A residential phone system behaves much like telephone systems
at a traditional office, able to accept calls coming in on multiple
lines, patch calls to different telephone extensions, and facilitate
intercom conversations between telephone stations. Features like speakerphone,
auto-redial and call hold, and services like caller ID and call waiting,
have become commonplace in the home. A phone system can also provide
phone-to-phone functions such as intercom and call transfer. Each
phone is assigned an extension number allowing the phones to call
each other or to transfer calls to each other.
KSU vs. non-KSU
There are two types of high-end residential telephone systems available:
KSU (key system unit) systems and KSU-less systems. Both KSU and
KSU-less systems come in a variety of different sizes, measured
by the number of outside lines (CO lines) and telephone extensions
they can handle. While one system might support three incoming lines
and eight extensions, another might be able to manage as many as
12 incoming lines and 24 extensions.
KSU systems, similar to security systems, use a central control unit that maintains
the programming, connects available outside lines to extension phones,
and handles all phone functions such as transferring calls between
extensions, call hold, and so on. Control units can be programmed
to assign system features selectively to each extension,
KSU systems also use
special, proprietary, phones designed to work with the specific
control unit. These phones have numerous special functions buttons
to handle tasks such as selecting a specific outside line, transferring
the call to another extension, hold, flash, and so on.
KSU phones require two
or three pairs in the 4-pair TP cable to the extension outlet. One
of the pairs is used for voice signals and the other one or two
pairs are used for control functions between the phone and the system
control unit. KSU phones typically have dedicated buttons for each
phone function as well as the standard numeric keys and an LCD display
to show everything from caller-ID information to the time and date.
KSU systems require that
the system control unit be installed at a central location (usually
mounted next to the structured wiring center) and that every telephone
cable be wired to that box. These systems rely on either a structured
wiring system with home-run wiring to each outlet or home run outlet
cables to the control unit. This usually limits installation to
KSU system is designed to work with proprietary phones but can also
use "standard" telephone handsets and even cordless phones
although all the control unit functions may not be accessible from
the nonproprietary phone. For example, most of the functions that
are accessible by a special button on the system phone may be accessed
by pressing the * or # key and then a numeric key on a regular phone.
Also, accessing an outside line may require dialing 9 first.
The second type of phone
system, referred to as a "KSU-less" telephone system,
does away with the special controller and cabling, usually making
it a better option for existing homes. KSU-less systems rely on
special proprietary phones that have the functions of a KSU system
built -in to each phone. These phones have buttons specifically
for functions such as call hold and transfer, intercom, conferencing
and speakerphone and are designed to operate between themselves.
Functions like intercom and call transfer will only work between
like phones attached to the same telephone wiring.
KSU-less system phones
can also operate on existing parallel or "daisy-chain"
telephone wiring and use only one pair (the line-1 pair) of wires
for all functions although many support 2 or 3 CO lines.
Note that there are NO
standards in the phone system industry and that handsets or components
from one manufacturer will not work with components from another
manufacturer. There is also limited interoperability between components
by the same manufacturer so be sure and check the interoperability
with the company or representative before selecting handsets or
Typical KSU telephone system components
originates at the KSU control unit. It contains all the switching
and interface electronics, programming and intelligence for the
system. Depending on the size of the system a residential sized
control unit will handle between 4 and 6 incoming (CO) lines, and
12 to 24 extension lines. If needed, commercial versions will handle
much larger configurations.
lines may be wired directly to the control unit or routed through
a structured wiring panel for added flexibility in connecting outlets
to extension lines. Either proprietary phone handsets designed to
work with the system or, if it is a hybrid system, standard handsets
are attached at extension outlets.
provide a computer interface (EIA-232) for system setting up, programming
or monitoring system operation. The interface can be connected to
a computer, a CRT terminal, or possibly a home automation system.
Most systems can provide full voice message processing including
voice-mail for each family member by an add-on voice processing
system. Voice processing systems (VPS) can provide several voice
mailboxes for different family members capable of storing hundreds
of incoming messages digitally. The VPS will also auto-answer incoming
calls with 20 or more custom greetings (some programmed to only
play on specific days) and routes them to the appropriate extension
mailbox. Using caller-ID the VPS can also announce incoming callers
by prerecorded identification using the speakerphone feature of
the system phones.
Messages can be retrieved
by pressing a message-waiting key on in-home phones or remotely
by entering a numeric password. Live call-screening from any extension
is also provided so an incoming call can be monitored while it is
A line-level music input
is also provided to supply music on-hold for incoming calls or at
an extension line when it is put on hold.
Some systems have door intercoms and door locks that attach to the
control unit. Pressing a button on the door intercom will announce
someone is at the door on the intercom function of the system and
allow hands-free conversion with the door intercom from a speakerphone
extension. Any extension can also "buzz" the door open
from the phone keypad.
KSU Handset Wiring
KSU handsets need two or three pairs (typically two) from the outlet
to the system controller. Line one (the blue pair on pins 4-5) is
used for normal analog voice and one or two other lines are used
for digital communications from the phone to the system controller
and to supply DC operating power to the handset. One of the advantages
of KSU systems is that the handsets do not require separate power
modules at each phone location.
In hybrid-KSU systems,
line-1 from the system controller also provides normal phone line
operating voltages, ring signals, busy signals, etc. to allow a
standard phone to operate normally.
Figure 4.5.2 illustrates how a KSU-less system is installed using
proprietary "smart phones".
Typical KSU-less phone system.
All phone systems allow various features, options, and special functions
to be programmed for each installation. Programming is usually done
in one of two ways: by using one of the proprietary phones (with
an LCD display and full key set), or by attaching a computer using
an EIA-232 interface. Using a handset, features can be turned of/off
or set to a value by using the keypad and LCD display. When using
a PC, the PC must either run set-up software provided by the manufacturer
or a "dumb terminal" software that allows the PC to emulate
a CRT terminal function.
the system will retain the programming during power outages.
KSU system controllers must always be located in a conditioned space.
Operating limits are typically 40 - 90 degrees F. The most convenient
mounting location is next to the structured cabling system panel
to allow easy wiring between the units. The KSU controller will
also require access to 110 V AC power and a good ground conductor
to the building grounding electrode (typically installed for the
structured cabling system).
diagram of a telephone line wiring of a KSU system when used with
a structured cabling system.
KSU integrated with a structured cabling
system. CO lines go from cross-connect to CO line input on KSU.
This example shows line 4 dedicated to a DSL modem. Extension lines
from the KSU terminate on a line distribution block in the structured
cabling panel. Voice outlet cables connect to the desired extension
CO lines are wired from the NID to the auxiliary disconnect on the
structured cabling system. Depending on the particular installation,
the CO lines will connect from the structured cabling system cross-connect
to the CO line inputs on the KSU system controller.
Lines can be shared with dedicated telecom applications such as
fax machines or analog modems, but many KSU systems will attempt
to connect to the line, even if it is in use (off-hook) by another
device, causing possible data loss and disruption.
a DSL modem is used, one CO line may be dedicated to the DSL modem
or the line may be shared with a KSU system line. In which case,
the DSL modem must be wired to the CO line side of the KSU, never
to one of the extension lines.
extension lines from the system controller to the outlet jack should
be wired according to TIA-570A specifications using an 8-pin modular
jack and plugs wired per T568A (see Section 3). A KSU system will
use the blue, orange, and optionally the green pair for KSU handsets.
extension lines are connected from the KSU controller to a structured
cabling voice line panel or module. Extension cables to outlets
are then plugged-in (or punched down) to the appropriate extension
jack. This allows changes to be made in the structured cabling system
instead of the KSU controller-usually a more convenient arrangement.
Wiring for optional
system features such as music-on-hold, door intercoms, and so on
will require home run wiring from the KSU controller to the optional
of KSU-less system phones with other phone-line applications
Some KSU-less system handsets may not be compatible with other services
that use the active phone-lines in the house such as DSL modem signals
or HomePNA home networking signals. Check carefully with the manufacturer
to determine if these services can coexist. Special filters used
to isolate phone devices from the phone lines that DSL signals or
HomePNA signals use may defeat the KSU functions of KSU-less smart