Editing SMS (section)

=== Message size ===
Transmission of short messages between the SMSC and the handset is done whenever using the [[Mobile Application Part]] (MAP) of the [[Signaling System 7|SS7]] protocol.<ref>{{cite web|url=http://services.eng.uts.edu.au/userpages/kumbes/public_html/ra/sms/|access-date=October 5, 2013 |last=Amri |first=Kuross |title=Communication Networks |archive-url=https://web.archive.org/web/20160511143408/http://services.eng.uts.edu.au/userpages/kumbes/public%5Fhtml/ra/sms/|archive-date=May 11, 2016 |url-status=dead}}</ref>  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 [[byte]]s (140 bytes × 8 bits / byte = 1120 bits).

Short messages can be encoded using a variety of alphabets: the default [[GSM 03.38|GSM 7-bit alphabet]], [[GSM 03.38#GSM 8 bit data encoding|the 8-bit data alphabet]], and the 16-bit [[UCS-2]] or [[UTF-16]] alphabets.<ref>{{cite web|url=https://www.twilio.com/engineering/2012/11/08/adventures-in-unicode-sms|title=Adventures in Unicode SMS|date=2012-11-08|publisher=Twilio|author=Chad Selph|access-date=2015-08-28|archive-url=https://web.archive.org/web/20150908104520/https://www.twilio.com/engineering/2012/11/08/adventures-in-unicode-sms|archive-date=2015-09-08|url-status=dead}}</ref><ref name="3GPP 23.038">{{cite web |url=https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=745 |title=Alphabets and language-specific information |id=3GPP TS 23.038}}</ref> 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. GSM 7-bit alphabet support is mandatory for GSM handsets and network elements.<ref name="3GPP 23.038"/>

Larger content ([[concatenated SMS]], multipart or segmented SMS, or "long SMS") can be sent using multiple messages, in which case each message will start with a [[User Data Header]] (UDH) containing segmentation information. Since UDH is part of the payload, the number of available characters per segment is lower: 153 for 7-bit encoding, 134 for 8-bit encoding and 67 for 16-bit encoding. The receiving handset is then responsible for reassembling the message and presenting it to the user as one long message. While the standard theoretically permits up to 255 segments,<ref>{{Cite book|last=Groves|first=Ian|url=https://www.worldcat.org/oclc/847640648|title=Mobile systems|date=1998|publisher=Springer|isbn=978-1-4615-6377-8|edition=1st|location=London|pages=70, 79, 163–66|oclc=847640648|access-date=2021-09-23|archive-date=2024-01-11|archive-url=https://web.archive.org/web/20240111044135/https://search.worldcat.org/title/847640648|url-status=live}}</ref> 10 segments is the practical maximum with some carriers,<ref>{{Cite web|url=https://help.twilio.com/articles/223181508-Does-Twilio-support-concatenated-SMS-messages-or-messages-over-160-characters|title=Does Twilio support concatenated SMS messages or messages over 160 characters?|website=Twilio Help Center|language=en-US|access-date=28 September 2024|quote=when sending very long SMS messages (longer than 10 segments with Unicode characters) some mobile carriers may have trouble handling these messages.}}</ref> and long messages are often billed as equivalent to multiple SMS messages. In some cases 127 segments are supported.<ref>{{Cite web|title=Simplifying Unicode punctuation for SMS|url=https://ssb22.user.srcf.net/adjuster/sms.html|access-date=July 15, 2021|website=ssb22.user.srcf.net|archive-date=July 15, 2021|archive-url=https://web.archive.org/web/20210715062556/https://ssb22.user.srcf.net/adjuster/sms.html|url-status=live}}</ref>