Thread: Why are SMS short text messages so darned short?
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Old 03-03-2006, 04:36 PM   #2
Bob Russell
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My curiousity got the best of me, so I did look into this a bit. What better place to start than Wikipedia ? Here's what they have to say:

Quote:
Transmission of the short messages between SMSC and phone is via SS7 within the standard GSM MAP framework. Messages are sent with the additional MAP operation forward_short_message, whose payload length is limited by the constraints of the signalling protocol to precisely 140 bytes. In practice, this translates to either 160 7-bit characters, 140 8-bit characters, or 70 2-byte characters in languages such as Arabic, Chinese, Korean, Japanese or Slavonic languages (e.g. Russian) when encoded using 2-byte UTF-16 character encoding (see Unicode). This does not include routing data and other metadata, which is additional to the payload size.

Larger content (known as long SMS or concatenated SMS) can be sent segmented over multiple messages, in which case each message will start with a user data header (UDH) containing segmentation information. Since UDH is inside the payload, the number of characters per segment is lower: 153 for 7 bit encoding, 134 for 8 bit encoding and 67 for 16 bit encoding. The receiving phone 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, 3 to 4 segment messages are the practical maximum, and long messages are billed as equivalent to multiple SMS messages.
After a short look (just enough to make me dizzy), I've concluded that this means there is a limitation of the signaling protocol that accounts for 160 7-bit characters. Carriers could extend the message length under SMS, but they would have to concatenate the pieces, which apparently it's not worthwhile to do, and may need other supporting changes such as in the handsets themselves.

At this site, they say that you can get some more characters by using compression also:

Quote:
Ways of sending multiple short messages are available. SMS concatenation (stringing several short messages together) and SMS compression (getting more than 160 characters of information within a single short message) have been defined and incorporated in the GSM SMS standards.
But it still seems like quite a shame to me!
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