UCS-2: definition and how it works
Published and updated
UCS-2 is the 16-bit character encoding SMS falls back to when a message contains any character outside the GSM-7 alphabet, which limits one segment to 70 characters.
How it works
SMS was designed around a 7-bit alphabet that covers Latin text and little else. To carry other scripts, the standards in 3GPP TS 23.038 allow the user data to be encoded as UCS-2, a fixed two-byte-per-character encoding that covers the Unicode basic multilingual plane. Handsets and networks treat it as the universal fallback.
The switch is all or nothing. A message is either entirely GSM-7 or entirely UCS-2; the sender's software picks the encoding by scanning the text. One character that is not in the GSM-7 tables, such as an emoji, a smart quote, a Chinese or Arabic character, or a letter with an accent outside the small GSM set, flips the whole message to UCS-2.
At two bytes per character, the 140 octets of a single SMS hold 70 characters. When the text runs longer, concatenation splits it into segments of 67 characters each, because six octets of every segment go to the header that lets the receiving phone reassemble the parts.
Emoji are the common trap. Most emoji sit outside the basic multilingual plane and take two UCS-2 code units, so a single emoji costs two of the 70 characters and forces the encoding at the same time. The cheapest fix is to keep emoji out of templates and to check any user-supplied text before sending.
- 3GPP TS 23.038, Alphabets and language-specific information, specifies UCS2 as the 16 bit encoding for SMS user data. portal.3gpp.org, checked on September 5, 2026
How this applies with textbee
A UCS-2 message costs more segments, and each segment is one SMS on the SIM plan. Paste a template into the segment calculator to see which character tipped it over, and swap curly quotes and dashes for their plain equivalents.
Related terms
- GSM-7: GSM-7 is the default 7-bit character set for SMS, defined in 3GPP TS 23.038, which fits 160 characters into the 140 bytes of a single text message.
- SMS segment: An SMS segment is one physical text message of up to 140 bytes, the unit that networks carry and providers bill, so a long or non-Latin message counts as several segments.
- Concatenated SMS: A concatenated SMS is a message longer than one segment that the sender splits into several SMS carrying a shared header, so the receiving phone can reassemble them into one text.
Sources
- UCS-2, portal.3gpp.org, checked on September 5, 2026