[LINK] short messages [was: Mindless negativity about Twitter (was Re: Forum on high speed bandwidth in Australia - this Thursday)

Scott Howard scott at doc.net.au
Tue May 5 09:51:40 EST 2009


On Mon, May 4, 2009 at 4:08 PM, Ivan Trundle <ivan at itrundle.com> wrote:

>
> http://latimesblogs.latimes.com/technology/2009/05/invented-text-messaging.html
>
> Why text messages are limited to 160 characters


Whilst there's probably some truth in this story, it does miss the fact that
there was another reason for the particular length - the limitations of the
underlying protocol.

SMS messages are normally carried across the network as SS7 messages.  SS7
is the protocol which is responsible for setup and tear-down of calls on a
phone network - histically (pre-SMS) it didn't carry any actual contact
beyond basic details like who a call is to/from, billing information, etc.
As a result, it's message size is small - maxing out at about 250, 8-bit
bytes.

Take off headers, etc, and you end up with only about 140 bytes available
for the SMS message without having to split it into 2 packets.

In an effort to squeeze in more than 140 bytes, SMS messages are 7-bits,
which means that you can actually fit 160 7-bit characters into a 140 byte
block.

Thus, 160 characters.

  Scott.


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