Message concatenation
How a long SMS splits into segments, why most routes bill each one, and how encoding changes the count.
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Message concatenation splits a long SMS into segments that the handset reassembles into one message. Each segment carries a user data header, and most routes bill each one separately.
Encoding sets the budget: a concatenated GSM-7 segment holds about 153 characters, a UCS-2 segment 67.
Encoding and the segment count
- Encoding
- GSM-7
- Characters
- 224
- Units
- 224
- Segments
- 2
Text description
The figure lists the encoding chosen for one sample text, its character count, its size in encoding units and the number of segments that size needs.
How the segments are built
The sender divides the text and adds a user data header to each segment. The header takes six octets with an 8-bit reference and seven with a 16-bit one, so a GSM-7 segment holds about 153 characters rather than 160.
On most routes every segment is a billable event and a separate receipt. Order is not preserved, and a lost segment can leave the message undisplayed even when the other receipts said delivered. The SMS segment calculator counts segments before you send.