Naked Science Forum
Non Life Sciences => Geek Speak => Topic started by: Chemistry4me on 06/03/2009 23:38:52
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What is the difference between broadband and dial-up? What makes one faster than the other? And why are some broadbands 'faster' than other broadbands (if you get what I mean)? [:)]
How can we improve the speed even more? [:-\]
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Because telephone lines were only originally designed to handle voice, and because there is a trade-off between the quality of the voice signal and expense, they have a limited bandwidth. Dial-up services send digital data over this limited bandwidth voice link by modulating an audio signal down the telephone lines, just like voice, and early modems (modulator-demodulator) actually used an Acoustic Coupler to connect the telephone hand-set to the modem. This consisted of a microphone to receive data from the hand-set ear-piece and a loudspeaker to send data via the hand-set mouth-piece. Because the bandwidth is so limited though, the data rates are relatively low.
However, it was eventually found that there were ways of increasing the bandwidth of those same telephone lines, by using new equipment at each end of the line combined with new data communications techniques and protocols to deal with a fairly wide range of line quality. This is what has become known as Broadband.
The actual speed you can get from a broadband connection depends on a number of factors. Probably the most important of these is the quality of the signal between your home and the exchange; this limits the maximum speed of the broadband link. The other factors are mostly to do with the cost limited bandwidth infrastructure constraints between exchanges and the bandwidth that each ISP can handle; fibre-optic links between exchanges can boost the infrastructure bandwidth between them, but it costs money not only to lay the fibre, but also for the equipment that is to be connected to the fibre at each end. Similarly, each ISP needs to invest in the equipment to handle the bandwidth they've bought from the infrastructure providers.
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Thanks for your time and explanation LeeE [:)]
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The basic transmission system over dial up lines is 64Kb/s it is amazing that modern modems can achieve a data rate of 56Kb/s over what are dedicated speech circuits.
When I first started working with line communications modems used 1200 audio channels modulated a 32 different levels to achieve 9600Kb/s and this was considered the ultimate limit now by incorporating high performance micro processors and subtle software this has been extended to 56Kb/s.
Now we throw these modems in the trash bin because broadband has become readily available on only wish I had Had one 40 years ago!
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Yes - dialup uses the existing voice infrastructure, which has for a couple of decades now been digital sampling at 8-bit/8kHz therefore with an absolute maximum information-carrying capacity of 64kbps (kilo-bits-per-second). It is quite remarkable that modern modulation schemes managed to achieve 40-56kbps even down the analog "last mile" of cable.
Dialup uses frequencies (in the telephone line) below 4kHz.
"Broadband" or more specifically ADSL (broadband over a normal telephone wire) uses frequencies from 8kHz up to around 1MHz. This is a whole lot more "bandwidth" which means you can get much more data throughput than on dialup.
30 years ago, ADSL would have been considered impossible - there was no viable way to spread the information efficiently over all the frequencies available, and the nature of the wiring would introduce all sorts of echoes into any signal carried using the technology of the day.
'Bog standard' twin-pair telephone cable, complete with ad-hoc rats-nest junctions in street cabinets is far from the ideal arrangement for carrying high frequency signals. But it is relatively cheap, and is already in place!
For both the higher dialup speeds, and ADSL (as well as digital terrestrial TV and DAB radio) we have to thank the invention of OFDM (orthogonal frequency division multiplexing) - a modern modulation method based on Fourier transforms.
This technique uses a very large number of carriers (in the case of ADSL, at 4kHz intervals from 8kHz upwards), each (phase and amplitude) modulated at a very low data rate, known as the symbol rate. Because the symbol rate is surprisingly low, the system is extremely tolerant of echos and reflections in the cable or transmission path.
For any given transmission channel (limited by frequency range and noise) OFDM allows data transmission very close to the theoretical maximum possible throughput. Neat.
As you get further from the telephone exchange, the absorbtion (or strictly attenuation) in the cable increases, especially at high frequencies. Consequently the modem system is forced to reduce the "precision" (or information-carrying capacity) of the modulation of the carriers towards the 1MHz top-end of the range, or even abandon those carriers altogether. The total available throughput of your telephone line thus decreases, and your maximum download speeds are correspondingly reduced.
It's worth noting that the touted ADSL 2/2+ systems mostly just allow use of higher frequencies, so will only allow faster datarates for people already getting close to the existing 5-8Mbps maximum, ie those for whom the telephone line itself is not already the limiting factor.
French mathematician Joseph Fourier (the 250th anniversary of his birth comes around in 2018) would be most impressed.
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The basic transmission system over dial up lines is 64Kb/s it is amazing that modern modems can achieve a data rate of 56Kb/s over what are dedicated speech circuits.
Isn't 56Kb/s slower than 64Kb/s? [???]
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Best of my knowledge broad band is much faster then dial up, 64Kbps is faster,but using dial up connection is hell there is lot of problem, 56kbps is downloading speed or browsing speed? [???], now broadband speed starts from 120KBPS, if you have broad band, simultaneously u can attend telephone call and browse too, its not possible in dialup.
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Chemistry4me
Of course 56Kb/s is less than 64Kb/s but you must bear in mind that the dial up modem can't feed digital directly into the line it must feed in a cunningly generated audio signal that gets converted to a 64kb/s data stream as soon as it gets to the exchange.
At the receiving exchange this is converted back into an audio stream and sent on to the receiving modem which then converts it back to a digital stream.
This requires some very clever hardware and software.
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At the receiving exchange this is converted back into an audio stream and sent on to the receiving modem which then converts it back to a digital stream.
This requires some very clever hardware and software.
Actually, I think one of the tricks with "56k" modems is that it's all-digital at the ISP end of the chain.
That's one reason why they can get 46-56kbps downstream (it's digitally pure to start with) but only c.33kbps upstream from the customer (whose modem is asynchronous to the exchange's digital sampling)
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What I had in mind was direct modem to modem communication and for which you say only 33Kb/s is possible which I find unsurprising.
My main use of modems was for the transmission of wireFax pictures before the days of ISP,s.
My present use of a modem is one rated at 28Kb/s for Fax that works fine.
PS I used to send pictures to America dial-up where the trick was to add 73 to the end of the number you dialed to make sure you got a cable link not a satellite
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Thanks for that fellas [:)]