When you send an SMS message all of the characters in the message are encoded as 7-bit GSM code or Unicode Character Set-2 (UCS-2). The message is then sent in one or more Protocol Data Units (PDU). A PDU contains information about the control, address and data to deliver an SMS message.

The 7-bit encoding cannot represent all characters. UCS-2 includes all of the characters within the Basic Multilingual Plane, but it reduces the number of characters in one PDU from 160 to 70. Users are often charged by the PDU. A message encoded using UCS-2 can be more expensive than the one encoded using 7-bit GSM. There are four types of SMS encodings to encode an SMS message:

The following character converters are used in the SMS encodings: