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The modern telephone system will usually have the following
components regardless of the manufacturer and size of the
system.
There will be a central unit to which all connections and
equipment are ultimately attached. This is referred to variously
as the Central Control Unit (CCU), Key System Unit (KSU) or
just The System. Whatever the designation used its function
is to provide switching between the lines and extensions and
service for the specialist functions it posses.
There will be extension telephones, either proprietary or
simple telephones. See the relevant sections of this book
for more details about these.
The wiring for the system will be routed between a number
of interconnection points where larger capacity cables are
split down for different areas of the installation on all
but the smallest systems. In the past the wiring for the telephones
was separate to the rest of the building wiring. Modern installations
tend towards using the category 5 data cabling system often
sharing the wiring with the customer's computer network.
The lines will terminate from the network on a Network Test
and Termination Point (NTTP) or Network Termination Unit (NTU)
this is the property of the network provider and the point
beyond which the wiring and equipment become the responsibility
of the customer. The extension wiring will originate from
the system's Main Distribution Frame (MDF) usually an integral
part of the main system unit. The wiring is then routed to
an Intermediate Distribution Frame (IDF) or if category 5
cabling is being used a Multiple Access Unit (MAU) or patch
panel from where the circuits are interconnected, or patched,
to the appropriate sockets and outlets.
In addition there are numerous optional items of equipment,
which may be connected to the main system unit to provide
the users with specialist functions and facilities. In many
cases it is necessary to have several devices to provide power
and signalling for these facilities, the precise requirements
vary from system to system. Some systems will have some of
these fitted as standard. The more common ones are outlined
below:
| Music On Hold interface |
For connection of an external music source which will
play to callers on hold and in some
systems provide background music to the users of the system
via the handsets or paging
system. |
| Paging System interface |
For connection of an external paging system allowing
users to broadcast messages over
the external paging system speakers from their handsets. |
| Call Logging interface |
For connection to a call management system which records
the call traffic through the
system and allows the system manager to see how the system
is being used. |
| Private Circuit interface |
For connection of the system to a private network or
long distance interoffice link. |
| Doorphone interface |
For connecting and controlling door access units via
the system. |
Figure 7 shows how these parts are typically related on a
generalised telephone system.
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CCU
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Music on Hold I/F |
| External Paging I/F |
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MDF
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Call Logging I/F |
| Private Circuit I/F |
Network
Resposibility |
Customer
Responsibility |
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Line &
Extension connections |
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IDF
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NTTP
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Building wiring to extensions
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Figure 7 - Telephone
System Components
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The following sections describe these and other components
in more detail.
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