SMS

SMS, for Short Message Service, is the short text messaging service defined by the GSM standard in the late 1980s. It was designed to fit inside the network's signalling channels, which is where its historic 160-character constraint comes from.

An SMS does not travel directly from one handset to another: it is handed to a message centre (SMS-C) that stores it and forwards it as soon as the recipient is reachable. This store-and-forward mechanism is what makes SMS reliable even when the phone is switched off.