Random Facts: Why SMS Is Only 160 Characters

from the that's-a-bit-refreshing dept

The LA Times has tracked down the reasoning for why SMS text messaging is limited to 160 characters. Basically, one guy working on the project figured that was plenty after typing a bunch of sentences out and noticing that most were less than 160 characters. There was no serious additional research done on it. It just sorta stuck once implemented. In an age where so many things are user-tested to death, it’s kind of nice to know this was almost an accident of history, based on the reasoning of one guy.

Filed Under: , ,

Rate this comment as insightful
Rate this comment as funny
You have rated this comment as insightful
You have rated this comment as funny
Flag this comment as abusive/trolling/spam
You have flagged this comment
The first word has already been claimed
The last word has already been claimed
Insightful Lightbulb icon Funny Laughing icon Abusive/trolling/spam Flag icon Insightful badge Lightbulb icon Funny badge Laughing icon Comments icon

Comments on “Random Facts: Why SMS Is Only 160 Characters”

Subscribe: RSS Leave a comment
Anonymous Coward says:


It’s also the most expensive method to transmit information. I read somewhere that at 10-cents a text, it weighed in at about $1,300 a megabyte, making the $17/megabyte cost to receive pictures from the Hubble Telescope a real bargain.

But yes, SMS used excess signaling capacity in the BCCH if I recall, and for the most part, all the result of a series of mistakes, but adopted from the bottom-up instead of top-down.

In it’s infancy before T9 prediction technology, it was seen as too difficult for the older folks, but adopted very quickly by the younger ones.

Lawrence D'Oliveiro says:

Data Efficiency

Note that 160 bytes is equivalent to 80 Unicode characters. In an ideographic script (e.g. Chinese, Japanese) that’s about 80 words, which is way more than you can manage with an alphabetic script. So the East Asians get to say more in each txt than anybody else does. 🙂

Why don’t they include some compression in the text encoding? English text only has an information content of about 1.2 bits per character. You know why people leave vowels out of their txts? They’re basically making up for the lack of compression. But why not let the computers do the work for us?

Derek Kerton (profile) says:

Re: Data Efficiency

People leave out letters for three reasons:

1) manual compression, as you said
2) entering each character historically can require a triple-tap, so each skipped character saves more than one keystroke. That’s a big speed improvement.
3) overall space is limited, both the 160 limit, and the size of the screen for composing and reading. Shorter fits better.

Allan Masri (profile) says:

Re: Data Efficiency

So the East Asians get to say more in each txt than anybody else does.

Uh…no, there is not a one-to-one ratio between words and characters in Asian languages. More like one-to-two in Chinese, but Japanese is even less condensed since it uses many particles and spells out many words using syllabaries. Irrelevant, I admit, but amusing.

Derek Kerton (profile) says:

Glamorous Story, But Not Quite True

This LA Times article is missing critical information. SMS was not designed so deliberately by one man. It was mostly an accident of the technical specs of a GSM cellular network.

The specs defined the size of packets that were to be sent from the network, over the SS7 signaling network, and then wirelessly over the control channel from a tower to the phones connected to it. The tower needs to communicate with the phones to get their ID, check their credentials, tell them when to switch to another tower, tell them if they have a voicemail waiting, tell them to ring, accept their dialing call requests, etc. These are all “control” events.

Voice calls take place on TWO separate radio channels, one for voice in and one for voice out, and the radio is only activated to do this when needed. This saves on bandwidth and on battery. The control channel, on the other hand, is more or less always on. Phones use this channel to set up the calls and activate the voice channel. THIS is the original purpose of this channel in the GSM standard, and the size and length of control instruction packets was defined around these tasks.

So later on, some people decided to “tack on” the ability to send a message inside this same channel. It WAS smart, and it does make sense. They simply looked at the packet size they had available, and made the best of it. This made it into the GSM standard, and since GSM is a rigid standard, all the terminal vendors built handsets that complied.

It was a great “invention”, but it was invention by committee, not one guy (and more than one guy claims credit). That’s how the GSM Association works. Also, it was not as “free form” as the LA Times would have you believe – some guy typing messages out to see how long they were, and deciding on 160 characters. Maybe it’s true that he did that as well, but if so, he did it just to test whether 160 would be adequate. The SMS specs were already constrained to 140 bytes by the real priority technical specs, and the SMS function was just a “nice to have” add in.

You see, the network control signals were sent between tower and handset in a packet that included header information plus about 140 bytes of payload. If you wanted to piggyback on this existing channel, you had 140 bytes to work with – no matter what your typewriter experiment concluded.

Hillebrand, or someone, then took the 140 byte limit (1 byte = 8 bits = 1 character), and figured an easy way to use a reduced character set of 7 bits each to squeeze 160 characters out of the constraint (140 * 8 / 7 = 160)

Therefore, an SMS message isn’t limited to 160 characters because of some dude’s typewriter experiment. It is limited because the signaling channel’s standard packet size payload was 140 bytes. It’s a less glamorous reality.

Add Your Comment

Your email address will not be published. Required fields are marked *

Have a Techdirt Account? Sign in now. Want one? Register here

Comment Options:

Make this the or (get credits or sign in to see balance) what's this?

What's this?

Techdirt community members with Techdirt Credits can spotlight a comment as either the "First Word" or "Last Word" on a particular comment thread. Credits can be purchased at the Techdirt Insider Shop »

Follow Techdirt

Techdirt Daily Newsletter

Techdirt Deals
Techdirt Insider Discord
The latest chatter on the Techdirt Insider Discord channel...