SMS Encoding and Segmentation

Core Concepts

When an SMS message exceeds the character limit of a single message, it is segmented into multiple parts for transmission and reassembled at the receiving device. This is known as a Concatenated SMS or Long SMS. Billing is typically based on the number of segments.

The character limit is determined by the encoding scheme used, primarily falling into two categories:

1. GSM-7 Encoding

   • Supports a basic character set of approximately 128 characters, including English letters, numbers, and basic Latin-based punctuation.

   • Single Message Limit: 160 characters.

   • For long messages, each segment of 153 characters uses a 7-byte User Data Header (UDH) to manage reassembly.

2. UCS-2 (UTF-16) Encoding

   • Used for messages containing characters outside the GSM-7 character set, such as non-Latin scripts (e.g., Chinese, Cyrillic, Arabic), complex symbols, or emojis.

   • Single Message Limit: 70 characters.

   • For long messages, each segment of 67 characters uses a UDH for reassembly.

Long SMS Segmentation Details

GSM-7 Encoding (e.g., Pure English Text)

UCS-2 Encoding (e.g., Non-English Characters)

Character Set Reference:

The GSM-7 default alphabet includes (but is not limited to):
@£$¥èéùìòÇ\nØø\rÅåΔ_ΦΓΛΩΠΨΣΘΞÆæßÉ !"#¤%&'()*+,-./0123456789:;<=>?¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑܧ¿abcdefghijklmnopqrstuvwxyzäöñüà^{}\[~]|€

Any character not found in this set will typically cause the message to be encoded in UCS-2, reducing the per-segment character capacity.