GSM 03.38
In mobile telephony GSM 03.38 or 3GPP 23.038 is a character encoding used in GSM networks for SMS, CB and USSD. The 3GPP TS 23.038 standard defines GSM 7-bit default alphabet which is mandatory for GSM handsets and network elements, but the character set is suitable only for English and a number of Western-European languages. Languages such as Chinese, Korean or Japanese must be transferred using the 16-bit UCS-2 character encoding. A limited number of languages, like Portuguese, Spanish, Turkish and a number of languages used in India written with a Brahmic scripts may use 7-bit encoding with national language shift table defined in 3GPP 23.038. For binary messages, 8-bit encoding is used.
GSM 7-bit default alphabet and extension table of [3GPP] TS 23.038 / GSM 03.38
The standard encoding for GSM messages is the 7-bit default alphabet as defined in the 23.038 recommendation.Seven-bit characters must be encoded into octets following one of three packing modes:
- CBS: using this encoding, it is possible to send up to 93 characters in one SMS message in a Cell Broadcast Service.
- SMS: using this encoding, it is possible to send up to 160 characters in one SMS message in the GSM network.
- USSD: using this encoding, it is possible to send up to 182 characters in one SMS message of Unstructured Supplementary Service Data.
Note that the second part of the table is only accessible if the GSM device supports the 7-bit extension mechanism, using the ESC character prefix. Otherwise, the ESC code itself is interpreted as a space, and the following character will be treated as if there was no leading ESC code.
Most of the high part of the table is not used in the default character set, but the GSM standard defines some language code indicators that allows the system to identify national variants of this part, to support more characters than those displayed in the above table.
In a standard GSM text message, all characters are encoded using 7-bit code units, packed together to fill all bits of octets. So, for example, the 140-octet envelope of an SMS, with no other language indicator but only the standard class prefix, can transport up to /7=160, that is 160 GSM 7-bit characters.
Longer messages may be sent, but will require a continuation prefix and a sequence number on subsequent SMS messages.
When there are 1 to 6 spare bits in the last octet of a message, these bits are set to zero. When there are 7 spare bits in the last octet of a message, these bits are set to the 7-bit code of the CR control instead of being set to zero.
This 7-bit encoding allows the transport of texts encoded in the Basic Latin subset of ASCII, as well as some characters of the ISO Latin 1 character set. It also allows the encoding of texts written in the Greek script, but only capitals; for such use in Greek, the Latin capital letters that look like the Greek letters are reused with the same code, so that the above character set is complete only for modern monotonic Greek restricted to capital letters. A complete support for the Greek alphabet requires a national version of the shifted 7-bit table, or an unspecified proprietary 8-bit encoding, or the use of the UCS-2 encoding.
Note that the special code marked SS2 in the table above has also been assigned to allow using another alternate 7-bit shift table. But this mechanism has never been used and the UCS-2 encoding has been preferred.
GSM 8-bit data encoding
8-bit data encoding mode treats the information as raw data. According to the standard, the alphabet for this encoding is user-specific.UCS-2 Encoding
This encoding allows use of a greater range of characters and languages. UCS-2 can represent the most commonly used Latin and eastern characters at the cost of a greater space expense. Strictly speaking, UCS-2 is limited to characters in the Basic Multilingual Plane. However, since modern programming environments do not provide encoders or decoders for UCS-2, some cell phones use UTF-16 instead of UCS-2. This works, because for characters in the Basic Multilingual Plane UCS-2 and UTF-16 encodings are identical. To encode characters outside of the BMP, such as Emoji, UTF-16 uses surrogate pairs, which when decoded with UCS-2 would appear as two valid but unmapped code points.A single SMS GSM message using this encoding can have at most 70 characters.
Note that on many GSM cell phones, there's no specific preselection of the UCS-2 encoding. The default is to use the 7-bit encoding described above, until one enters a character that is not present in the GSM 7-bit table. In that case, the whole message gets reencoded using the UCS-2 encoding, and the maximum length of the message sent in a single SMS is immediately reduced to 70 characters, instead of 160. Others vary based on the choice and configuration of SMS application, and the length of the message.
To avoid unexpected costs for senders that have a subscription for a limited pack of sent SMS, applications should display the number of character used and the maximum number of characters in the composed SMS. When a message exceeds this maximum, the message will be sent as multiple successive SMS containing parts of the message ; these parts are intended to be reassembled later by the recipient.
Some applications alert the user when a message will require splitting, or even send a longer message as a multimedia message.
National language shift tables
Since release 8 of the 3GPP 23.038 standard of March 2008, additional characters sets can be accessed through the use of a National Language Shift Tables.These tables allow using of different character sets according to the language the text is going to be written. The choice of table for a given message is selected in the User Data Header section of an SMS message and can be specified for the whole text or a single character. Locking and Single shift tables together in the same message are possible, if both standard default alphabet table and default alphabet extension table are to be replaced.
Using a shift table, a message can still use 7-bit encoding for the characters, but a different set can be chosen to correctly show accented and language specific characters. This allows up to 155 characters, encoded in 136 octets. With both Locking and Single shift tables, up to 150 characters are allowed, encoded in 132 octets.
Characters from any locking shift table take one septet, characters from the single shift table take two septets.
Initially, shift tables only for Turkish were specified; Spanish and Portuguese were added in later revisions of release 8. Release 9 introduced 10 languages used in India written with a Brahmic scripts and Urdu.
There is still no defined national language shift table for French, Greek, Russian, Bulgarian, Arabic, Hebrew and most Central European languages that need a better coverage than the default 7-bit standard character set and its default 7-bit extension character set: if ever any character is composed that cannot be represented in those default GSM 7-bit sets, the message will be automatically reencoded using UCS-2, with the effect of dividing by more than two the maximum length in characters of messages that can be sent at the price of a single SMS.
Although a revision of GSM 03.38 has defined Data Coding Scheme values for Cell Broadcast System for German, English, Italian, French, Spanish, Dutch, Swedish, Danish, Finnish, Norwegian, Greek and Turkish; with Hungarian, Polish, Czech, Hebrew, Arabic, Russian and Icelandic added in later revisions, no coding tables were defined for these languages. The purpose of this field was purely to identify the language of the message.
There's also no language shift table for Japanese written in basic kanas, or for Korean written in Hangul jamos, or for Chinese written in the Han script. This is often not a problem in Japan, because it uses other standards than GSM and WAP for messaging. The two other languages also have too many distinct characters to fit into a 7-bit shift table.
Spanish language (Latin script)
There's no specific Locking Shift Character Set for the Spanish language. Uses the default Basic Character Set.Portuguese language (Latin script)
Turkish language (Latin script)
Urdu language (Arabic and basic Latin scripts)
It may also be used for the Sindhi language also written in the Arabic script.Sometimes it may be used for Arabic language as well, but the Eastern digits won't be used in that case because standard Arabic prefer its traditional Eastern Arabic digits, and will frequently be replaced by Western Arabic digits which are also used now frequently in Urdu as well. However, in India, phones recognizing the Arabic language indication may substitute the Persian-Hindu variants of the Eastern Arabic digits by the traditional Eastern Arabic digits.