Learn about SMS

The most important concepts in understanding SMS are - maximum payload size, character sets, and the length of the message. You must learn about SMS regulations and types of SMS message interactions. You can also view a video about SMS.

Maximum payload size for an SMS message

The maximum payload of an SMS message is 140 bytes, or more specifically 1120 bits. The number of characters that can fit into a payload depends on the character set that is used for SMS and also on the length of the message text.

Message length

For SMS, message length is dictated by the number of bytes the message text is. A single payload can only be 140 bytes. If a message text is longer than 140 bytes then it becomes a concatenated or multi-part message. In a multi-part message, some of the allocated bytes from the payload are used to create a user data header (UDH). This provides identification and ordering information so that the receiving device knows how to order the separate payloads, which may arrive at the handset out of order into a single readable message.

The UDH takes 6 bytes or 48 bits. This therefore reduces the space for how many characters can be in each message part.

Character sets

SMS in Acoustic Campaign primarily supports two character sets - GSM-7 and UCS-2/UTF-16*.

Although Acoustic Campaign can send up to a maximum of 800 GSM characters or 400 UCS-2 characters in six parts, some markets may limit the maximum number of concatenated messages. Certain markets may have specific requirements or limitations. Check with the governing bodies of the market to obtain further information.

GSM-7

GSM-7 character set supports most but not all characters for languages that use the Latin-based alphabet such as English, Spanish, French, and so on. The GSM character encoding uses seven bits to represent each character similar to ASCII.

Therefore GSM-7 messages break down as follows:
  • Standard single SMS messages - For GSM phones with 7-bit character encoding, a standard SMS message can contain a maximum of 160 characters. That is 1120 bits / (7 bits/character) = 160 characters for a single SMS message.
  • GSM-7 multi-part or concatenated messages - When the message text is longer than 160 GSM characters, the message is concatenated and sent. When a message is concatenated, the user data header (UDH) consumes 6 bytes or 48 bits. This reduces the maximum number of characters in each message part:

    1120 bits - 48 bits = 1072 bits

    1072 bits / (7 bits/character) = 153 characters per message part.

    Note: 153 characters * (7 bits/character) = 1071 bits. However, the extra bit cannot be used to represent a full character. It is added as padding so that the actual 7-bit encoding data begins on a septet boundary—the 50th bit.

UCS-2/UTF-16

This character set is used for non-Latin based alphabet languages such as Arabic, Chinese, Cyrillic, and so on. As the characters for these languages are supported within Unicode, 16 bits per character is used instead of 7 bits per character.

The two types of UCS-2/UTF-16 messages are:
  • Standard single SMS message (Unicode) - For Unicode phones with 16-bit character encoding, a standard SMS message can contain up to 70 characters. That is 1120 bits / (16 bits/character) = 70 characters.
  • UCS-2/UTF-16 multi-part or concatenated messages - When the message text is longer than 70 UCS-2 characters, the message is concatenated and sent. When a message is concatenated, the user data header (UDH) consumes 6 bytes or 48 bits. This reduces the maximum number of characters in each message part:

    1120 bits - 48 bits = 1072 bits

    1072 bits / (16 bits/character) = 67 characters per message part

Note: Acoustic Campaign utilizes the best character set dependent on what characters are detected within the message. When a character that is not supported by GSM-7 is detected, the entire message is converted to Unicode. Therefore every character present uses the UTF-16 equivalent. SMS messages can only ever be in one particular encoding. Either fully GSM-7 or fully UTF-16.

UTF-8

Acoustic Campaign does support and transmit SMS message as UTF-8, but how the message is processed depends on capabilities of the mobile phones, mobile carriers, and local SMS gateways or vendors.

Though UTF-8 is a common Unicode character set that supports characters for many languages. it is not widely supported across the SMS technology field. Most mobile phones are configured to support GSM-7 or UCS-2 (UTF-16) or both. Even if a few phones do support UTF-8, most mobile carriers do not support UTF-8 as they favor the much more robust UTF-16 format. The local SMS gateway (the vendor or the aggregator) also must support UTF-8. Many do support UTF-8 but may not support it in all the markets.

In the instance where UTF-8 is fully supported, an SMS message breaks down as follows:
  • Standard single SMS message (Unicode) - For Unicode phones with 8-bit character encoding, a standard SMS message can contain a maximum of 140 characters. That is 1120 bits / (8 bits/character) = 140 characters.
  • UTF-8 multi-part or concatenated messages - When the message text is longer than 140 UTF-8 characters, the message is concatenated and sent. When a message is concatenated, the user data header (UDH) consumes 6 bytes or 48 bits. This reduces the maximum number of characters in each message part:

    1120 bits - 48 bits = 1072 bits

    1072 bits / (8 bits/character) = 134 characters per message part

Table 1. Maximum character count for supported encoding systems
Message type Max character count for GSM Max character count for UTF-16 Max character count for UTF-8
Standard single SMS message 160 70 140
Two concatenated SMS messages 306 134 268
Three concatenated SMS messages 459 201 402
Four concatenated SMS messages 612 268 536
Five concatenated SMS messages 765 335 670
Six concatenated SMS messages 800 400  

SMS Regulations

SMS users must abide by their countries SMS legal rules and guidelines. It is not IBM®'s responsibility to maintain how SMS users are communicating to their contact base by using SMS messaging.

Types of SMS Message Interactions

The Provisioning team enables SMS for your organization.

Most of the programs that you create are two-way SMS messages, where you send a message to your users and you can receive a reply message. You can also send a one-way SMS message by using a Sender ID. However, the user cannot directly reply to the Sender ID.

Acoustic Campaign also supports Free to End-User (FTEU) types of SMS messaging for the USA market.

Video

See an overview of SMS:

For more information, watch this video: http://cdn.training.silverpop.com/training/eng/mobile/sms2/smsoverview/index.html