The SMS message, as specified by the Etsi organization (documents GSM 03.40 and GSM 03.38), can be up to 160 characters long, where each character is 7 bits according to the 7-bit default alphabet. Eight-bit messages (max 140 characters) are usually not viewable by the phones as text messages; instead they are used for data in e.g. smart messaging (images and ringing tones) and OTA provisioning of WAP settings. 16-bit messages (max 70 characters) are used for Unicode (UCS2) text messages, viewable by most phones. A 16-bit text message of class 0 will on some phones appear as a Flash SMS (aka blinking SMS or alert SMS).
User Data header was added to the SMS format specification to add new features. Originally SMS was made to send single small binary files or text messages, with a maximum of 140 bytes or 160 7-bit characters. But now mobile devices need to distinguish between different files, for example ringtones, operator logos and wap-push messages. Also users want to send larger messages which was impossible with the original SMS format specification.
Each short message has a flag to indicate if the message part includes a User Data Header or not. If this flag is set to 1 (or true), then the first few bytes of the message are the User Data Header, followed by the message text or data. A very general use of UDH is for plain concatinated messages, where UDHI is set.
User Data header was added to the SMS format specification to add new features. Originally SMS was made to send single small binary files or text messages, with a maximum of 140 bytes or 160 7-bit characters. But now mobile devices need to distinguish between different files, for example ringtones, operator logos and wap-push messages. Also users want to send larger messages which was impossible with the original SMS format specification.
Each short message has a flag to indicate if the message part includes a User Data Header or not. If this flag is set to 1 (or true), then the first few bytes of the message are the User Data Header, followed by the message text or data. A very general use of UDH is for plain concatinated messages, where UDHI is set.
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