Friday, February 11, 2005

Why TDM Voice is Hard to Beat

In the world of telephone technology, TDM or time division multiplexing is the gold standard. It's the technology currently used for most long distance phone calls and any calls that go between different telephone company offices. Yet, TDM seems to be on the way out. Why? Let's see.

A standard phone line from your home, office or PBX system to the telephone company is an analog line. It's the same varying current signal that was invented by Alexander Graham Bell. You can have one phone or modem call per pair of telephone wires with this scheme. The phone companies originally used a scheme called FDM or frequency division multiplexing to stack multiple calls on phone line trunks or microwave relay stations. Unless everything works perfectly, these calls can interfere with each other or "crosstalk." Years ago you could often hear strange noises and even other calls in the background when you made a long distance call.

TDM solved that. The TDM or digital trunk system digitizes each phone call and gives it a private channel on a T1 or larger digital trunk line. A T1 line has 24 channels available. A larger T3 line has 672 channels. Now here's why it works so well. The digitizing process uses what is called a waveform coder that creates an exact representation of the analog phone signal in a digital format. It stays in that format so that it isn't subject to noise and distortion until it is reconverted back to analog at the far end by a waveform decoder. All along the way, the call has its own channel all to itself. Nothing interferes with it.

This carefully controlled process, which gave us crystal clear phone conversations on undersea fiber optic cables, has the seeds of its own downfall. The problem is that when you are not using all channels available, they are still being transmitted from point to point empty. Like empty boxcars on a train. There are other coding schemes that reduce bandwidth by anywhere from half to ninety percent, but the TDM channels are a fixed size and can't take advantage of newer developments.

The main competitor to TDM is VoIP. VoIP calls are transmitted as a series of packets that are sent through the network but they don't have their own channels assigned. Instead, all packets go down the same pipe. If they are sent over the Internet they may take differing routes to the same destination. VoIP calls are digitized like TDM, but can select from a variety of coders and decoders (codecs) to trade off voice quality for bandwidth savings.

VoIP can share the same network lines as computers rather than needing a specialized telephone network. That's a big potential cost savings. Another savings comes from being able to use bandwidth for data anytime it isn't needed for voice. That makes for better utilization of resources.

VoIP's challenge is to maintain voice quality while enjoying a cost savings over TDM. To do that, some of the TDM quality control measures need to be added to the VoIP system. For instance, you can select the same codec for VoIP that is used for TDM, or at least one that sounds as good to the callers. There needs to be enough bandwidth at all times for voice and data or voice quality will suffer. If bandwidth is limited, routers need to give priority to telephone calls over data transfers during periods of congestion. Private IP networks are more controllable than the Internet, although there are big advantages to be gained by using the almost universally available broadband Internet connections. This is especially true for teleworkers, mobile workers and virtual offices.

Wireline TDM is still an excellent choice for business telephone use, especially for traditional key telephone units and PBX systems that were designed for this technology. VoIP and various wireless standards are coming on strong, though, and have the opportunity to perform as well as TDM service with potential cost savings and additional features that come with new technology.

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