PBX switches are installed in large business campuses to
relieve the public telephone company switches from having to switch local calls.
When you call a coworker locally in your office campus, the PBX switches the
call locally instead of having to rely on the public CO switch. The existence of
PBX switches also limits the number of trunks needed to connect to the telephone
company's CO switch. With a PBX installed, not every office desktop telephone
needs its own trunk to the CO switch. Rather, the trunks are shared among all
users.
Small organizations and branch offices often use a key
telephone system, as shown in Figure 1-5,
because a PBX offers functionality and extra features that they may not require.
A key system offers small businesses distributed answering from any telephone,
unlike the central answering position required for a PBX. Notice in Figure 1-5 that telephones interconnect to a
key system via connector blocks, while trunks
coming in from the local exchange interconnect to the key system via termination blocks.
Today, key
telephone systems are either analog or digital and are microprocessor based. Key
systems are typically used in offices with 30 to 40 users, but can be scaled to
support over 100 users.
A key system has three major components:
-
Key service unit A key service
unit (KSU) holds the system switching components, power supply, intercom, line
and station cards, and the system logic.
-
System software System
software provides the operating system and calling-feature software.
-
Telephones (instruments or
handsets) Telephones allow the user to choose a free line and dial out,
usually by pressing a button on the telephone.
Larger companies use proprietary telephone networks with PBXs.
In a key telephone system, each telephone has multiple lines that allow users to
access outside lines to their CO. When a call comes into the company, a line or
a key lights up on the telephone and indicates that a particular line is in use.
Users can call another extension or let another person know where to pick up a
call by using an intercom function, such as an overhead paging system or
speakerphone.
Key telephone system functionality has evolved over time to
include a class called hybrid telephone systems.
The hybrid system adds many features that were previously available only in
PBXs. There is no single definition of the functions and features that are
classified as a hybrid system because all vendors provide a mix that they
believe gives them a competitive advantage.
The main difference between a key telephone system and a hybrid
telephone system is whether a single-line telephone can access a single CO local
loop or trunk only (key telephone system) or whether the single-line telephone
can access a pool of CO local loops or trunks (hybrid telephone system).
Sections
Comments (0 posted)
Syndication
