How SMS works

JUST when we’re finally used to seeing everybody constantly talking on their cell phone, it suddenly seems like no one is talking any more. 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. 

JUST when we’re finally used to seeing everybody constantly talking on their cell phone, it suddenly seems like no one is talking any more. 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. 

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.

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.

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 channel 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.

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 email and instant messaging (IM) do – although some phones are equipped for mobile email 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.

 

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