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Why is 56k the quickest dialup modem velocity?

Why is 56k the quickest dialup modem velocity?

2023-11-25 08:28:11

by Alex Freeman

If you happen to’ve ever had dialup web service, or nonetheless do, or simply know somebody that does, you could have in all probability heard phrases like “56k modem”. “56k” has grow to be nearly synonymous with dialup Web entry. But it surely’s such an arbitrary quantity. It isn’t divisible by ten, it is not an influence of two… so why was it chosen because the quickest dialup velocity? For the reply, we must journey again in time fairly some time.

Our guests from Google must be warned that this isn’t a “stripped down” clarification; it’s meant for comparatively technical readers. However when you actually need to know the place this magic quantity comes from, it’s essential to perceive a few of the technical background. As we will see, “56k” was not simply pulled out of a hat.

Setting the baseline

First, let’s be particular about what we imply. A “56k” (typically the ‘okay’ is capitalized and typically not; the explanations should not actually related right here) dialup modem can obtain information at as much as 56 kilobits per second. That may be a theoretical most of the {hardware}; for varied causes (and sure, we’ll get there) this velocity can by no means truly be achieved. Add speeds are significantly slower.

This was all codified in 1998 within the ITU V.90 suggestion, which merged two competing requirements (X2 from 3COM and K56flex from Rockwell Semiconductor). V.92 got here barely later, and launched varied tweaks; principally, it elevated the utmost add velocity. Any fashionable dialup modem will conform to certainly one of these requirements.

Acquired all that?

Lengthy, way back

Bur story begins a lot earlier. Way back, when the cellphone networks have been nonetheless fully analog, when the primary actual cellphone networks have been being constructed, the engineers that have been designing them confronted a choice. They wanted to find out how a lot bandwidth their networks needed to have to be able to guarantee satisfactory voice high quality. Too little bandwidth, and customers would not be capable to acknowledge the voice of the particular person on the opposite finish. However the extra bandwidth the system was specified for, the harder (and costly) the community was to construct.

Finally, they settled on roughly 3.2 kHz. The implementation for that is fairly easy: the cellphone traces in particular person homes run to bins owned by the cellphone firm, which move all voice visitors by a lowpass filter. This filter has a passband of three.2 kHz, and a stopband beginning at 4 kHz. Your complete sign at 3.2 kHz and beneath is preserved, every little thing above 4 kHz is attenuated closely, and as for the frequencies in between, nicely, we simply don’t be concerned about these. Downside solved.


Quick-forward to 1962. Ma Bell begins utilizing T-1s to attach its switching facilities. T-1s are a digital communications hyperlink, not analog. Your complete cellphone community goes digital. In a contemporary cellphone community, the one analog communication is between a person home’s cellphone line and the cellphone firm’s field.

T-1s are a specific type of DS-1 (Digital Sign 1). They implement the DS-1 protocol, with an AMI line code (alternate mark inversion; a “0” is 0 volts, “1”s alternate between logic excessive and logic low), over twisted pair cable, for a distance of as much as 6000 toes (very roughly 1 mile). Now that we have gotten all exact, I’ll play a bit free with terminology and name this “a DS-1” as a result of it makes what follows simpler to grasp. And it is very almost appropriate.

A DS-1 is made by multiplexing 24 DS-0 streams. Every DS-0 corresponds to a single cellphone line; that’s, a typical home will get one DS-0. Why 24 of them in every DS-1? No thought. Almost certainly that was that was the utmost attainable with out overstressing the gear that was out there on the time. Probably the most comparable European commonplace, E-1, multiplexes 32 channels collectively (1 is used for inside signaling functions).

In an effort to get your voice information onto this digital community, it should first be digitized. So now as an alternative of simply going by a lowpass filter, it goes by a lowpass filter, then is sampled, after which is run into an analog-to-digital converter that assigns a numerical worth to the amplitude of the sign on the on the spot it’s sampled. Easy, proper?

As we have mentioned, the lowpass filter cuts off all frequencies above 4 kHz. Which means that the filtered sign might be sampled at 8 kHz (which is to say, it’s sampled 8000 occasions per second) with no constancy loss. That is the Nyquist Fee; any sign sampled at a charge equal to twice its highest frequency element might be fully reconstructed. So now we’ve got turned our analog sign right into a discrete-time sign working alongside at 8000 samples/second.

Subsequent we need to quantize our sign in order that we’ve got a purely digital sign. For this, an 8-bit analog-to-digital converter (ADC) is used. Right here there’s some constancy loss, however not sufficient to be detectable to human ears. Constancy could possibly be improved by utilizing, say, a 16-bit ADC. However that might value extra to fabricate (on the time, a lot extra) , and would imply that each one the opposite {hardware} needs to be constructed to transmit twice as a lot information. So 8-bit it’s.

For these of you retaining rating at house, meaning we now have a sign that transmits information 8000 occasions every second, and transmits 8 bits of information for every of these samples. 8 bits/pattern * 8000 samples/second yields 64 kbit/s (64 kilobits per second). So why cannot dialup modems go as much as 64 kbit/s? The brief reply could be “historic causes”.

We’re with the band

The extra full reply is “management information”. Most communications networks must transmit two varieties of data. The primary is information; for cellphone networks, that is voice information. The second is signaling data, the management information that lets the community talk standing data with itself. There are two methods to ship management information: “in band” (the place it’s packed in with the information) and “out of band” (the place it will get its personal separate channel, probably in a wholly totally different bodily medium).

After we have created the 64 kbit/s information stream that fills up our DS-0, it must be multiplexed right into a DS-1 for longer-distance transmission to the cellphone firm’s switching station. And the DS-1 has management information to hold. The Bell engineers determined to ship that data in band.

However how will we go about packing this information in? A method is to “steal” some information bits and substitute them with management information. And that’s precisely what the Bell engineers did.

Petty theft

A DS-1 transmits information in frames. Every body accommodates one full pattern from every DS-0, plus a framing bit. Since there are 24 DS-0s multiplexed to make every DS-1, and a single pattern from a DS-0 is 8 bits of information. 24 * 8 makes 192 bits. Toss within the framing bit and you’ve got 193 bits.

So we’ve got our 193-bit body, and we need to pack some management information into it. How? Each 6 frames, the least vital bit of every voice channel is “stolen”. The info bit is thrown out, and management information is shipped as an alternative. As a result of it’s the least vital bit that’s misplaced, this very slight alteration to the voice sign is just not detectable by human ears. All people’s completely happy.

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Enter the modem

That system labored high quality till folks wished to transmit extra than simply voice on these networks. If you happen to’re sending voice, dropping the least vital bit of each sixth byte is not any massive deal. If you’re sending information, it’s a very massive deal. Neither modem has any technique to know which bytes will probably be affected. Perhaps this pattern, possibly the subsequent one. It is a drawback.

The answer is easy sufficient: assume you might be all the time going to lose that least vital bit. Assume which you can solely reliably ship or obtain 7 bits with every pattern. When the receiver will get its information, it merely throws away the least vital bit of each byte. That yields a most information charge of seven bit/pattern * 8000 samples/s, or 56 kbit/s.

And that is the place the time period “56k” comes from.

So why cannot I truly join at 56 kbit/s?

Anybody that has ever used a dialup modem is aware of full nicely that they do not truly get to attach at that velocity, although. And that their connection velocity varies every time they dial in. There are two components at work right here.

The primary is the FCC. If you’re in the US, the FCC locations a restriction on the ability output of units linked to the cellphone community. The result’s that you’ll by no means be capable to join at a velocity quicker than 53.3 kbit/s.

The second is the general complexity of the cellphone community. 56 kbit/s (or 53.3 kbit/s) requires superb working situations, as it’s actually working past the paramaters of what the cellphone community is required to be able to. Working at these speeds requires that there solely be one ADC between the person and their ISP (which isn’t assured to be true, however usually is), and that the copper wiring within the person’s “native loop” have superb electrical properties. A part of the dialup course of that’s used to provoke a connection is an analysis of the general high quality of the connection; whether it is decided to be missing, the modem will mechanically drop right down to a decrease information charge.

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Alex Freeman is the creator of, and has totally an excessive amount of free time.

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