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Why Text Messages are limited to 160 Characters

May 5th

Posted by CodeBubba

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I always wondered this myself.  I had certain ideas like maybe 160 out of 255 characters in an Ethernet Packet or some other method of squeezing the data in.  Turns out that my thoughts on that were not all that far-fetched…

Looking for a data pipeline that would fit these micro messages, Hillebrand came up with the idea to harness a secondary radio channel that already existed on mobile networks. This smaller data lane had been used only to alert a cell phone about reception strength and to supply it with bits of information regarding incoming calls. Voice communication itself had taken place via a separate signal.

Read the rest of the article here or click on the image above.

-CB

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  • srvctec
    Very interesting! I've always wondered this myself, but never got around to checking into it.

    Here's a question for you- why in the world did Twitter decide on a limit of 140 characters when the devices used to make that message, i.e. cell phone text messages, have a limit of 160 characters? I just don't get it.

    Just an FYI- I had to use IE to log in here, since Firefox 3.0.10 wouldn't ever show me as logged in even though I tried it 4 or 5 times- just kept taking me back to the log in screen.
  • CodeBubba
    srvctec,

    My guess is that Twitter decided to allow for the extended character set. Notice that in the article he described how they got it up to 160-characters, they went with a 7-bit character set instead of 8-bit. An 8-bit character set can display 255 different characters whereas a 7-bit one is limited to 128. By doing the 7-bit characters, they can be bit-encoded (Multiplexed) for transmission and De-Multiplexed (unpacked) at the receiving end; but you can't use any of the upper 128 in the SMS system. For most routine text messaging no one misses those anyway.

    -CB :)
  • The 8-bit/7-bit difference makes sense. I always assumed (not having ever used Twitter) that they stuck some additional information in the message (the sender's ID, perhaps).

    Of course, 8 bits can represent 256 characters, not 255. (I wouldn't say "display" because many of the ASCII characters aren't really displayable -- 0-31 and 255, if I recall correctly.)

    Steve
  • CodeBubba
    Steve,

    Well yeah ... you're right, 8-bits can hold 256 discrete values but I said 255 characters because 0 is considered the null-character in the ASCII character set. When constructing string characters (in languages such as 'C') the null character (0) is never considered part of the string, it is used as the string terminator.

    -bruce
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