Why SMS are limited to 160 characters - Human Readable

Or, why when you use special letters (ï, ê, à) you end up with less room in SMSs.

This is a post is probably 10 years too late. But it is a topic that interests me, and I wanted to explain it for the uninformed in a technical but human readable way.

The issue is that it is a limitation of the GSM system. GSM is the name for the way in which almost all mobile phones communicate. The actual length of a text message is 256 characters or letters (the term characters are more exact because numbers are not letters and neither is % or #).

Now an SMS needs to have contained within itself who sent the message, when it was sent and who it is going too, these are called overheads and take up exactly 96 characters, which is why you end up with 160 characters left. (256-96=160).

On a side note, each character is sent as an 8bit binary number -- you know those 1s and 0s everyone is talking about -- meaning you have a maximum of 256 types of characters. This gives you just enough room for all upper and lower case letters (2 * 26), numbers (0-9), a range of symbols ({}[]-+£$%) and some control characters (space, return, null). This is an internationally recognised standard specified as the ASCII standard.

For those of you keeping count that is 2048 bits sent (256 * 8). GSM runs at 9600bps meaning a full SMS with no protocol overheads (such as having to ask if it is allowed to send an SMS) takes 0.2 (1/5th) of a second to send.

Extra-special characters used in languages such as Welsh, Swahili, Flemish, whatever, use one extra control characters to specify that the next character is going be a bit weird and treat it as such. Therefore using those extra-special characters will take up double the room of a normal character.

Same goes for return too, I noticed on my 6600 that if you are close to your character limit and you use a lot of returns, it will tell you when you send the message that it is going to have to take 2 messages because it falsely counted returns as 1 character (or maybe no characters, I don't remember). What you can deduce from this is that return takes 2 characters, not an uncommon technique.

SMS messages weren't originally expected to be a popular communication medium and were originally designed to be used for control messages for your phone case in point: WAP Settings. They figured 256 characters was enough. The reason it won't change because GSM is so prevalent and so many things would need to be updated, phones, SIM cards, and SMS message centres, where the messages are stored before being sent on to your phone. It is expensive, and will never happen, which is why MMS was invented.

MMS, which we all know sends photos and videos, but is perfectly capable of sending text too, much like an e-mail. I know on my plan one MMS costs 3x SMS so if I want to send a message of 4x SMS, it is cheaper as an MMS.

So there you have it. I hope this was easy enough to follow even for the intermediate mobile phone user, and now you understand why the ridiculously small limit on SMSs and how best to deal with them.