|What kinds of phones can I add to a
business phone system?
Although IP phones are catching up, the most
common telephones used on PBXs are digital. Digital phone systems
sample your voice using a method called Time Division Multiplexing or TDM.
When you speak into a digital telephone, your voice is digitally sampled
into time slots so that a conversation doesn’t have to use the entire
bandwidth of a circuit. The system then uses a clock to synchronize the
digital samples and turn them back in to voice. Whereas analog telephone
stations can only handle one conversation at a time, digital phone stations
can compress more than one conversation and other features onto a single
pair of wire. This provides for more features, less wiring, and more
efficient communication than a simple analog circuit.
Most manufacturers have acronyms to describe
their digital protocols. These protocols are all some form of TDM.
Avaya 2420 Digital Phone
|A multi-line digital phone with large programmable
display and fixed feature buttons using DCP, a proprietary form of
- Avaya – DCP – Digital Communications
Protocol, MLX – Multi Line Digital, TDL
- Norstar – TCM – Time Compression Multiplex
- Toshiba - DKT
|Analog phones do not use
digital sampling to compress your voice. When you use an analog telephone,
the sound waves of your voice are converted into electrical waves by a
microphone and conducted down copper wire. The telephone of the person you
call then uses those electrical signals to vibrate a speaker in their
phone’s handset. When you make a call with an analog phone over an analog
phone line, you seize the entire circuit. No one else can use that circuit
while you are conversing. The same is true for analog telephone stations
connected to a business phone system. They can only handle one conversation
at a time.
There aren’t any completely analog
business telephone systems manufactured anymore. The AT&T Merlin phone
|2500 Series Analog
|A typical single line analog telephone used in
common areas like lobbies, conference rooms, and break rooms.
probably the most popular analog phone systems ever produced. The majority
of these were installed in the 1980’s, and there are still many of them in
service today. Merlin phone systems used an ATL (analog telephone line)
standard of 4-pair (8 wires) copper wiring that connected the Merlin control
unit to each telephone. Most digital phone systems only use a single pair
wiring scheme that can handle both voice and signaling information.
Digital phone systems still accommodate
analog phones and analog devices like fax machines. Single line analog
phones are usually seen in common areas like a warehouse, lobby, or lunch
|Over the past decade Voice over
IP (VoIP) has become very popular. IP phones are now the most common
choice for new PBX installations. By converting voice into data packets, IP
phones can send voice conversations over a business’ data (IP) network.
Whereas digital and analog phones require dedicated wiring to each station,
IP phones share the same wiring as the data network. There is less
investment in wiring infrastructure and conceivably one person or department
can manage both the phone system and computer network. The telephone system
becomes just another application on the data network.
The use of VoIP has also bred new
applications and enhanced current ones. One of the most important
applications provided by today’s IP capable PBXs is telecommuting.
An employee no longer
|Cisco 7940G IP Phone
|A common IP phone
station used on Cisco unified communication systems.
needs to be in the office to use the phone system. IP
phones can connect to the business phone system via VPN (virtual private
network) over the the public Internet. The employee can make and receive
calls just as if he were in the office.