How SMS Works
Cute Girl • onDid You Know 13 years ago • 9 min read

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.

What is SMS? SMS AttacksRecently 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. 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 ring tone. 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, see this page. In the next section we'll learn about some of the uses and advantages of SMS.

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.

Why use SMS?
SMS has several advantages. It is more discreet than a phone conversation, making it the ideal form for communicating when you don't want to be overheard. It is often less time-consuming to send a text message than to make a phone call or send an e-mail. SMS doesn't require you to be at your computer like e-mail and instant messaging (IM) do -- although some phones are equipped for mobile e-mail and IM services.

SMS is also a convenient way for deaf and hearing-impaired people to communicate. SMS is a store-and-forward service, meaning that when you send a text message to a friend, the message does not go directly to your friend's cell phone. The advantage of this method is that your friend's cell phone doesn't have to be active or in range for you to send a message. The message is stored in the SMSC (for days if necessary) until your friend turns his cell phone on or moves into range, at which point the message is delivered. The message will remain stored on your friend's SIM card until he deletes it. In addition to person-to-person messages, SMS can be used to send a message to a large number of people at a time, either from a list of contacts or to all the users within a particular area. This service is called broadcasting and is used by companies to contact groups of employees or by online services to distribute news and other information to subscribers.

In a 2004 University of Plymouth study on the psychology of SMS users, researchers found that mobile phone users were primarily either "texters" or "talkers" [ref]. Compared to the talkers, the texters sent nearly double the number of SMS messages and made less than half as many voice calls per month. The texters preferred SMS to voice calls for its convenience as well as for the ability to review a message before sending it. Companies have come up with many uses for the service beyond just your typical person-to-person message. Because SMS doesn't overload the network as much as phone calls, it is frequently used by TV shows to let viewers vote on a poll topic or for a contestant. As a promotional tool, wireless carriers put up giant screens at concerts and other large-scale events to display text messages from people in the audience.

You can use text messaging subscription services to get medication reminders sent to your phone, along with weather alerts, news headlines or even novels broken into 160-character "chapters." Internet search engines such as Yahoo! and Google have short messaging services that enable users to get information such as driving directions, movie showtimes or local business listings just by texting a query to the search engine's phone number. Social networking services such as Dodgeball use SMS to alert people who live in big cities when their friends or crushes are nearby. The possibilities for integrating SMS into your lifestyle seem endless. Naturally, SMS has limitations, and there are some people who feel it has outlived its usefulness. In the next section, we'll look at the disadvantages of SMS and some of the alternatives out there.

SMS HistorySMS 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].

SMS Criticism and Alternatives SMS in the NewsBecause of the impersonal nature of SMS, it raises certain questions of etiquette -- namely, what kind of information is OK to send in a text instead of delivering it in person? Recently, several people have sought legal action after they were fired or notified of divorce proceedings via SMS. Broadcast text messages have been used to rally political activists in Beijing and to mobilize young people for riots in Belfast. Recently, a contest pitted the efficiency of SMS against Morse code (the Morse coders won).

Despite their popularity, short messaging services have recieved some criticism. Here are a few of the disadvantages:

You have to pay for it. Most wireless plans charge for a certain number of text messages a month. Some only charge for user-originated messages, while others charge for incoming messages as well. If you exceed your message allowance, you may be charged 10 cents per message, and those little charges can add up.

Speedy message delivery is not guaranteed. During periods of high traffic, it might be minutes or even hours before a message gets through.

It's strictly for sending text messages. SMS does not support sending pictures, video or music files. 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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  • Sunny 13 years ago
    This is really informative, good information on how SMS works. Thanks for posting this.