Dissecting a Telephony Network

When you pick up your home phone's handset and place a call to a friend, many operations occur in the background. This section walks through a basic phone call step by step, assuming you have a regular analog phone.


Traditional analog phones are called POTS devices. POTS stands for "Plain Old Telephone Service."

When you pick up the handset of your phone, you hear a dial tone in the receiver, and you can press keys on the touchtone keypad to dial digits. However, on close inspection of your phone, you notice that there doesn't seem to be a battery in the phone, and it's not being powered from an alternating current (AC) wall outlet. So, the question is, "Where is the phone getting its power?" The answer is the phone company. The telephone switch at the phone company's central office (CO) applies -48 volts of direct current (DC) across the wires coming out of the telephone wall jack. That voltage enables the phone to dial digits or even illuminate the keypad (in case you need to dial in the dark), for example.


The telephone jack in the wall of your home (assuming you live in North America) is an RJ-11 jack, where RJ means Registered Jack. This RJ-11 jack has up to six conductors, although a phone that has only a single line uses just two of these wires.

You dial your friend's phone number of 555-1212. The tones created from the digits you dial travel over a pair of wires leaving your phone, into your RJ-11 wall jack, and back to the telephone switch at your local CO. These two wires leaving your phone are called the tip and ring wires.


Did you ever notice that in the movies or on television, almost all phone numbers begin with 555? That's because originally, there were no residential or business seven-digit phone numbers in the North American Dial Plan beginning with 555. (Although some numbers outside the 555-0100 through 555-0199 range are now released for actual assignment.) Examples include 555-1212 for the Brady Bunch's home and 555-0267 for the Townsend Agency in Charlie's Angels. (It doesn't work. I tried.) The producers don't want to create another situation like the one caused by the song "Jenny." Remember, eight six seven five three oh nine.

The names tip and ring come from the plug used by the operators of yesteryear to interconnect calls. As you can see in Figure 1-1, the plug used by these operators resembles the plug you might use to connect your headphones to your home stereo equipment. There are three conductors on this plug. The conductor (that is, wire) connected to the tip of the plug is called the tip, and the conductor connected to the ring in the middle of the plug is called the ring.

Figure 1-1. Tip and Ring

The tip and ring wires take on a new name after they leave your home. The connection from your home to the local CO is called the local loop. When your dialed digits, carried over the local loop, reach your local CO, the local loop connects into a phone switch. As the name suggests, a phone switch is responsible for interconnecting the endpoints (that is, the phones) that make up a phone call. The phone switch can recognize the digits you dialed by listening to the tones generated by your telephony keypad, or by interpreting pulses generated by a rotary phone. The phone switch can then determine to where your call should be forwarded, based on those dialed digits. If your friend lives just across town, perhaps your friend's phone connects into the same CO that your home phone connects to. If that's the case, then the phone switch sends ringing voltage over the local loop connecting to your friend's home. If your friend does not live nearby, then the phone switch might need to forward your call to a phone switch in a different CO. A connection from one phone switch to the other is called an interoffice trunk.

Your friend's phone begins to ring, and he picks it up. Your friend's local CO telephone switch notices that he picked up his phone's handset and stops sending ringing voltage. At this point, you and your friend can begin to talk with one another.

This chapter digs deeper into the inner workings of the phone call just described. For example, several types of signaling occurred to make the phone call possible. When you heard the dial tone and your friend heard his phone ringing, that was information signaling. When you dialed digits, that was address signaling.

Figure 1-2 illustrates some of the components that make up a traditional telephony network.

Figure 1-2. Components of a Telephony Network

The list that follows defines the traditional telephony network components illustrated in Figure 1-2:

  • Edge devices (for example, phones)Connect into a telephony network.

  • Local loops Connect callers to a local CO over a pair of wires called tip and ring.

  • Phone switches Make it possible for one phone to connect to another phone by dialing a phone number. The switch interprets the dialed digits and interconnects the dialing phone's local loop with the destination phone's local loop.

  • Trunks Interconnect phone switches. Unlike a local loop, a trunk typically transports multiple simultaneous conversations.

The preceding example considered a phone switch located in your local CO. However, for businesses, that is not always the case. Businesses might have their own phone switches. The next few lessons in this chapter compare and contrast these various types of phone switches, beginning with phone switches that make up the public switched telephone network (PSTN).

Voice over IP First-Step
Voice over IP First-Step
ISBN: 1587201569
EAN: 2147483647
Year: 2005
Pages: 138
Authors: Kevin Wallace

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