FREQUENTLY ASKED QUESTIONS
Digital business telephone systems can be based on one of two important technology standards. These are TDM or Time Division Multiplexing vs VoIP or Voice over Internet Protocol. Both are digital standards, as opposed to the analog single line or Key telephone systems found in very small phone systems. Generally, TDM is associated with PBX or Private Branch Exchange systems traditionally found in medium and larger size businesses. VoIP is the newer technology thought to need completely different equipment and more likely chosen for brand new installations. But there is more of an overlap in these technologies than you might think
Legacy PBX phone systems are often designed with very specialized hardware and programming chosen for high reliability. The phone sets may be either analog or digital and are individually connected to the PBX system on their own wiring network. The advantage of having a PBX system is that it is your own little phone company within the company. Users can call each other using just 3 or 4 digits and there are no line or per minute charges. It's only when someone makes a call outside the company that you incur local or long distance charges. Those outside calls are made using a group of shared phone lines on a first come, first served basis.
The outside phone lines are most often supplied to the PBX system using a T1 PRI digital line service. T1 PRI bundles up to 23 local or long distance lines together into one digital line service. There are actually 24 channels, with one reserved for signaling and data such as Caller ID. PRI is also known as primary rate interface. What's important about T1 PRI is that all of the channels are completely separate just like analog phone lines. If a channel isn't being used, it just sits there idling until needed. This channelization prevents crosstalk of one call into another and degradation of voice quality or dropped calls from signal interference. T1 PRI is the highest call quality you can get by virtue of it's strict design characteristics.
If T1 PRI is the gold standard, then why have anything else? The reason is cost savings. Remember that unused T1 PRI channels do nothing although you are paying for the entire line by the month. The other cost that might be saved is having a separate telephone network with its own wiring plus an incompatible computer network. Combining the two networks is called convergence. The slower speed telephone network is converted to a format compatible with the higher speed computer network so all of the signals can travel together on one set of wire. That format is called Ethernet or IP for Internet Protocol. As you might guess, IP is also the standard used on the actual Internet.
The conversion from regular telephone signals to VoIP can be made by an IP PBX system or can be done right inside the phone set, often called an IP phone or SIP phone. SIP is a particular protocol used by many but not all IP PBX systems.
Notice that in a VoIP phone system the PBX has become an IP PBX. Some systems use other terminology such as voice gateway or soft switch. When VoIP calls connect to standard telephones, those called need to be terminated to the PSTN or Public Switched Telephone Network. The termination can be analog phone lines or even T1 PRI digital phone lines connected to electronic interface cards that plug into the IP PBX. Two IP PBX systems are connected together using an unchannelized T1 line.
Just to mix things up a little further, legacy PBX systems can also be converged onto computer networks by adding a IP conversion card to the PBX system. The resulting IP data stream can be conveyed on a unchannelized T1 line across long distances or connected to a LAN router or switch. Again, once converted from TDM to IP format, voice quality is no longer automatic but must be engineering into the local or wide area network