How SMS Work

Just when we're finally used to seeing everybody constantly talking on their cell phones, it suddenly seems like no one is talking at all. Instead, they're typing away on tiny numerical pads, using their cell phones to send quick messages. SMS, or text messaging, has replaced talking on the phone for a new "thumb generation" of texters.




In this article, we'll find out how text messaging works, explore its uses and learn why it sometimes takes a while for your text message to get to its recipient.

SMS stands for short message service. Simply put, it is a method of communication that sends text between cell phones, or from a PC or handheld to a cell phone. The "short" part refers to the maximum size of the text messages: 160 characters (letters, numbers or symbols in the Latin alphabet). For other alphabets, such as Chinese, the maximum SMS size is 70 characters.

But how do SMS messages actually get to your phone? If you have read How Cell Phones Work, you can actually see what is happening.

Even if you are not talking on your cell phone, your phone is constantly sending and receiving information. It is talking to its cell phone tower over a pathway called a control channel. The reason for this chatter is so that the cell phone system knows which cell your phone is in, and so that your phone can change cells as you move around. Every so often, your phone and the tower will exchange a packet of data that lets both of them know that everything is OK.

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. The tower also gives your phone a pair of voice channel frequencies to use for the call.


The control channel also provides the pathway for SMS messages. 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 the same way, when you send a message, your phone sends it to the tower on the control channel and it goes from the tower to the SMSC and from there to its destination.

The actual data format for the message includes things like the length of the message, a time stamp, the destination phone number, the format, etc. For a complete byte-by-byte breakdown of the message format,




SMS History

SMS was created during the late 1980s to work with a digital technology called GSM (global system for mobile communications), which is the basis for most modern cell phones. The Norwegian engineers who invented it wanted a very simple messaging system that worked when users' mobile phones were turned off or out of signal range. Most sources agree that the first SMS message was sent in the UK in 1992.

As SMS was born in Europe, it's not surprising that it took a little longer to make its way to the United States. Even today, texting enjoys much greater popularity in Europe, though its stateside use is on the rise. A July 2005 study found that 37 percent of U.S. mobile phone owners had sent or received at least one text message in the previous month [ref]. [/]

[i]SMS Attacks

Recently it has been suggested that SMS messages could be used to attack a cell phone system. The basic idea is very simple. If a large number of SMS messages were sent by computers to phones in a small geographical area (like a city), these messages would overwhelm the control channels and make it impossible for the cell phone system to set up calls. Now that cell phone providers know about the possibility of this threat, they can design systems to throttle messages coming from the SMSC onto the network.


Why 160 Characters?

SMS was designed to deliver short bursts of data such as numerical pages. To avoid overloading the system with more than the standard forward-and-response operation, the inventors of SMS agreed on a 160-character maximum message size.

But the 160-character limit is not absolute. Length limitations may vary depending on the network, phone model and wireless carrier. Some phones don't allow you to keep typing once the 160-character limit is reached. You must send your message before continuing. However, some services will automatically break any message you send into chunks of 160 characters or less. So, you can type and send a long message, but it will be delivered as several messages.

lternatives to SMS

Alternative messaging services allow for more elaborate types of messages. With EMS (Enhanced Messaging Service), you can send formatted text, sound effects, small pictures and icons. MMS (Multimedia Messaging Service) allows you to send animations, audio and video files in addition to text. If your mobile phone is EMS- or MMS-enabled, you can use these standards just as you would SMS. However, the cost per message will be higher.

Another alternative to using SMS is using an instant messaging program, such as AOL IM, on your cell phone. This can be in the form of software that's pre-installed on your phone, or you can use WAP (Wireless Application Protocol) to access the Internet and sign into your IM account. WAP is a protocol that gives you small, simplified versions of web pages that are easily navigable on your mobile phone or PDA (check out How WAP Works for more information). You can use it to send instant messages or actual e-mails from your phone.

A common complaint about SMS is its inefficient delivery structure -- when the message center is backed up, messages take longer to reach their destination. To make message delivery faster, networks are using more new next-generation technologies such as GPRS (General Packet Radio Service).

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