SMS is a short message service. The short part refers to the maximum size of the text messages 160 characters. The term originally referred to messages sent using the SMS. Text messaging is the act of composing and sending electronic messages, typically consisting of alphabetic and numeric
Your phone also uses the control channel for call setup. When someone tries to call you, the tower sends your phone a message over the control channel that tells your phone to play its ringtone.
When a friend sends you an SMS message, the message flows through the SMSC, then to the tower, and the tower sends the message to your phone as a little packet of data on the control channel.
In some countries, text messages can be used to contact emergency services. In the UK, text messages can be used to call emergency services only after registering with the emergency SMS service.
A similar system known as Emergency Alert is used in Australia to notify the public of impending disasters through both SMS and landline phone calls.
These messages can be sent based on either the location of the phone or the address to which the handset is registered.