How SMS Works
by Jennifer Hord
Browse the article How SMS Works
Introduction to How SMS Works
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 Text messaging has replaced
of the text messages: 160 characters (letters, numbers or symbols in talking on the phone for some
the Latin alphabet). For other alphabets, such as Chinese, the users. See more cell phone
pictures.
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.
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.
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, see this page.
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.
Advantages of 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.
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].
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.
Next, we'll discuss the disadvantages of SMS and look at some alternative communication technologies.
SMS Criticism and Alternatives
Despite their popularity, short messaging services have received some SMS in the News
Because of the impersonal
criticism. Here are a few of the disadvantages of SMS:
nature of SMS, it raises certain
You have to pay for it. Most wireless plans charge for a certain questions of etiquette -- namely,
what kind of information is OK to
number of text messages a month. Some only charge for user-
send in a text instead of
originated messages, while others charge for incoming delivering it in person?
messages as well. If you exceed your message allowance, you Recently, several people have
may be charged 10 cents per message, and those little charges sought legal action after they
can add up. were fired or notified of divorce
proceedings via SMS.
Speedy message delivery is not guaranteed. During periods of
high traffic, it might be minutes or even hours before a message Broadcast text messages have
gets through. been used to rally political
It's strictly for sending text messages. SMS does not support activists in Beijing and to
sending pictures, video or music files. mobilize young people for riots
in Belfast. Recently, a contest
Alternatives to SMS
pitted the efficiency of SMS
Alternative messaging services allow for more elaborate types of
against Morse code (the Morse
messages. With EMS (Enhanced Messaging Service), you can send
coders won).
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).
To learn more about SMS and other forms of mobile communication, check out the links on the following
page.
Lots More Information
Related HowStuffWorks Articles
How Cell Phones Work
How Cell-phone Viruses Work
How Ringtones Work
Cell Phone Buying Guide
How Radio Works
How Routers Work
How Instant Messaging Works
How E-mail Works
How WAP Works
What does GSM mean in a cell phone?
What is a packet?
More Great Links
SMS Tutorial
160 Characters
Textually.org
Dodgeball
Sources
Barkhuus, Louise and Anna Vallgarda. "Saying it All in 160 Characters: Four Classes of SMS
Conversations."
IT University of Copenhagen, April 2004.
http://www1.itu.dk/graphics/ITU-
library/Internet/Forskning/Technical_Reports
/ITU-TR-2004-45.pdf
Burns, Enid. "Teens, College Students Are Most Active Cell Phone Users."
Click Z Network, August 29, 2005.
http://www.clickz.com/stats/sectors/wireless/article.php/3530886
Clements, Tom. "SMS - Short But Sweet."
http://developers.sun.com/techtopics/mobility/midp/articles/sms
Coll, Steve. "In the Gulf, Dissidence Goes Digital." Washington Post, March 29, 2005.
http://www.washingtonpost.com/ac2/
wp-dyn/A8175-2005Mar28?language=printer
Dybwad, Barb. "Morse Code Trumps SMS in Head-to-Head Speed Texting Combat."
Engadget, May 6, 2005.
http://engadget.com/entry/1234000463042528/
Dybwad, Barb. "Only 25 Percent of Americans Using SMS." Engadget, March 21, 2005.
http://www.engadget.com/entry/1234000560036891/
Gupta, Puneet. "Short Message Service: What, How and Where?"
http://www.wirelessdevnet.com/channels/sms/features/sms.htm
"Lecturer May Sue After SMS Sacking." Independent Online, September 13, 2005.
http://www.iol.co.za/index.php?set_id=1&
click_id=29&art_id=qw1126585085708B215
"Linked By Their Phones, Chinese Take to the Streets." Textually.org, April 25, 2005.
http://www.textually.org/textually/archives/2005/04/008069.htm
Reid, Donna and Fraser Reid. "Insights into the Social and Psychological Effects of SMS Text
Messaging."
http://www.160characters.org/documents/
SocialEffectsofTextMessaging.pdf
Trosby, Finn. "SMS, The Strange Duckling of GSM."
http://www.telenor.com/telektronnik/volumes/pdf/3.2004/Page_187-194.pdf