We've learned a lot about GSM radios and communicatng with them while designing products for industrial applications. And we've answered a lot of questions. Here are some of the most common questions people ask us.
Click on a question to view the answer

• What is GSM?

GSM stands for Global System for Mobile communications. It's a European Digital Cellular System standard that was developed by a committee of European telecommunications companies.It operate at 4 differant frequencies; 850MHz & 1900MHz in America, and 900MHZ and 1800MHZ in most other countries.

• What is GPRS?

GPRS stands for General Packet Radio Service. It's a GSM protocol to deliver high speed packet data services to mobile terminals. GPRS allows many users to share the same channel and allows users to stay virtually 'on line' all of the time; radio resources are used only when data is actually being transmitted or received. Call setup is almost instantaneous and users may be charged on the basis of actual data transmitted, rather than connection time.

• What is SMS?

Short Message Service(SMS) is a GSM communication service that allows the interchange of short Text Messages between mobile devices.

• What features should I look for in a GSM Radio?

• How much power does it draw; can it be battery powered for long periods of time
• Does it support all 4 of the GSM Frequencies(QUAD), you need 850MHz support in the US for good reception
• Is the output power high enough to ensure good network connection (1 watt at 1800 & 1900 MHz and 2 watts at 850 & 900 MHz)
• Can you easily mount it on your PCB board, many modules require expensive connectors or are difficult to solder
• Is it PTCRB Certified, without certification you are not guaranteed that it will work in your country or on your providers networks
• What development tools are available for it

• What causes the a GSM Radio to return a 'CMS ERROR 515' error?

• Sending a SMS when the SIM initialization is going on (after rebooting the module)
• Reading a SMS immediately after booting the module when SIM initialization is not yet over
• Writing a SMS to the memory before SIM initialization is complete
• Sending more than one SMS at a time or sending a SMS when the response (OK or ERROR) for a previously sent SMS has not been received

• Why does my radio shut off as soon as it tries to connect to the network?

The most common cause of this problem is a problem with the power supply. The radio draws significant power (as much as 2A) when it transmits to the cell tower and the power supply has to be capable of supplying the proper voltage at that time:
• It is not well filtered - make sure you have the proper filter caps close to the radio power pins
• It does not have good load-regulation and the voltage is dropping when the radio is transmitting
• The voltage level is too low to allow for dips when transmitting.

• What is an 'APN' and where do I get it?

GSM networks use an APN (Access Point Name) to determine how the carrier network passes IP traffic from the device to the host network. It determines what IP addresses are assigned to the device, which security methods are utilized and how the GSM data network connects to the customer’s network. APNs can be general-purpose (available to multiple customers) or can be specific to a customer. You can get the APN for your network and plan from your service provider.

• How do I send the SMS message 'Hello There' to a phone number?

Send the following commands to the module - Each line must be terminated with a 'Carriage Return':
AT+CMGF=1   - Set the module into Text mode
AT+CSCS='GSM'   - Set the character mode to GSM
AT+CSCA='###'   - Set the SMS message center number, ### = number for your service provider
AT+CMGS='###'   - Start sending SMS, ### = phone number to send message to
   - The module will return'>'to indicate it is ready for the text
Hello There   - Send the text, followed by ctrl-Z and Carriage Return

• How do I make a GPRS connection and send an HTTP Request?

First you need to open a PDP context by sending the following commands:
AT+CGATT=1   - Attach to GPRS Service
AT+CGDCONT=1,'IP','wap.cingular'   - Define PDP Context (cid, PDP type, APN)
AT+CDNSCFG='208.67.222.222','208.67.220.220'   - Configure Domain Name Server (primary DNS, secondary DNS)
AT+CSTT='wap.cingular','wap(at)cingulargprs.com','cingular1' - Start Task & set APN, User ID, and password
AT+CIICR   - Bring up wireless connection with GPRS(THIS MAY TAKE A WHILE)
AT+CIFSR   - Get Local IP address (returns IP address assigned to your module)
AT+CIPSTATUS   - Get Connection Status (needs to be 'IP STATUS' before you can connect to a server)

After you have a context, you need to make a connection to the server and then send your data: AT+CDNSORIP=1   - Indicates whether connection request will be IP address (0), or domain name (1)
AT+CIPSTART='TCP','www.google.com','80' - Start up TCP connection (mode, IP address/name, port)
   - returns 'CONNECT OK' to indicate you've connected to the server
   - IT MAY TAKE A WHILE FOR THIS TO BE RETURNED
AT+CIPSEND   - Issue Send Command (wait for module to return '>' to indicate it's ready to receive data
GET / HTTP/1.1   - Send data - this example is an HTTP request for the default (home) page of Google
Host: www.google.com
Accept: */*
Accept-Language: en-us
   - Send a blank-line and then a Ctrl-Z
   - Data from the remote server will be sent by the module