Fundamentals BroadNet answer

What is GSM-7 encoding?

Last reviewed July 22, 20261 min readReviewed by BroadNet engineering

Quick answer

GSM-7 is the default 7-bit character encoding for SMS, defined in 3GPP TS 23.038. It covers the basic Latin alphabet, digits, and punctuation — roughly 128 characters — and fits 160 characters into a single SMS segment. Any character outside the GSM-7 set (emoji, Arabic, Cyrillic, accented Latin such as é or ñ) forces the whole message into UCS-2, which drops the limit to 70 characters per segment.

Plain-language definitionVendor-neutralReviewed July 22, 2026

The long answer

Everything worth knowing, nothing that isn't.

§01

Why the character set drives cost

SMS is billed per segment. Nine 'extended-table' characters — | ^ € { } [ ] ~ and backslash — each cost two of the 7-bit characters. Worse, a single non-GSM-7 character (one emoji, one Arabic letter) switches the entire message to UCS-2 and can double your segment count. Model the split before you send with the /tools/sms-segment-calculator.

§02

Concatenation and the 153 / 67 rule

When a message spans multiple segments, a 6-byte User Data Header (UDH) is added to each part for reassembly, stealing space. So a concatenated GSM-7 message holds 153 characters per segment (not 160), and a concatenated UCS-2 message holds 67 (not 70). See /answers/what-is-ucs-2-sms and /answers/what-is-udh-sms.

BNWritten by the BroadNet engineering team·Operating carrier-grade messaging since 2003, headquartered in Dubai·Editorial standards

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