Computers and telecommunications breakthroughs such as microwave towers and, more recently, satellites have also changed the old telephone system, which is over 100 years old, but it is still generally a wire from a switching office to your home that makes the connection. The phone company switching offices, which have varied from old fashioned plugs in holes and relays to modern computer switches over time, still make a solid connection to your phone, send a special ringing voltage down the line to alert you to a call, and work pretty much as they did a hundred years ago. The switching office has changed with digital and multiplexed lines, but the POTS (Plain Old Telephone Service) is little changed.
Cells, on the other hand are so … well … portable. They are really radios that are in constant communication with nearby towers. These towers are then interconnected with computers and POTS. There is a basic signal that the cell phone exchanges with the towers relatively continuously. It is called the “Control Signal.”
When you first power up the phone, it listens for an SID (System Identification Code — a unique 5-digit number that is assigned to each carrier by the FCC) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.
When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system. That is how your phone knows if it is on a Verizon or an AT&T or whatever network.
Along with the SID, the phone also transmits a registration request, and the telephone switching office keeps track of your phone's location in a database — this way, the switching office knows which cell you are in when it wants to ring your phone.
The switching office gets the call, and it tries to find you. It looks in its database to see which cell you are in. Once found, the computer picks a frequency pair that your phone will use in that cell to take the call.
The switching office communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. Now, you are talking by two-way radio to a friend.
As you move toward the edge of your cell, your cell's base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own) sees your phone's signal strength increasing. The two base stations coordinate with each other via computer, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell. As you travel, the signal is passed from cell to cell.
The range of cell phone towers varies depending on many things including tall building in your area, but is typically 5 to 20 miles … greater outside the city. Cell phone signals are very high frequency in the microwaves and limited to line of sight. Plus cell phones are relatively low power. The secret to the continuous communications compared to other radio services is the large number of towers that have been built to relay the communications.
Let's say you're on the phone and you move from one cell to another — but the cell you move into is covered by another service provider, not yours. Instead of dropping the call, it'll actually be handed off to the other service provider.
If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The switching office of the cell that you are roaming in contacts the switching office of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local switching, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds. Of course, you can set your phone to "not allow roaming" … something I highly recommend for those on a budget.
So the control channel is key to your cell phone provider keeping track of just where you are and contacting you with a call. If a call is sent to your phone, the control channel is used to send the ring signal that “wakes your phone up” to receive the call.
The result is that cell phones are in constant communication with their cell phone towers over pathways known as control channels. The communication has to remain constant so that the cell phone system knows what cell your phone is in as you move around. Your phone and the tower for the cell you're in exchange packets of data over the control channel from time to time; this helps the cell do its job of routing calls to you and ensuring there's a frequency available for your service. The control channel was designed to be simple, not use a lot of power, and keep the network clear for actual calls.
Texting and SMS
It was just twenty years ago that engineers decided the control channel could be used to send additional short messages without adversely affecting the network. An agreement for communications, called a “protocol” was developed and implemented.
This protocol is called SMS (which stands for Short Message Service). The first message was sent from Neil Papworth of Sema Group from his computer to Richard Jarvis of Vodafone in the United Kingdom. It read, simply, “Merry Christmas.” After all, it was December.
A merry Christmas it must have been. That text message was the harbinger of a revolution in communication that has redefined how people talk to each other. The old telecommunications standby, the phone call, has decreased in relevance since the rise of SMS and most young people these days prefer the text over a call.
SMS was instrumental in the popularization of the Internet-based pidgin language as people across the world replaced numbers for words and truncated whole sentences to fit into 160-character messages. Love it or hate it, but the pervasiveness of SMS has forever ingrained “LOL,” "ROTFLOL," and “OMG” into the English lexicon “4ever.”
However, enough is never enough. Soon smart phone users were clambering to send more than short text messages. They wanted to “instant message” pictures and even audio files. A new system called MMS or Multimedia Messaging Service was designed and implemented, but it doesn’t use the simple Control Channel method of communications. It is more of a full blown Internet communications, although it is tied to telephone numbers rather than IP addresses.
So, on this twenty-year anniversary of SMS and Texting, like black and white television and the US Post Office, there are new and better ways to go, rapidly shoving the old technology aside.
However, since the limited SMS message length of 160 characters max spawned another Internet service called "Twitter," I suspect that "OMG" and "WTH" will be with us for a long, long time … IMHO.
So, happy birthday SMS, LOL, you’re getting long in the tooth old gal. Eventually texting is going to be replaced by the data plan on your smartphone. There will probably always be an SMS icon somewhere on your phone, but in the years and decades to come, it will become a relic of a bygone era, a reminder of the technology that helped spur this Mobile Revolution and change the very nature of communication.
You’ll be able to tell your grandkids about smoke signals, dial telephones, and SMS. They’ll give you that look that kids always give old people and ask if you had to walk to school, through the snow, uphill, both ways. Yeah, that’s what it will be like … LOL.