Dial Tones and Switches

When you pick your phone you get what is known as a “dial-tone”.  A dial-tone simply tells the caller that the switch is ready to receive a phone number so that it can connect two parties.  If the caller does not respond in a short amount of time, the switch will send a very loud and annoying tone that if not responded to, will cut you off.  Only so many callers can gain access to the switch at any particular moment.

Most new phones work like a cell phone; that is, you dial the number then press send.  This eliminates the lag between dial tone and numbers dialed—which resolves the problem of too many people taking too long to dial a number.

“Where is the switch?”

It is housed in a centrally located building in your neighborhood called the Central Office. It usually looks like a large, mostly windowless fortress. The buildings are usually constructed to withstand natural—and unnatural—disasters to protect the heart of the local network.  Your phone line is connected by a vast array of cables, wires and equipment, that terminate at the central office.  This is where your phone line is connected to the rest of the world.

The Central Office - (CO)

The Central Office (or CO—what the industry calls it) —houses the switch, in-coming and out-going cables, cable and equipment racks and a bank of batteries with power generators in case the local power goes out. Years ago it used to be a noisy place where mechanical switches opened and closed 24/7. But now the switch is an electronic device that only hums to the music of telephone traffic.

The switch interprets who you are calling based on the numbers dialed and sends the call down the correct cable. If you dial an area code that is different from yours, the switch knows you want to call someone outside your area. The next three numbers are called the exchange. This further tells the switch where in the neighborhood that person is and finally, the last four numbers identify who you are calling.

The cables leaving the central office are designed geographically in an orderly fashion. The cable leaving to the north is designated as Route 1, to the east Route 2, to the South Route 3 and Route 4 goes west.  These cables are designed in groups of wire pairs and those groups have numbers to identify them. A pair of wires is required for every individual telephone number—in most cases—depending on your service.

Fiber Optic Cable

Fiber optic cable has various configurations but they also have letters and numbers for the identification of each fiber.  These cables are connected to the switch through a dizzying number of connections on racks where each pair of wires or optic fiber is terminated.  Somewhere in there is your pair of wires or light pulse.  The same two wires that enter your phone must travel from your home or office to the racks.  Some of these racks house fiber optic equipment that interprets light pulse transmissions from the fiber optic cables.

Let’s follow a copper cable out from the central office and see what happens to it.  (See Figure 1.)

The Route 1 cable we will call “Cable 1”.  It is huge because it is going to end up at every service location in the northern part of the neighborhood—which could be miles away.

For the purpose of explanation, let’s assume this cable is 36 hundred pairs of wires (noted as 1, 1-3600).  It travels north underground through conduit and manholes—large underground concrete cable vaults that a man could stand in—and when it exits a manhole, it may go up a telephone pole or is directly buried.

It then branches off into smaller cables, like the limbs of a tree, and then goes through distribution cabinets to further divide the route until your pair enters your residence or business. The same thing occurs on the other routes. These 4 cable routes and central office that services it is called a wire center.

So how does your call get to your friend several miles away in another wire center?

Central offices are connected by a trunk cable. It is called a “trunk” because it is only used to send traffic between central offices or over long distances, and not directly to a customer.

See below.

When you dial a telephone number, the switch “knows” that the first three numbers are not in your wire center.  Your call is switched to the trunk cable that goes to the other wire center.  Once “switched” to that central office, the second switch interprets the whole number and sends a ring to the person being called.

All this switching occurs at a fantastic speed.  The labyrinth of central office to central office connections handles your calls to a larger area around your home or office.

Long Distance Calling: Phoenix to New York

Switching through every central office between Phoenix and New York is going to take too long and jam up local traffic. Never fear! The brains at AT&T worked out something to handle it.

An area code is a larger geographic area containing many wire centers. When you dial the area code for New York, your central office switch recognizes that your call is long distance.  The switch directs your call onto what is known as the AT&T Long Lines system.  It is no longer called
AT&T Long Lines but the concept is the same.

The figure below  shows a simplistic diagram of a Long Lines route. You can think of it as getting on the interstate highway instead of plodding along the
surface streets and stopping at every red light. Your call could travel through a number of Long Lines switching stations, but this is just an example.  These switching stations either pass the call to the next Long Lines station or send it to a local central office for routing, as described earlier.

To reach  another continent your call will need to cross oceans and other bodies of water.  Intercontinental cables were placed many decades ago to connect the U.S. with Europe, the Caribbean, and East Asia. A company branch of AT&T dropped huge lead-protected submarine cables off the back of a ship from one coast to the other—splicing them together as they went. These cables have since been replaced by fiber optic cables.

Telecommunication satellites are used to bounce intercontinental calls from one place to another. Once the call gets to the other continent, that country’s version of Long Lines picks up the call and switches it along as described previously.

Great—now we’re all connected!

Telecom Networks 101: Part 1

About the Author:

Saundra O’Neill began working for AT&T, and has spent the last fifteen years in utility design engineering for construction.  During this time her work in several fields of engineering (Civil, Structural and Telecom) established real world knowledge and experience in a wide variety of processes in the building industry with an emphasis on telecom Outside Plant and its processes.  Her experience and a desire to communicate/teach others, has expanded into technical writing - including software documentation and a variety of articles covering the telecommunications industry. Visit Sandy’s simple telecommunications blog.