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Transmission of short messages between the SMSC and the handset is done using the Mobile Application Part (MAP) of the SS7 protocol.
Messages are sent with the MAP mo- and mt-ForwardSM operations, whose payload length is limited by the constraints of the signaling protocol to precisely 140 octets (140 octets = 140 * 8 bits = 1120 bits). Short messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet (shown below), the 8-bit data alphabet, and the 16-bit UTF-16/UCS-2 alphabet.
Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual Short Message sizes of 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters.
Support of the GSM 7-bit alphabet is mandatory for GSM handsets and network elements,[18] but characters in languages such as Arabic, Chinese, Korean, Japanese or Cyrillic alphabet languages (e.g. Russian) must be encoded using the 16-bit UCS-2 character encoding (see Unicode). Routing data and other meta data is additional to the payload size.
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7 6 5 4 3 2 1 0 Meaning
0 0 0 0 0 0 0 0 SMSC Default Alphabet
0 0 0 0 0 0 0 1 IA5 (CCITT T.50)/ASCII (ANSI X3.4)
0 0 0 0 0 0 1 0 Octet unspecified (8-bit binary)
0 0 0 0 0 0 1 1 Latin 1 (ISO-8859-1)
0 0 0 0 0 1 0 0 Octet unspecified (8-bit binary)
0 0 0 0 0 1 0 1 JIS (X 0208-1990)
0 0 0 0 0 1 1 0 Cyrllic (ISO-8859-5)
0 0 0 0 0 1 1 1 Latin/Hebrew (ISO-8859-8)
0 0 0 0 1 0 0 0 UCS2 (ISO/IEC-10646)
0 0 0 0 1 0 0 1 Pictogram Encoding
0 0 0 0 1 0 1 0 ISO-2022-JP (Music Codes)
0 0 0 0 1 1 0 1 Extended Kanji JIS(X 0212-1990)
0 0 0 0 1 1 1 0 KS C 5601
1 1 0 0 x x x x GSM MWI control - see [GSM 03.38]
1 1 0 1 x x x x GSM MWI control - see [GSM 03.38]
1 1 1 1 x x x x GSM message class control - see [GSM 03.38]
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Mimer Group ApS - Denmark - khh@mimergroup.com - +45 70 20 33 33