you cannot find it and you are using Windows 98, then probably you have not installed it. You can go to Control Panel -> Add/Remove Programs -> Windows Setup tab -> Communications list box item -> Details button to install MS HyperTerminal.

Before you start programming your SMS application, you may want to check if your mobile phone, GSM/GPRS modem and SIM card are working properly first. The MS HyperTerminal is a handy tool when it comes to testing your GSM devices. It is a good idea to test your GSM devices beforehand. When a problem occurs, sometimes it is difficult to tell what causes the problem. The cause can be your program, the GSM device or the SIM card. If you test your GSM device and SIM card with MS HyperTerminal and they operate properly, then it is very likely that the problem is caused by your program.

For Linux users, minicom can be used instead of HyperTerminal.

13.2. The Procedure for Sending AT Commands to a Mobile Phone or GSM/GPRS Modem Using MS HyperTerminal

To use MS HyperTerminal to send AT commands to your mobile phone or GSM/GPRS modem, you can follow the procedure below:

1. Put a valid SIM card into the mobile phone or GSM/GPRS modem. You can obtain a SIM card by subscribing to the GSM service of a wireless network operator.

2. Connect your mobile phone or GSM/GPRS modem to a computer and set up the corresponding wireless modem driver. You should find the wireless modem driver in the CD or disk that was provided by the manufacturer. If the manufacturer does not provide such CD or disk with your mobile phone or GSM/GPRS modem, you can go to the manufacturer's web site and see whether the wireless modem driver can be downloaded there. If the wireless modem driver cannot be found on the web site, you can still use Windows' standard modem driver.

3. Run MS HyperTerminal by selecting Start -> Programs -> Accessories -> Communications -> HyperTerminal.

4. In the Connection Description dialog box, enter a name and choose an icon you like for the connection. Then click the OK button.

Figure 5. The screenshot of MS HyperTerminal's Connection Description dialog box in Windows 98.

5. In the Connect To dialog box, choose the COM port that your mobile phone or GSM/GPRS modem is connecting to in the Connect using combo box. For example, choose COM1 if your mobile phone or GSM/GPRS modem is connecting to the COM1 port. Then click the OK button.
   
   (Sometimes there will have more than one COM port in the Connect using combo box. To know which COM port is used by your mobile phone or GSM/GPRS modem, follow the procedure below:
   
   In Windows 98:
   Go to Control Panel -> Modem. Then click the Diagnostics tab. In the list box, you can see which COM port the mobile phone or GSM/GPRS modem is connected to.
   
   In Windows 2000 and Windows XP:
   Go to Control Panel -> Phone and Modem Options. Then click the Modems tab. In the list box, you can see which COM port the mobile phone or GSM/GPRS modem is connected to.)

Figure 6. The screenshot of MS HyperTerminal's Connect To dialog box in Windows 98.

6. The Properties dialog box comes out. Enter the correct port settings for your mobile phone or GSM/GPRS modem. Then click the OK button.
   
   (To find the correct port settings that should be used with your mobile phone or GSM/GPRS modem, one way is to consult the manual of your mobile phone or GSM/GPRS modem. Another way is to check the port settings used by the wireless modem driver that you installed earlier.
   
   To check the port settings used by the wireless modem driver on Windows 98, follow these steps:
   a. Go to Control Panel -> Modem.
   b. Select your mobile phone or GSM/GPRS modem in the list box.
   c. Click the Properties button.
   d. The Properties dialog box appears. The Maximum speeds field on the General tab corresponds to HyperTerminal's Bits per second field. Click the Connection tab and you can find the settings for data bits, parity and stop bits. Click the Advanced button and you can find the setting for flow control.
   
   To check the port settings used by the wireless modem driver on Windows 2000 and Windows XP, follow these steps:
   a. Go to Control Panel -> Phone and Modem Options -> Modems tab.
   b. Select your mobile phone or GSM/GPRS modem in the list box.
   c. Click the Properties button.
   d. The Properties dialog box appears. Click the Advanced tab and then click the Change Default Preferences button.
   e. The Change Default Preferences dialog box appears. The Port speed field on the General tab corresponds to HyperTerminal's Bits per second field. You can also find the setting for flow control on the General tab. On the Advanced tab, you can find the settings for data bits, parity and stop bits.)

Figure 7. The screenshot of MS HyperTerminal's Properties dialog box in Windows 98.

7. Type "AT" in the main window. A response "OK" should be returned from the mobile phone or GSM/GPRS modem.
   Type "AT+CPIN?" in the main window. The AT command "AT+CPIN?" is used to query whether the mobile phone or GSM/GPRS modem is waiting for a PIN (personal identification number, i.e. password). If the response is "+CPIN: READY", it means the SIM card does not require a PIN and it is ready for use. If your SIM card requires a PIN, you need to set the PIN with the AT command "AT+CPIN=".

Figure 8. The screenshot of MS HyperTerminal's main window in Windows 98.

If you get the responses above, your mobile phone or GSM/GPRS modem is working properly. You can start typing your own AT commands to control the mobile phone or GSM/GPRS modem.

Further details about how to use AT commands to send and receive SMS messages will be provided in the following sections.

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Introduction to GSM / GPRS Wireless Modems - Tutorial(12)

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 radio waves.

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.

What is a GPRS Modem?

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.

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 them.

• 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.

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How to Receive SMS Messages Using a Computer / PC? - Tutorial(11)

In general, there are three ways to receive SMS messages using your computer / PC:

1. Connect a mobile phone or GSM/GPRS modem to a computer / PC. Then use the computer / PC and AT commands to get the received SMS messages from the mobile phone or GSM/GPRS modem.

2. Get access to the SMS center (SMSC) or SMS gateway of a wireless carrier. Any SMS messages received will be forwarded to your computer / PC using a protocol / interface supported by the SMSC or SMS gateway.

3. Get access to the SMS gateway of an SMS service provider. Any SMS messages received will be forwarded to your computer / PC using a protocol / interface supported by the SMS gateway.

If you do not want to develop SMS software or applications but just want to use your computer / PC to receive text messages, you may want to read our Quick Guide for Non-Developers.

The 1st Way: Using a Computer to Receive SMS Messages through a Mobile Phone or GSM/GPRS Modem

Receiving SMS messages through a mobile phone or GSM/GPRS modem has a major advantage over the other two ways -- wireless carriers usually do not charge any fees for receiving incoming SMS messages with their SIM cards. The disadvantage of receiving SMS messages this way is that a mobile phone or GSM/GPRS modem cannot handle a large amount of SMS traffic. One way to overcome this is to load balance the SMS traffic with a pool of mobile phones or GSM/GPRS modems. Each mobile phone or GSM/GPRS modem will have its own SIM card and mobile phone number.

In terms of programming, sending and receiving SMS messages through a mobile phone or GSM/GPRS modem are similar. What you need to do is to send instructions (in the form of AT commands) to the mobile phone or GSM/GPRS modem.

The following table lists the AT commands that are related to the receiving and reading of SMS messages:

AT command	Meaning
+CNMI	New message indications
+CMGL	List messages
+CMGR	Read messages
+CNMA	New message acknowledgement

Below shows a simple example that demonstrates how to use AT commands and the HyperTerminal program of Microsoft Windows to read SMS text messages received by a GSM / GPRS modem or mobile phone. The lines in bold type are the command lines that should be entered in HyperTerminal. The other lines are responses returned from the GSM / GPRS modem or mobile phone.

AT
OK
AT+CMGF=1
OK
AT+CMGL="ALL"
+CMGL: 1,"REC READ","+85291234567",,"06/11/11,00:30:29+32"
Hello, welcome to our SMS tutorial.
+CMGL: 2,"REC READ","+85291234567",,"06/11/11,00:32:20+32"
A simple demo of SMS text messaging.

OK

Here is a description of what is done in the above example:

• Line 1: "AT" is sent to the GSM / GPRS modem to test the connection. The GSM / GPRS modem sends back the result code "OK" (line 2), which means the connection between the HyperTerminal program and the GSM / GPRS modem works fine.

• Line 3: The AT command +CMGF is used to instruct the GSM / GPRS modem to operate in SMS text mode. The result code "OK" is returned (line 4), which indicates the command line "AT+CMGF=1" has been executed successfully. If the result code "ERROR" is returned, it is likely that the GSM / GPRS modem does not support the SMS text mode. To confirm, type "AT+CMGF=?" in the HyperTerminal program. If the response is "+CMGF: (0,1)" (0=PDU mode and 1=text mode), then SMS text mode is supported. If the response is "+CMGF: (0)", then SMS text mode is not supported.

• Line 5-9: The AT command +CMGL is used to list all SMS text messages in the message storage of the GSM / GPRS modem. There are two SMS text messages in the message storage: "Hello, welcome to our SMS tutorial." and "A simple demo of SMS text messaging.". "+85291234567" is the sender mobile phone number. "06/11/11,00:30:29+32" and "06/11/11,00:32:20+32" tell us when the SMS text messages were received by the SMSC. "+32" is the time zone. Note that the unit is a quarter of an hour. So, +32 means GMT+8 hours, since 32 quarters of an hour = 8 hours. "REC READ" indicates both of the SMS text messages have been read before.

• Line 11: The result code "OK" indicates the execution of the AT command +CMGL is successful.

To enable an application to receive SMS messages, you have to write the source code for connecting to and sending AT commands to the mobile phone or GSM/GPRS modem, just like what a terminal program (such as HyperTerminal of Microsoft Windows) does. You can write the source code in C, C++, Java, Visual Basic, Delphi or other programming languages you like.

However, like what we have discussed in the earlier section "The 1st Way: Sending SMS Messages from a Computer Using a Mobile Phone or GSM/GPRS Modem", usually a better solution is to use a high-level SMS messaging API (Application programming interface) / SDK (Software development kit) / library instead of writing your own code for interacting with the mobile phone or GSM/GPRS modem via AT commands. The API / SDK / library encapsulates the low-level details. So, an SMS application developer does not need to know AT commands and the composition of SMS messages in the bit-level. Some SMS messaging APIs / SDKs / libraries support SMSC protocols in addition to AT commands. To move from a wireless-modem-based SMS solution to a SMSC-based SMS solution, usually you just need to modify a configuration file / property file or make a few changes to your SMS messaging application's source code. The links to some open source and free SMS messaging libraries can be found in the article "Free Libraries/Tools for Sending/Receiving SMS with a Computer".

Another high-level solution is to place an SMS gateway between the SMS messaging application and the mobile phone or GSM/GPRS modem. The SMS messaging application can then use simple protocols such as HTTP / HTTPS for receiving SMS messages. If an SMSC protocol (e.g. SMPP, CIMD, etc) is used for communicating with the SMS gateway instead of HTTP / HTTPS, an SMS messaging API / SDK / library can be very helpful to you since it encapsulates the SMSC protocol's details.

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How to Send SMS Messages from a Computer / PC? - Tutorial(10)

In general, there are two ways to send SMS messages from a computer / PC to a mobile phone:

1. Connect a mobile phone or GSM/GPRS modem to a computer / PC. Then use the computer / PC and AT commands to instruct the mobile phone or GSM/GPRS modem to send SMS messages.

2. Connect the computer / PC to the SMS center (SMSC) or SMS gateway of a wireless carrier or SMS service provider. Then send SMS messages using a protocol / interface supported by the SMSC or SMS gateway.

If you do not want to develop SMS software or applications but just want to use your computer / PC to send text messages, you may want to read our Quick Guide for Non-Developers.

The 1st Way: Sending SMS Messages from a Computer Using a Mobile Phone or GSM/GPRS Modem

The SMS specification has defined a way for a computer to send SMS messages through a mobile phone or GSM/GPRS modem. A GSM/GPRS modem is a wireless modem that works with GSM/GPRS wireless networks. A wireless modem is similar to a dial-up modem. The main difference is that a wireless modem transmits data through a wireless network whereas a dial-up modem transmits data through a copper telephone line. More information about GSM/GPRS modems will be provided in the section "Introduction to GSM / GPRS Wireless Modems". Most mobile phones can be used as a wireless modem. However, some mobile phones have certain limitations comparing to GSM/GPRS modems. This will be discussed in the section "Which is Better: Mobile Phone or GSM / GPRS Modem" later.

To send SMS messages, first place a valid SIM card from a wireless carrier into a mobile phone or GSM/GPRS modem, which is then connected to a computer. There are several ways to connect a mobile phone or GSM/GPRS modem to a computer. For example, they can be connected through a serial cable, a USB cable, a Bluetooth link or an infrared link. The actual way to use depends on the capability of the mobile phone or GSM/GPRS modem. For example, if a mobile phone does not support Bluetooth, it cannot connect to the computer through a Bluetooth link.

After connecting a mobile phone or GSM/GPRS modem to a computer, you can control the mobile phone or GSM/GPRS modem by sending instructions to it. The instructions used for controlling the mobile phone or GSM/GPRS modem are called AT commands. (AT commands are also used to control dial-up modems for wired telephone system.) Dial-up modems, mobile phones and GSM/GPRS modems support a common set of standard AT commands. In addition to this common set of standard AT commands, mobile phones and GSM/GPRS modems support an extended set of AT commands. One use of the extended AT commands is to control the sending and receiving of SMS messages.

The following table lists the AT commands that are related to the writing and sending of SMS messages:

AT command	Meaning
+CMGS	Send message
+CMSS	Send message from storage
+CMGW	Write message to memory
+CMGD	Delete message
+CMGC	Send command
+CMMS	More messages to send

One way to send AT commands to a mobile phone or GSM/GPRS modem is to use a terminal program. A terminal program's function is like this: It sends the characters you typed to the mobile phone or GSM/GPRS modem. It then displays the response it receives from the mobile phone or GSM/GPRS modem on the screen. The terminal program on Microsoft Windows is called HyperTerminal. More details about the use of Microsoft HyperTerminal can be found in the "How to Use Microsoft HyperTerminal to Send AT Commands to a Mobile Phone or GSM/GPRS Modem" section of this SMS tutorial.

Below shows a simple example that demonstrates how to use AT commands and the HyperTerminal program of Microsoft Windows to send an SMS text message. The lines in bold type are the command lines that should be entered in HyperTerminal. The other lines are responses returned from the GSM / GPRS modem or mobile phone.

AT
OK
AT+CMGF=1
OK
AT+CMGW="+85291234567"
> A simple demo of SMS text messaging.
+CMGW: 1

OK
AT+CMSS=1
+CMSS: 20

OK

Here is a description of what is done in the above example:

• Line 1: "AT" is sent to the GSM / GPRS modem to test the connection. The GSM / GPRS modem sends back the result code "OK" (line 2), which means the connection between the HyperTerminal program and the GSM / GPRS modem works fine.

• Line 3: The AT command +CMGF is used to instruct the GSM / GPRS modem to operate in SMS text mode. The result code "OK" is returned (line 4), which indicates the command line "AT+CMGF=1" has been executed successfully. If the result code "ERROR" is returned, it is likely that the GSM / GPRS modem does not support the SMS text mode. To confirm, type "AT+CMGF=?" in the HyperTerminal program. If the response is "+CMGF: (0,1)" (0=PDU mode and 1=text mode), then SMS text mode is supported. If the response is "+CMGF: (0)", then SMS text mode is not supported.

• Line 5 and 6: The AT command +CMGW is used to write an SMS text message to the message storage of the GSM / GPRS modem. "+85291234567" is the recipient mobile phone number. After typing the recipient mobile phone number, you should press the Enter button of the keyboard. The GSM / GPRS modem will then return a prompt "> " and you can start typing the SMS text message "A simple demo of SMS text messaging.". When finished, press Ctrl+z of the keyboard.

• Line 7: "+CMGW: 1" tells us that the index assigned to the SMS text message is 1. It indicates the location of the SMS text message in the message storage.

• Line 9: The result code "OK" indicates the execution of the AT command +CMGW is successful.

• Line 10: The AT command +CMSS is used to send the SMS text message from the message storage of the GSM / GPRS modem. "1" is the index of the SMS text message obtained from line 7.

• Line 11: "+CMSS: 20" tells us that the reference number assigned to the SMS text message is 20.

• Line 13: The result code "OK" indicates the execution of the AT command +CMSS is successful.

To send SMS messages from an application, you have to write the source code for connecting to and sending AT commands to the mobile phone or GSM/GPRS modem, just like what a terminal program does. You can write the source code in C, C++, Java, Visual Basic, Delphi or other programming languages you like. However, writing your own code has a few disadvantages:

• You have to learn how to use AT commands.

• You have to learn how to compose the bits and bytes of an SMS message. For example, to specify the character encoding (e.g. 7-bit encoding and 16-bit Unicode encoding) of an SMS message, you need to know which bits in the message header should be modified and what value should be assigned.

• Sending SMS messages with a mobile phone or GSM/GPRS modem has a drawback -- the SMS transmission speed is low. As your SMS messaging application becomes more popular, it has to handle a larger amount of SMS traffic and finally the mobile phone or GSM/GPRS modem will not be able to take the load. To obtain a high SMS transmission speed, a direct connection to an SMSC or SMS gateway of a wireless carrier or SMS service provider is needed. However, AT commands are not used for communicating with an SMS center or SMS gateway. This means your have to make a big change to your SMS messaging application in order to move from a wireless-modem-based solution to a SMSC-based solution.

In most cases, instead of writing your own code for interacting with the mobile phone or GSM/GPRS modem via AT commands, a better solution is to use a high-level SMS messaging API (Application programming interface) / SDK (Software development kit) / library. The API / SDK / library encapsulates the low-level details. So, an SMS application developer does not need to know AT commands and the composition of SMS messages in the bit-level. Some SMS messaging APIs / SDKs / libraries support SMSC protocols in addition to AT commands. To move from a wireless-modem-based SMS solution to a SMSC-based SMS solution, usually you just need to modify a configuration file / property file or make a few changes to your SMS messaging application's source code.

The links to some open source and free SMS messaging libraries can be found in the article "Free Libraries/Tools for Sending/Receiving SMS with a Computer".

Another way to hide the low-level AT command layer is to place an SMS gateway between the SMS messaging application and the mobile phone or GSM/GPRS modem. (This has been described in the section "What is an SMS Gateway?" earlier.) Simple protocols such as HTTP / HTTPS can then be used for sending SMS messages in the application. If an SMSC protocol (e.g. SMPP, CIMD, etc) is used for communicating with the SMS gateway instead of HTTP / HTTPS, an SMS messaging API / SDK / library can be very helpful to you since it encapsulates the SMSC protocol's details.

Usually a list of supported / unsupported mobile phones or wireless modems is provided on the web site of an SMS messaging API / SDK / library or an SMS gateway software package. Remember to check the list if you are going to use an SMS messaging API / SDK / library or an SMS gateway software package.

Major Drawback of Sending SMS Messages through a Mobile Phone or GSM/GPRS Modem -- Low SMS Sending Rate

Using a mobile phone or GSM/GPRS modem to send SMS messages has a major drawback, that is the SMS sending rate is too low. Only 6-10 SMS messages can be sent per minute (when the "SMS over GSM" mode is used). The performance is not affected by the connection between the computer and the mobile phone or GSM/GPRS modem (i.e. the SMS sending rate is about the same no matter the mobile phone or GSM/GPRS modem is connected to the computer through a serial cable, USB cable, Bluetooth link or infrared link) and does not depend on whether a mobile phone or GSM/GPRS modem is used (i.e. the SMS sending rate is about the same no matter a mobile phone or a GSM/GPRS modem is used). The determining factor for the SMS sending rate is the wireless network.

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