GSM-7 is a character encoding standard which packs the most commonly used letters and symbols in many languages into 7 bits each for usage on GSM networks.  As SMS messages are transmitted 140 8-bit octets at a time, GSM-7 encoded SMS messages can carry up to 160 characters.

GSM-7 Encoding Quirks

GSM-7 is the standard alphabet for SMS messages, written up in the standard GSM 03.38.  It is always supported on GSM networks.  In languages with more than 128 commonly used symbols, GSM-7 is mandated but local language support is implemented with shift tables or by changing text encoding to (16-bit) UCS-2 encoding.

The basic character set for GSM-7 can be found here.

For some characters, such as '{' and ']', an escape code is required - so even in a GSM-7 encoded message these characters will be encoded using two characters.

As SMS messages contain 140 8-bit octets, up to 160 GSM-7 characters may be transmitted: (140*8)/7 = 160.

How Twilio Encodes Your Messages

When sending SMS messages with Twilio, we'll automatically send messages in the most compact encoding possible.  If you include any non GSM-7 characters in your message body, we will automatically fall back to UCS-2 encoding (which will limit message bodies to 70 characters each).  Additionally, Twilio prepends a User Data Header of 6 Bytes (this instructs the receiving device on how to assemble messages), leaving 153 GSM-7 characters or 67 UCS-2 characters for your message.

Note that this may cause more messages to be sent than you expect - a body with 152 GSM-7-compatible characters and a single unicode character will be split into 3 messages when encoded in UCS-2. This will incur charges for 3 outgoing messages against your account.

How Do I Check if My Message Can Be Encoded in GSM-7?

This page contains an interactive tool which can check if encoding your message in GSM-7 is possible, or if UCS-2 is needed.

How Can I Avoid My Messages Being Split When I Expect Them to be in GSM-7?

Unfortunately, GSM-7 is not a supported character encoding in many text editors.  Even setting encoding to ASCII (or US_ASCII, or UTF-8) will not guarantee that text you write will be limited to GSM-7.  You can use the above linked tool to quickly check the number of segments - that is, total messages - some text will be divided into.

If you are writing in an editor with Unicode support you'll need to be particularly careful.  Text editors designed for writing might automatically add angled smart quotes, non-standard spaces, or punctuation which looks similar to GSM-7 but is a different Unicode character.  We've discussed a few of these issues on our blog.

Ready to Try Twilio Programmable SMS and SMS - With GSM-7 and UCS-2 Support?

Sign up for a free Twilio trial account today - you'll have enough credit to explore the two major encodings we use, and a lot more.

More Information on GSM-7 Encoding and Twilio