Understanding Traditional Telephony

The PSTN, or Public Switched Telephone Network, is made up of Central Office switches to which subscriber lines are connected. The CO switch is programmed so that it knows which phone number (subscriber line) is attached to a particular port. If the number called is not on the local switch, the call is routed over an interoffice trunk to another switch, which may have the called subscriber line connected directly to it or may in turn route the call to other CO switches. Telephone numbering plans are organized so that calls are routed efficiently through the switch system to the correct destination switch.

Note that for our purposes, a line connects to a single phone number and supports one call at a time, whereas a trunk interconnects two switches and supports multiple calls at a time.


Business Telephony Systems

Businesses have more elaborate requirements of the telephone beyond simply placing calls. Over time, two main types of business systems have evolved: the PBX and the Key System. Both have their place, and both offer calling features that make it easier to carry on business both internally and externally with staff, customers, and suppliers.


Business telephone systems often use a Private Branch Exchange (PBX) switch, usually located in their building. The PBX is configured in much the same way as the PSTN CO switch: it holds the dial plan for all numbers within the business, and external calls are routed over a CO trunk to the PSTN CO switch if the called number is not on the PBX. As a business grows, it is common to install another PBX in another location or building and set up a special trunk (called a tie-line or tie-trunk) between the PBXs so that calls to the remote location are still internal numbers (typically 4- or 5- digit numbers) instead of PSTN calls.

A PBX consists of a control plane (the "brain"), a terminal interface that connects phones to the features they want to use, a switching engine that determines which port to route a call out, line cards to connect to phones, and trunk cards to connect to the PSTN or to tie trunks to other PBXs. PBXs come in a variety of sizes, supporting from 10 to 20,000 phones. All PBXs offer basic calling features, with additional advanced features optional based on hardware capability and licensing. These features typically include Hold, Transfer, Conference, Park, Voice Mail, and so forth.

Key Systems
Smaller businesses will sometimes use a key system. A key system is like a PBX in that it controls a group of local phones, but key systems tend to have fewer features than PBXs. One characteristic of key systems that many businesses specifically request is distributed answering from any phone; that is, all the phones ring at once, and whoever is able to pick up Line 2 (for example) can push the Line 2 button on any phone and take the call. PBXs don't normally do this; they have a central answering point (a receptionist or Auto Attendant) and Direct Inward Dial numbers (DIDs) if needed.


Telephony Signaling

Telephony signaling refers to the messages that must be sent to set up and tear down a phone call—that is, anything other than the actual voice. Following are the three types of telephony signaling:

• Supervisory: Communicates the current state of the telephony device. There are three types of supervisory signals:

• On-Hook: The phone is hung up. Only the ringer is active in this state. (Note that if the speakerphone button is pressed, this is the same as being off-hook.)

• Off-Hook: The phone receiver is out of the cradle. This signals the phone switch (PSTN, PBX, or Key) that the phone wants to make a call; the switch sends a dial tone to indicate that it is ready to receive digits.

• Ringing: The switch sends voltage to the phone to make it ring, alerting the user that there is an inbound call. The other end of the call hears a ringback tone.

• Address: Communicates the digits that were dialed. Address signaling is most commonly done using Dual Tone Multi Frequency (DTMF) tones, commonly known as TouchTone dialing. The combination of tones tells the switch what number was pressed. Older systems also support pulse dialing, which is what the old-fashioned rotary dial phones used. Pulse dialing works by repeatedly opening and closing the circuit to the phone switch; the switch counts the number of pulses and interprets that as the number dialed. You might have seen in really old movies when someone picks up the phone and taps the receiver cradle repeatedly; this was how you got the attention of the operator.

• Informational: Communicates the call status to participants in the call. Informational signals include dial tone, ringback tone, and reorder tone. These tones, and others not mentioned here, will vary from country to country. In England, for example, ringback tone sounds very different from what would be heard in North America.

Signaling System 7 (SS7)

SS7 is a global telephony standard that allows a phone call to be routed between CO switches, between long-distance carriers, and even between national telephone providers in other countries. SS7's primary role is to complete the setup and teardown of phone calls; this is quite a distinct process from the actual transport of the voice signal. In fact, the call control information in an SS7 network must traverse an entirely separate network from the voice path. The capabilities of SS7 have allowed the introduction of relatively complex value-added services, such as call screening, number portability, and prepaid calling cards.

PSTN Call Setup

To make a PSTN call, several steps occur that the caller is unaware of. The following steps refer to Figure 7.


1. The calling phone goes off-hook, closing the circuit to the local CO switch.

2. The local CO switch detects that current is flowing over the closed circuit and sends a dial tone to the calling phone.

3. Address signals (DTMF or pulse) are sent as the calling party dials the called number.

4. The local CO switch collects the digits and makes its routing decision; in this example, it uses an SS7 lookup to locate the destination CO switch.

5. Supervisory signaling indicates to the far-end trunk that a call is inbound.

6. The PBX determines which internal line the call should go to and causes the connected phone to ring.

7. The ringback tone is heard at the calling party end.

8. The called party goes off-hook, and a voice circuit is established end-to-end.

The fact that all this happens with very high reliability billions of times every day is pretty impressive. It also provides some insight into how complex it is to duplicate these functions in a VoIP system. More on that later.