Logical View of Direct Inward Dialing
PSTN
PBX
Receiver
51706
52013
707-555-1234
Caller
Configuration within the phone
company's switch directs all calls to
415-995-xxxx to the PBX. Extension
51706 maps to 415-995-1706.
Caller believes receiver is on PSTN
network, however, Receiver is actually
mapped across a T-1 circuit that
services 10,000 numbers and myriad
phones. Please note that a single
PBX/trunk cannot handle 10,000
stations. This example is illustrative only.
Coordinated Dial Plans (CDP). (As we mentioned earlier in this section, if you
are entering the world of telephony from the Cisco router, you are no doubt
thinking Cisco Discovery Protocol for CDP.The acronym CDP stands for both
actually, depending on your perspective.) A coordinated dial plan is analogous to
addressing in IP routing—the dial plan defines what numbers exist on your network
and how callers will reach phones outside your company. (For example, a
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Figure 1.2 Logical View of Direct Inward Dialing
PSTN
PBX
Receiver
51706
52013
707-555-1234
Caller
Configuration within the phone
company's switch directs all calls to
415-995-xxxx to the PBX. Extension
51706 maps to 415-995-1706.
Caller believes receiver is on PSTN
network, however, Receiver is actually
mapped across a T-1 circuit that
services 10,000 numbers and myriad
phones. Please note that a single
PBX/trunk cannot handle 10,000
stations. This example is illustrative only.
Old World Technologies • Chapter 1 15
coordinated dial plan may require a nine to be dialed before an external number.)
The term call routing has two meanings, however, that overlap slightly.The first
context is the physical act of routing a call through the network. For example,
calls to 312 are destined for Chicago, which is a long-distance call requiring a
service provider beyond the PBX.The second meaning involves the act of processing
that call—there may be three alternate paths to Chicago, and, based on
availability, price, and preference, an administrator can route the call along any of
those paths.
NOTE
Routing in PBX systems is static, unlike data networks, which typically
use dynamic routing.
In the voice world, telecommunications services are billed at various amounts
based on the tariff involved. A flat rate structure removes per-minute charges
from the billing calculation. Other tariffs can remove distance or other parameters
from the calculation.
It is also important to consider the historical import of which end is which in
the voice world. For example, you may hear the term tip-and-ring in single pair
copper connections, which relates to which end supplies the voltage on the wire.
In the same manner, there are also foreign exchanges, which have slightly different
meanings depending on your background.
NOTE
Most voice copper wires are described in pairs—thus, a two-wire connection
is a single pair, while a four-wire connection is a double pair, or two
pair. Traditional Category 5 wiring would be four pair.
For our purposes, we will describe a Foreign Exchange Station (FXS) as a link
between the switch and an extension.This term is sometimes used to describe a
connection that services an analog device within the company attached to the
PBX, such as a fax machine or modem. If you’ve worked within the phone company,
this term may be defined differently; however, this definition is best in the
context of AVVID. In contrast, a Foreign Exchange Office (FXO) link is between
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16 Chapter 1 • Old World Technologies
the PBX and the central office. It is a DS-0 and analog, and it is tariffed at a flatrate.
There may be instances when local services are desired, but ISDN or T-1
bandwidths are not needed. FXO connections can be used to service these situations,
and can also be used to provide local 911 service in the event all other calls
traverse the private network to a main site in another location