Introduction To Line Telecommunications

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10. The Telephone System


10. - The Telephone System


10.3 - Telephone System Components

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
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.

Music on Hold I/F
External Paging I/F
Call Logging I/F
Private Circuit I/F
  Line &
Extension connections
Building wiring to extensions
Figure 7 - Telephone System Components

The following sections describe these and other components in more detail.




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Extracted from

Introduction To Line Telecommunications
Copyright Panasonic Business Systems UK Ltd 2000