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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.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
Understanding Traditional Telephony
This section introduces traditional telephony systems, concepts, and applications.
The PSTN
FIGURE 5 Public Switched Telephone Network
A Representation of
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.
PBX Systems
FIGURE 6
A Representation of a
PBX System
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
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
CCNA Voice Quick Reference by Michael Valentine
Understanding Traditional Telephony
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
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
CCNA Voice Quick Reference by Michael Valentine
Understanding Traditional Telephony
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.
FIGURE 7
PSTN Call Setup
0 Customer
Telephone
NOTE
Codes do not always
need to be dialed; Local
numbers must always be
dialed.
Numbering Plans
A numbering plan is an organized distribution of telephone numbers administered by a regional or national authority. The
plan defines the rules that allocate numbers according to an established international telecommunications standard. For
example, the North American Numbering Plan defines a country code of 1, followed by a three-digit area code, a threedigit
office code, and a four-digit local number. There are numerous other numbering plans for other countries or regions
of the world.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
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.
NOTE
Several other ranges are
reserved for specialized
purposes. One commonly
recognized one is 555-
01XX, which is used in
film and TV, demonstrations,
or education. No
actual customer is
assigned these numbers,
so calling a number seen
in a movie will not pose a
nuisance to anyone. When
Tommy Tutone recorded
"867-5309/Jenny," he
immediately annoyed
thousands of phone
customers worldwide.
It is very important to note that the "N" represents any digit in the range 2 through 9, and the "X" represents any digit 0
through 9. You will never find an office or area code of OXX or 1XX; those numbers are either reserved for specialized
purposes or would interfere with things like operator access numbers. Several ranges are also reserved for Easily
Recognizable Codes (ERCs); these are numbers where the second and third numbers of the area code are the same. They
are used for special services—for example, 800, 888, 877, and 866 are toll-free numbers. Another recognizable assignment
is the " N i l " series: this includes 411, 611, and 911 numbers that are not used as area codes but for other special
assignments, such as information or emergency services.
E.164 Addressing
The E.164 addressing scheme is an international standard for telephone numbering plans, originally developed by the
International Telecommunication Union. An E.164 number contains the following components:
CC—Country Code
NDC—National Designation Code
SN—Subscriber Number
An E.164 number is standardized at 15 digits, generating over 100 trillion unique strings. In theory, it's possible to direct
dial any conventional phone in the world from any other conventional phone.
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