The encyclopedia · Software & IT · Technical decision · 1984–1992
SMS won by riding the phone's free signalling channel, 160 characters at a time.
Designers piggybacked text onto the SS7 control channel that calls already used, so a message cost almost nothing and texting became universal.
Friedhelm Hillebrand · Bernard Ghillebaert · GSM group
the move
In the early 1980s mobile phones used radio channels only to carry calls, and setting up and clearing each call used a signalling network that was already there. A text message, unlike a call, needs an instant to send and no dedicated traffic channel.
Designers in the GSM group, including Friedhelm Hillebrand and Bernard Ghillebaert, proposed sending short messages through that existing SS7 control network. To test how much text people actually need, Hillebrand typed random sentences and found almost everything fit in under 160 characters.
The SS7 signalling message was constrained to 140 octets, and with the GSM 7-bit alphabet that gave a maximum of 160 characters. Because the message piggybacked on the control channel, operators could carry it at almost no extra cost, and the first SMS went out in 1992.
why it works
- Riding the SS7 control channel meant one message did not need to hold a traffic channel, so it was nearly free to deliver.
- The 140-octet signalling limit, decoded with the 7-bit alphabet, gave a natural 160-character message.
- Texting worked on the phones and networks that already existed, so it needed no new infrastructure.
- Operators charged for each message, turning a tiny signalling payload into a real revenue line.
what transfers
When you cannot build a new network, hack the channel that already exists: a feature priced at the marginal cost of a signal can spread faster than one built on new infrastructure.
what came after
SMS became the world's default way to send a short message, and for years it was a major source of mobile revenue. Later mobile data and messaging apps took over most of the volume, but the original 160-character design survived as the format's signature.
references
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