How 56K Modems Work
- and Why They Sometimes Don't -
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- How do analog modems work?
- Why are analog modems limited to 33.6Kbps?
- How and what were 56K modems designed to do?
- What factors limit 56K connectivity?
- What kinds of rates are people getting from 56K modems?
- What are some basic things I can do to improve the chances of good 56K connectivity?
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| 1. How do analog modems work? |
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Traditional phone lines were designed to carry voices, not data. They're analog. Traditional
modems were designed, therefore, to convert the data that computers understand into audio tones
that can be safely carried across phones. They are also analog. When sending, data is translated
into audio tones through a process called modulation. When receiving, audio tones are converted
back into data through a process called demodulation. A modem is a modulator-demodulator which
is the source of the name.
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| 2. Why are analog modems limited to 33.6Kbps? |
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Analog modems are optimized for a situation in which both ends connect via analog lines, despite
the fact that portions of the telephone network are now digital. They're extremely robust, but
cannot make the most of the bandwidth that becomes available when one end of the connection
is completely digital. This is because they were designed under the assumption that both ends
of the connection suffer impairment due to quantization noise introduced by analog-to-digital
converters.
When analog information is translated into binary digits the waveforms that are generated
are continuous, although binary numbers are by nature discrete. When these digits are reconstructed
at the other end they can only approximate the original analog waveform. The difference between
the original waveform and the reconstructed quantized waveform is called quantization noise.
Quantization noise contributes to the ratio of signal-to-noise and limits analog modems to
a top speed of 33.6Kbps.
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| 3. How and what were 56K modems designed to do? |
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56K modems were designed to take advantage of the digital portions of the phone network and
of the fact that while analog-to-digital conversions are affected by quantization noise, digital-to-analog
conversions are not. This also explains why 56K modems are capable of delivering higher speed
on the downstream (receive) channel than on the upstream (send) channel. The downstream conversion
is a digital-to-analog one. The upstream conversion is analog-to-digital and thus limited by
quantization noise. In an ideal universe these modems deliver, but the universe isn't ideal
and a number of factors conspire to make this type of connectivity the joy we all know and love.
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| 4. What factors limit 56K connectivity? |
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FCC Regulations
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Sending a signal down a telephone wire requires electrical power. The FCC limits the
amount of power that phone companies can use to do this in order to prevent crosstalk.
Crosstalk is what you hear when you're talking on the phone and simultaneously hearing
someone else's phone conversation in the vague distance. The more electrical power you
apply to a telephone wire the more likely that crosstalk will occur. This cap on signal
strength limits data throughput to 53Kbps.
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PBX Telephone Systems
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Digital PBX telephone systems, which are used in many offices, also limit throughput.
If you have to dial 9 to get an outside line the phone system is a digital PBX. These
systems incorporate code that performs an analog-to-digital conversion so that calls can
be stored digitally on magnetic media, such as hard disks. This system provides features
like employee extensions and call forwarding, but the analog-to-digital conversion is
a limiting factor. On these systems throughput with a 56K modem is limited to a maximum
of about 35Kbps.
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Line Noise
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Line noise is also still a problem. Digital lines don't usually suffer from noise problems,
but analog wires do. Although the phone network is increasingly digital, the portion of
the network which connects homes to the CO is the slowest portion of the network to convert
to digital and will probably remain analog for quite awhile. This means that while someone
might get great 56K connectivity from their (non-PBX using) office, they're less likely
to be as successful when dialing from home. If there is buzzing or static on your telephone
headset, there's line noise. Caller ID, answering machines, and cordless phones add even
more noise to the lines even when they're in a different room. Line noise can also be
caused by nearby power lines or other environmental or structural factors. Splitters,
phone line extenders, surge supressors and other devices that prevent the modem from being
plugged directly into the phone jack in the wall also cause noise and other errors.
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CO Switches & Other Teleco Equipment Issues
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Connections between local CO switches can also be a problem since older equipment tends
to require analog termination, resulting in an analog-to-digital conversion as the call
goes through to the next switch. If the phone call made to an ISP gets routed through
these "partially analog" switches, 56K capability is lost.
Problems can also occur as data travels from the local CO to the home. Older phone
lines connect directly to the switch at the CO while newer lines go through a digital
loop carrier (DLC), a device which can combine 96 seperate lines into one before they
reach the CO switch. The use of DLC means the telephone company buries less expensive
copper wire thus increasing connection reliability and reducing expense. If DLC is digitally
connected to the switch it doesn't cause problems for 56K connectivity, but if a universal
connection is used there will be an analog-to-digital conversion we're back to square
one.
There can also be a pad between a home and the CO. This equipment balances volume on
both ends of the line when calls are made. When these sit between the home and CO another
analog-to-digital conversion happens and performance is downgraded.
Some local lines run through an amplifier called a load coil which boosts the signal
rates across longer distances. These cause some signal distortion which affects 56K
throughput potential.
The bottom line is that if there is more than one analog-to-digital conversion in a
local phone loop it is impossible to receive 56K connectivity no matter what you do.
It's a lot like the game you might've played in elementary school where one person whispers
something into the ear of the person next to them and so on until the end of the loop
at which point the last person to get the message says it out loud and everyone giggles
because it's so much different than the original one. In this case, think of it as the
data that is an e-mail message being whispered into a whole series of ears, some of
which belong to people who speak Hungarian, some of which belong to people who speak
English. In between these people who speak these different languages are people who
speak both languages and are translating the message back and forth from Hungarian to
English. If you can imagine what your original message might look like at the end of
such a chain of babble you'll have a better sense of why data transmission flakes out
if more than one conversion has to be made. |
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| 5. What kinds of rates are people getting from 56K modems? |
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Here are some estimates of the rates of what's happening as of September '99 with 56K modem
connections:
About 60-70% of modem owners are getting rates higher than 33.6K which means, obviously,
that 30-40% get rates of 33.6k or less. Of those getting rates higher than 33.6k, about 75%
get rates between 44-49.3k. 10% get rates of 50k or higher. About 15% get rates between 33.6-38.6k.
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| 6. What are some basic things I can do to improve the chances of good
56K connectivity? |
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The frustrating thing about 56K connectivity is the fact that so much of what can impact the
ability to achieve it are factors that an individual has no control over. Here are the things
that can be controlled by an individual:
- Be sure the phone cable connecting the modem is plugged directly into the jack with no
splitters or other devices between the modem and the jack.
- Be sure that the latest drivers (PC) or scripts (Macs) are installed.
- Be sure the modem has the latest firmware.
- Don't use a Winmodem or other 56K modem with known issues.
We have some more advanced 56K Modem Troubleshooting if you
would like to do some more involved troubleshooting.
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