Why you don't get 56k. by Katie Barns for
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
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
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
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.
Central office switch-ups
Connections between local central-office switches can sometimes be a problem. Old
equipment may require analog termination, resulting in an analog-to-digital conversion as
the call goes through to the next switch. If a local call to your ISP gets routed through
these "partially analog" switches, you'll lose 56k capability. If that happens,
the telephone company may be able to tell you which type of switches your call gets routed
through on the way to your ISP. (One such problem switch is the AT&T 1AESS. However,
the AT&T 5ESS and the Northern Telecom DMS-100/500 switches should both work fine.)
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
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
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.
Connect for success
Your ISP must have a fully digital connection to the telephone company's central office
for 56k technology to work. This means that the ISP must have either ISDN
or a T1 or T3 line.