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What kinds of phones can I add to a business phone system?

 

Digital phones
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 TDM.
  • Avaya  – DCP – Digital Communications Protocol, MLX – Multi Line Digital, TDL
  • Norstar – TCM – Time Compression Multiplex
  • Toshiba - DKT
Analog phones
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 Phone
A typical single line analog telephone used in common areas like lobbies, conference rooms, and break rooms.

systems were 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 room.

 
IP phones
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.



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