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.

When you make a long distance call, you can be sure it's traveling through digital switches only. The long distance network in the United States is, thankfully, a fully digital system. Transcontinental calls, however, use digital ADPCM encoding for voice compression, which doesn't work with 56k PCM encoding. You won't be able to get the higher throughput rates when calling another continent.

Trouble in the office-to-home commute
A number of problems can occur as data makes its way from the local central office to your home. Older telephone lines connect directly to the switch at the central office, and newer lines go through a digital loop carrier (DLC). These devices can combine 96 separate lines into one before they reach the central-office switch. By using DLC, the telephone company doesn't have to bury as much expensive copper wire, which saves money and increases connection reliability. But DLCs can wreak havoc with 56k. If the DLC is digitally connected to the switch, no problem--but if it uses a universal connection, an analog-to-digital conversion will occur, rendering your modem's 56-kbps capabilities useless.

There may also be a pad between you and the central office. A pad balances the volume on both ends of the line when you make a call. If the pad occurs before the signal is converted to analog, you'll see only a slight degradation in 56k performance. But if you encounter an analog pad between the central office and your home, up crops another analog-to-digital conversion to sabotage your 56k connection. In our tests, we introduced digital pad impairments to see how each 56-kbps modem handled them; for the most part, the products did not find them too bothersome.

Some local lines also run through an amplifier called a load coil to boost the signal rates across longer distances. Load coils cause some signal distortion and will detrimentally affect your modem's 56k throughput potential. During testing, we also tested a long local loop containing a load coil. Many of the products fell back to 28-kbps rates, while others handled this impairment without great performance loss.