Introducing SMS > Message Format
An SMS is a message delivered to or from a mobile phone. In the industry, messages originating from a phone are referred to as MO (mobile originated) or MT (mobile terminated ). As author or operator of a service that employs SMS, you don't need to know a lot about the specific protocols and pathways the carriers use to send these messages, but there are aspects of the format that may impact your use.
A text message is limited to 140 bytes, or 160 7-bit characters. If you are sending Arabic, Korean, or other languages that require 16-bit encoding, you will be limited to 70 characters .
It's possible to send 8-bit characters, but they are not typically viewable on phones (they are used instead for data such as ringtones and over the air provisioning of WAP settings). Additional information, such as the sender's number and the time stamp, are included as well, but are not counted against the 160 character limit.
MMS, or Multimedia Message Service, is an evolution of the SMS system that allows users to send photos, videos , and audio in addition to text. Unlike an SMS, there is no set limit on text length, either. Instead of being sent on the GSM signaling channel, MMSs are sent over an IP connection from an MMS-C to the phone. The IP connection to an MMS-C must always be initiated from the phone, however, a carrier will typically send an SMS to the phone that contains instructions for the phone to connect back to an MMS-C via IP to collect the MMS message.
The system for sending and receiving MMS messages is quite different from SMS, and varies somewhat by carrier, so it won't be covered in this guide.
Introducing SMS > Definitions
Before we get much further, it will be useful to establish some common terms that will come up throughout the guide. These include:
A 4- to 12-digit (in the U.S.) number to which mobile phones can send and receive text messages. While they are technically unique to each carrier, the Common Short Code Administration (http://www.usshortcodes.com) acts as the common registrar in the U.S., ensuring that the same short code is not issued by different carriers to different application providers.
Digital cellular technology used by virtually all carriers worldwide (but see below).
Digital cellular technology used in the U.S. by Sprint and Verizon.
Removable smart card found in GSM phones. Contains configuration data (e.g., phone number, SMS-C connectivity info ) as well as user data (e.g., portable phone book). Contains a small CPU that processes SMS messages.
Wireless Application Protocol. Open international standard for encoding Internet data for mobile devices. WAP sites are generally written in XHTML MP.
A service that provides an interface to application providers (that's you) to send and receive text messages. An SMS Aggregator typically exposes web interfaces to the application providers and connects directly to the SMS gateways (SMS-C) of multiple carriers to send and receive the messages.
A service that provides the content and logic behind an SMS service. This could be you!
A company that sells or rents telecommunication transmission services such as telephony and data communication. For example, T-Mobile, Cingular, and Sprint in the U.S., and Orange and British Telecom in Europe.