What causes the 56kbps limit on dial-up internet connections?

[[email protected]]

Hi:

What are the physical* reasons dial-up speeds cannot go above 56kbps?
One factor is that the phone line cuts of frequencies below 300 Hz and
above 3,000 Hz. What are the other physical causes of this limit?
Would this limit still exist if it weren't for the aforementioned
frequency-cutoffs?

bps = baud X number of bits per baud.

If only 1 baud is used, what is the maximum-bits-per-baud that can be
used on a phone line without the frequency of the analog electric
signal exceeding 3,000 Hz?

*NOTE: By "physical", I am referring to causes not associated with
legal regulations - such as limitations place by the FCC and
governments.


Thanks,

Radium

 

[Jeff Liebermann]

[email protected] hath wroth:

What are the physical* reasons dial-up speeds cannot go above 56kbps?
One factor is that the phone line cuts of frequencies below 300 Hz and
above 3,000 Hz. What are the other physical causes of this limit?
Would this limit still exist if it weren't for the aforementioned
frequency-cutoffs?

The speed limit is the point where the error rate drops to the point
where communications is futile within the audio bandwidth. The V.90
spec allows 56Kbits/sec download, but only 33.6Kbits/sec upload.
(V.92 does 48Kbit/sec upload).

bps = baud X number of bits per baud.

Yep. It would be 64Kbits/sec were it not for telco bit-robbing, where
every 6th frame is stolen for signaling. The audio channel can handle
the bandwidth, but when it hits the DS0 digital channels, bit robbing
limits the bandwidth to 56K. Therefore, you only get:
8Kbits/sec * 7 bits = 56 Kbits/sec.

What do you mean by a "band"? Are you perhaps thinking of bits per
baud?

If only 1 baud is used, what is the maximum-bits-per-baud that can be
used on a phone line without the frequency of the analog electric
signal exceeding 3,000 Hz?

*NOTE: By "physical", I am referring to causes not associated with
legal regulations - such as limitations place by the FCC and
governments.

You could go faster if you don't care about telco channel crosstalk or
hitting the line protectors thus creating distortion.

You may find the following of interest:

Radium

Oh, it's you again.

 

[[email protected]]

[email protected] hath wroth:

The speed limit is the point where the error rate drops to the point
where communications is futile within the audio bandwidth.

Usually, how much bps until this limit is reached?

The V.90
spec allows 56Kbits/sec download, but only 33.6Kbits/sec upload.
(V.92 does 48Kbit/sec upload).
Yup.

Yep. It would be 64Kbits/sec were it not for telco bit-robbing, where
every 6th frame is stolen for signaling. The audio channel can handle
the bandwidth, but when it hits the DS0 digital channels, bit robbing
limits the bandwidth to 56K. Therefore, you only get:
8Kbits/sec * 7 bits = 56 Kbits/sec.

I don't understand how you got that equation. Please clarify.

What do you mean by a "band"?

I never used the word "band".

Are you perhaps thinking of bits per
baud?
Yes.

You could go faster if you don't care about telco channel crosstalk or
hitting the line protectors thus creating distortion.

Okay, but if only 1 baud is used what is the maximum-bits-per-baud
that can be used on a phone line?

You may find the following of interest:
<http://www.LearnByDestroying.com/aty11/aty11.htm>
Hmmm... It's 7 years old. Maybe I should update it.

Thanks for the link.

 

[Phil Allison]

"Jeff Liebermann"


** The limit is set by the restricted bandwidth AND the available to noise
ratio of a telephone network voice circuit.

A guy named Shannon work out the maths, loooong ago.

It kinda like the inverse of the Nyquist sampling theorem.

http://en.wikipedia.org/wiki/Shannon–Hartley_theorem

The formula for when the s/n ratio >> unity is:

Bits/sec = 0.33 x bandwidth (Hz) x s/n (in dB)

For a voice circuit that becomes:

0.33 x 3000 x 45 ( typical good line)

= 45,000 b/s


The actual s/n ratio on a given voice circuit, which includes analogue
twisted wire lines and digital signals over co-ax or fibre varies - so
56K modems do an automatic line test to see just what capacity is available
and adjust themselves to that.


....... Phil

 

[Phil Allison]

"Phil Allison"


** My previous post was to:

" [email protected] "


Whose post does not show on my server.




......... Phil

 

[[email protected]]

** The limit is set by the restricted bandwidth AND the available to noise
ratio of a telephone network voice circuit.

A guy named Shannon work out the maths, loooong ago.

It kinda like the inverse of the Nyquist sampling theorem.

http://en.wikipedia.org/wiki/Shannon–Hartley_theorem
Okay.

The formula for when the s/n ratio >> unity is:

Bits/sec = 0.33 x bandwidth (Hz) x s/n (in dB)

What is the "0.33"?

For a voice circuit that becomes:

0.33 x 3000 x 45 ( typical good line)

= 45,000 b/s


The actual s/n ratio on a given voice circuit, which includes analogue
twisted wire lines and digital signals over co-ax or fibre varies - so
56K modems do an automatic line test to see just what capacity is available
and adjust themselves to that.

If the max a voice circuit can take is 45 kbps, then how can dial-up
speeds get above that?

Also if frequencies below 300 Hz and above 3,000 Hz are cut-off,
wouldn't the bandwidth of a phone line be 2,700 Hz?

 

[Eeyore]

Hi:

What are the physical* reasons dial-up speeds cannot go above 56kbps?
One factor is that the phone line cuts of frequencies below 300 Hz and
above 3,000 Hz. What are the other physical causes of this limit?

Noise performance (signal to noise ratio).

Would this limit still exist if it weren't for the aforementioned
frequency-cutoffs?

Not the same limit for sure.

Graham

 

[Eeyore]

What is the "0.33"?

I dare say if you could be bothered to read the link supplied or google a bit
yourself for sampling theory you'd find a more complete answer.

Your desire for totally 'potted answers' that onvolve no effort on your poart is
one of the things that annoys so much. If you were genuinely interested you'd be
prepared to do some 'homework'

If the max a voice circuit can take is 45 kbps, then how can dial-up
speeds get above that?

Are you really that STUPID ?

It's bloody onvious. The bandwidth is fixed and the 0.33 is fixed so what's the
remaining variable ? Hint - PA used the word *typical* to describe it.

I suggest you stick 56k into the equation and calculate the resulting answer.

Graham

 

[Jeff Liebermann]

[email protected] hath wroth:

Usually, how much bps until this limit is reached?

Look at a communications channel this way. You can get *ANY* speed
through a bandwidth limited channel, up to a given error rate. If
your application requires a very low error rate to function, then you
have to have a good signal to noise ratio, and your thruput will be
fairly small. However, if you have a mess of forward error
correction, small packets, and a very tolerant application, you migtht
be able to squeeze some more thruput through the same channel.

I'm not going to expound on how V.90 works in detail. It gets messy
fast. There are modulation schemes for increasing the base 600 baud
modulation rate (bits per baud) to much higher bits/baud. Then add,
adaptive equalizers, echo cancellers, error detection, error
correction, data compression, etc. Anything to squeeze more thruput
into a rather ugly looking POTS line. However, that's just between
the user and the CO (central office). Once at the CO, everything gets
converted to digital and the rules change.

I don't understand how you got that equation. Please clarify.

Nope. I don't want to get into how a DS0 (digital) line work. You
can get 64Kbits/sec out of a DS0 if you can use out of band
signalling. However, if you're using in band signalling, you're stuck
with 56Kbits/sec. Even if the analog part of the puzzle can go faster
than 56Kbits/sec, the digital part at the CO will limit the speed to
56Kbits/sec.

I never used the word "band".

Sorry. There was a fly splattered on my monitor.

Okay, but if only 1 baud is used what is the maximum-bits-per-baud
that can be used on a phone line?

56Kbits/sec. The limit is NOT all from the analog part of the line.
The analog modem glop gets converted to digital at the CO and that's
limited to 56Kbits/sec. I could easily (well maybe not so easily) get
more than 56Kbit/sec thruput going between my house and the CO, but
the digital thruput at the switch will limit thruput to 56Kbits/sec.

The problem is worse when dealing with SLC (subscriber line
concentrators) where the analog to digital conversion is done outside
the CO, such as with Pair Gain. The best you can do with those is
perhaps 28.8Kbit/sec, mostly because the digital audio filter cuts off
at a much lower frequency than the filters at the CO. Since most of
the energy is in the higher frequency part of the audio spectrum, the
loss of the higher frequencies is fatal to higher speed modem
operation.

Thanks for the link.

You really should be asking in comp.dcom.modems.

 

[John Larkin]

Hi:

What are the physical* reasons dial-up speeds cannot go above 56kbps?
One factor is that the phone line cuts of frequencies below 300 Hz and
above 3,000 Hz. What are the other physical causes of this limit?
Would this limit still exist if it weren't for the aforementioned
frequency-cutoffs?

bps = baud X number of bits per baud.

If only 1 baud is used, what is the maximum-bits-per-baud that can be
used on a phone line without the frequency of the analog electric
signal exceeding 3,000 Hz?

*NOTE: By "physical", I am referring to causes not associated with
legal regulations - such as limitations place by the FCC and
governments.


Thanks,

Radium

The fact that all modern telephone exchanges digitize the voice signal
to 8 bits at a sample rate of 8 KHz, which means that the digital
paths within and between phone offices absolutely cannot transport
more than 64 KBPS. Some overheads, nonlinearities, and error
correction things resulted in the 56K download number. Upload is
burdened by some additional issues.

If the phone system were still truly analog, the number of bps you
could jam into 3 KHz of bandwidth would depend on the s/n ratio, per
Shannon, and could in some cases be a lot higher.

John

 

[Jamie]

Hi:

What are the physical* reasons dial-up speeds cannot go above 56kbps?
One factor is that the phone line cuts of frequencies below 300 Hz and
above 3,000 Hz. What are the other physical causes of this limit?
Would this limit still exist if it weren't for the aforementioned
frequency-cutoffs?

bps = baud X number of bits per baud.

If only 1 baud is used, what is the maximum-bits-per-baud that can be
used on a phone line without the frequency of the analog electric
signal exceeding 3,000 Hz?

*NOTE: By "physical", I am referring to causes not associated with
legal regulations - such as limitations place by the FCC and
governments.


Thanks,

Radium

You see, the internet is like a bunch of pipes with water running
through it, think of water as your information.
Plumbing is old, Pipes are small, You can only get some much through
those pipes!
It's true!
One of our Senators even said so!

 

[Joerg]

The fact that all modern telephone exchanges digitize the voice signal
to 8 bits at a sample rate of 8 KHz, which means that the digital
paths within and between phone offices absolutely cannot transport
more than 64 KBPS. Some overheads, nonlinearities, and error
correction things resulted in the 56K download number. Upload is
burdened by some additional issues.

In reality I have rarely ever reached >30K in situations where I had to
use dial-up. Hotels and such.

If the phone system were still truly analog, the number of bps you
could jam into 3 KHz of bandwidth would depend on the s/n ratio, per
Shannon, and could in some cases be a lot higher.

Lately our phone line here has a constant and very audible noise hash on
them. Maybe they don't want folks using dial-up and buy the DSL service ;-)

 

[colin]

Hi:

What are the physical* reasons dial-up speeds cannot go above 56kbps?
One factor is that the phone line cuts of frequencies below 300 Hz and
above 3,000 Hz. What are the other physical causes of this limit?
Would this limit still exist if it weren't for the aforementioned
frequency-cutoffs?

bps = baud X number of bits per baud.

If only 1 baud is used, what is the maximum-bits-per-baud that can be
used on a phone line without the frequency of the analog electric
signal exceeding 3,000 Hz?

*NOTE: By "physical", I am referring to causes not associated with
legal regulations - such as limitations place by the FCC and
governments.

The phone lines are digitised nowadays,
the rate at wich they digitise them is I gues about 56kbps
therefore no encoding scheme is going to get round this.
the modems carefully tune the signal levels for each
encoded state so that it matches the digitising of the phone line
to as close to 1:1 as possible.

Colin =^.^=

 

[Phil Allison]

"Jeff Liebermann"

Look at a communications channel this way. You can get *ANY* speed
through a bandwidth limited channel, up to a given error rate.

** Bollocks.

You need to read up on Shannon's theorem - fool.

I'm not going to expound on how V.90 works in detail.

** Irrelevant to the inherent bit rate capacity of a voice communications
line.

Sorry. There was a fly splattered on my monitor.

** Fly spots all over you as well.

56Kbits/sec. The limit is NOT all from the analog part of the line.

** In truth - it is.

From the user's position a dial up voice circuit IS analogue.




........ Phil

 

[JosephKK]

[email protected] [email protected] posted to
sci.electronics.design:

Hi:

What are the physical* reasons dial-up speeds cannot go above
56kbps? One factor is that the phone line cuts of frequencies below
300 Hz and above 3,000 Hz. What are the other physical causes of
this limit? Would this limit still exist if it weren't for the
aforementioned frequency-cutoffs?

bps = baud X number of bits per baud.

If only 1 baud is used, what is the maximum-bits-per-baud that can
be used on a phone line without the frequency of the analog electric
signal exceeding 3,000 Hz?

*NOTE: By "physical", I am referring to causes not associated with
legal regulations - such as limitations place by the FCC and
governments.


Thanks,

Radium

The number 1 reason is end equipment. If the end equipment does not
support any better than POTS that is all you get. See also
Shannon's theorem, which relates usable bandwidth, signal to noise
ratio, and datarate. If, as usual, the end equipment is the limiting
factor on a twisted pair line then that is the limit.
If you are looking for the limits on just a Cat 3 copper twisted pair,
then you are looking at something on the order of 30 MB/(second*mile)
based on the equipment that i have seen for sale.

 

[neon]

There is a limit more inportant is YOUR limit set by your hardware and phone company. the phone company will tell you that the lines were set up for voice only never was set up for data. Basicly you get what you hardware is capable in the long run. not what you buy your modem for v90 or v92

 

[Joel Kolstad]

In reality I have rarely ever reached >30K in situations where I had to use
dial-up. Hotels and such.

There are a lot of old PBXes out there that really mess things up.

On good "direct line to the CO" connections, back when I used dial-up (>5
years ago now) I would routinely get ~45-48kbps. The only time I got the
fabled 53kbps was with an internal (modern ) PBX connection to an inside
modem pool .

45-48kbps is really quite good given the transmission medium used, I think.

Lately our phone line here has a constant and very audible noise hash on
them. Maybe they don't want folks using dial-up and buy the DSL service ;-)

A lot of phone companies officially only support something like 14.4kbps and
will do absolutely nothing to improve line quality if they can demonstrate a
successful 14.4kbps connection. I can't imagine that part of that policy
wasn't largely influenced by marketing... (Even so, of course, most lines
will do much better.)

On the other hand, I'm not sure there ever was a means of determining that
voice quality on a POTS line was "inadequate," was there?

I read an article the other day about some ham who was tracking down huge HF
band interference at night. He quickly figured out that it was a bad ballast
on a street lamp, but the power company made him go around and note all the
bad lights he could find in something like a 2-3 mile radius. Supposedly they
were "willing" to do this themselves, but the emissions/interference guys only
worked regular 9-5 hours and were "not allowed" to work overtime... such as at
night, when the street lamps would actually be *on*. What nonsense that is...
I have to imagine there's a union in there somewhere... what hiring manager
would ever hire someone to mitigate interference from streetlamps with the
understanding that they'd never have to work outside of 9-5 unless overtime
was paid?

---Joel

 

[Robert Baer]

[email protected] hath wroth:




The speed limit is the point where the error rate drops to the point
where communications is futile within the audio bandwidth. The V.90
spec allows 56Kbits/sec download, but only 33.6Kbits/sec upload.
(V.92 does 48Kbit/sec upload).




Yep. It would be 64Kbits/sec were it not for telco bit-robbing, where
every 6th frame is stolen for signaling. The audio channel can handle
the bandwidth, but when it hits the DS0 digital channels, bit robbing
limits the bandwidth to 56K. Therefore, you only get:
8Kbits/sec * 7 bits = 56 Kbits/sec.

What do you mean by a "band"? Are you perhaps thinking of bits per
baud?




You could go faster if you don't care about telco channel crosstalk or
hitting the line protectors thus creating distortion.

You may find the following of interest:
<http://www.LearnByDestroying.com/aty11/aty11.htm>
Hmmm... It's 7 years old. Maybe I should update it.




Oh, it's you again.

Theoretical as well as calculated limits may be nice to dream about,
but the fact is that almost every telco throttles the speed to about
48K, making them liars when they say that they support 56K.

 

[Don Bowey]

Theoretical as well as calculated limits may be nice to dream about,
but the fact is that almost every telco throttles the speed to about
48K, making them liars when they say that they support 56K.

What telco says they support 56 kbit/s? I think you may be the liar.

 

[Jeff Liebermann]

Theoretical as well as calculated limits may be nice to dream about,
but the fact is that almost every telco throttles the speed to about
48K, making them liars when they say that they support 56K.

Everyone lies, but that's ok because nobody listens.

I routinely get 49.3Kbits/sec on my dialup connections. Sometimes, it
goes to 50.3Kbits/sec. I guess the mythical throttle doesn't apply to
me.

The actual maximum speed is about 53Kbits/sec. The FCC limits the
line levels on dialup lines to prevent crosstalk. Without sufficient
line level, the S/N ratio drops, which causes the error rate to climb.
The modem compensates by reducing its speed.

Incidentally, the local telco only "supports" 1200 or 9600 (v.32)
baud, depending on whom I ask. Just try calling 611 with a modem
problem and see what happens.