A guide to VoIP telephony
Published: 26 Jul 2006
Alternatives to VoIP
VoIP isn’t the only game in town when it comes to Internet telephony: there’s also TDMoIP or Time Division Multiplexing over Internet Protocol. Most companies with more than two or three employees use a PBX or PABX (Private Branch eXchange or Private Automatic Branch eXchange) to handle their internal telephone system. Small PBXs can operate with multiple ordinary phone lines, but once the employee count rises and a company physically spreads out over a wide area it’s usual to switch to a Time Division Multiplexed PBX connected to one or more wideband lines. For example a T1 line has a bandwidth of 1.544Mbps and can handle up to 24 extensions and a T2 line has a bandwidth of 43.232Mbps and can handle up to 672 extensions.
Wideband lines are used to transfer the private exchange signals between localised branches -- for example, between a head office and a factory. These wideband lines are leased from the telephone company and are relatively expensive.
TDM samples the audio signals of each call for a fixed length of time. These samples are then strung together in sequence (multiplexed) for transfer via the wideband line.
The installation of a TDM PBX often represents a considerable infrastructure investment, which companies may not wish to write off by changing over to a VoIP-based system. However, it's still possible to save money on leased line costs by installing one, or more, links using TDMoIP gateways. A TDMoIP gateway is a piece of hardware that accepts the multiplexed feed out of a TDM PBX and has a broadband connection to the Internet. On the PBX side, these TDMoIP gateways can emulate all the varieties of wideband line normally used for multiplexed telecoms, from T1/E1 to T3/E3. This is often called ‘pseudowire emulation’ by the TDMoIP vendors.
Running TDM, a system that depends on accurate and dependable timing, over the Internet, which is inherently asynchronous, requires techniques that are too complex to go into here. TDMoIP gateway products are differentiated by how well they handle these timing problems. TDMoIP gateways can be purchased from Afar Communications or RAD Data Communications.
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