For all the buzz about how 56k will change your life, a lot can go wrong. Even the technology's name is little more than wishful thinking: you won't achieve 56 kbps, even under the best conditions, for technological and bureaucratic reasons. To help you understand why, we've assembled a list of all the potential barriers between you and that magic number. To understand some of these roadblocks, however, you need a handle on how 56k technology actually works.
Roadblocks on the way to 56k
All 56k specifications used today take advantage of nuances in the way the phone system is designed. In a standard call between two modems, your data must be translated into analog "tones" before it can be transmitted across the telephone network. This translation is called the digital-to-analog conversion. Once your data reaches a telephone company's central office, it's translated back to digital form by a coder/decoder (codec) for transmission across the phone company's digital backbone. Unfortunately, because the telephone network contains some random noise, the analog-to-digital conversion is only an approximation of the original digital signal. To ensure that data remains readable despite the effects of this quantization noise, transmission rates are currently limited to about 53 kbps.
However, because most ISPs connect directly to the phone company's digital backbone using routers, data coming from an ISP never need undergo an analog-to-digital conversion. Instead, the data can be encoded using pulse code modulation (PCM) so that it remains entirely digital until it gets to the central office. Once it arrives, the data is put through a digital-to-analog conversion before being sent across the analog phone lines to your modem. And because digital-to-analog conversions aren't affected by quantization noise, the result--in theory, at least--is throughput as high as 56 kbps from the ISP to you.
The bad news? Anything that adds noise to the telephone line or causes an analog-to-digital conversion between your ISP and your modem lessens the transmission's performance. Worse than that, if there's nasty noise on the phone line, your only solution may be to move. Scream all you want, but the telephone company is obligated to provide you with a clean enough line to get 4,800-bps data rates only. But those aren't the sum of your potential troubles.
The FCC says, "No more than 53 kbps!"
Although your modem says "56k," you won't get throughput that fast, thanks to a speed
limit set by the Federal Communications Commission (FCC). The reason for that regulation?
Sending a signal down a telephone wire requires electrical power. But the more power you apply, the greater the chance of a problem called crosstalk. You've encountered this annoyance if you've ever heard other people's conversation during a phone call. To help prevent crosstalk, the FCC limits the amount of power that phone companies can use to send signals over the network. And this cap on signal strength limits data throughput to a maximum of 53 kbps, regardless of what your modem can actually deliver. The FCC is currently reviewing this ruling and may overturn it later this year to enable true 56-kbps modem connections.
Office PBX systems
If you have to dial 9 to get an outside line, your office uses a digital PBX telephone system, which means you also won't be able to achieve 56k rates. A PBX system incorporates a codec that performs an analog-to-digital conversion so that your calls can be stored digitally on magnetic media, such as hard disks. This system gives you some great features, such as employee extensions and call forwarding, but it also limits your 56k calls to a maximum throughput of about 35 kbps.
Noisy analog lines
Digital lines usually don't suffer from noise problems, but the analog wires between the phone company's central office and your home are a different story. If you hear buzzing or static when you listen through your phone's headset, chances are you won't be able to achieve optimum modem speed. Caller ID, answering machines, and cordless phones can add even more noise to your line. To minimize the hum, try disconnecting these types of devices one by one and listening again to determine which, if any, are the source of the problem. If this doesn't work, your line noise may be caused by nearby power lines or other environmental or structural factors. In that case, call the phone company and complain (good luck). In our testing, we injected white noise called intermodulation distortion, which is similar to what you might encounter over analog lines.
