Dial-up modems, why only a top speed of 56K

A question occurred to me yesterday.
[li]A PSTN phone call is carried by a DS0 which is 64Kbps.[/li][li]A dial-up modem uses your phone line.[/li][li]56K is as high as a (non ISDN) dial-up modem can go. Why is that?[/li][/ol]

It’s the electronics capabilities usually and not the line, but the line can require replacement also. The telephone company equipment is only required to handle 28.8 speeds on regular telephone lines. I believe that regulation is a FCC regulation. Not only are you stuck with the 56k connections when DSL isn’t available, you my only get 28k speed and the phone company doesn’t have to upgrade your line.

IIRC, the way data is transmitted by the earliest dial-up modems is that data was turned from a digital datum to an analog sound on a phone line (MOdulating), then at the receiving end turned back into a digital datum (DEModulating).

The analog sound representing the data did so thus: when the frequency of the sound being transmitted changed, it indicated a different bit of data. That is, a 1 would be one frequency, and a 0 another. This is the simplistic approach; there are associated synchronization signals, acknowledgements, non-acknowledgements. But the basics of this method limited how much data could be transmitted, as it relied on the analog representation of these frequencies. Regular ol’ phone lines couldn’t faithfully reproduce the frequency shifts to get much faster that 300 baud (bits per second), or about 37.5 ASCII characters per second.

(That’s why in old movies, the text on computers spelled out at the same rate as a movie character read. It really did receive the text that slow.)

Newer protocols were developed that had 4 frequencies, where each frequency represented a pair of bits: 00 01 10 or 11. This effectively doubled the speed of possible transmission.

Other methods of packing more data were developed, but the limit of the speed was the limit of the sound quality of analog phone transmission.

To compare today’s screamin’-fast data transferr rates: I had a 28.8 kBaud modem at the advent of web pages. I went to check out a friend’s new web site. She had a complex graphic on the home page which took nearly an hour to download. Today, that image would be downloaded in about 1/2 second.

To add to the previous comments. There are a number of assumptions here. Your 56kb/s modem is actually designed to work over analog voice lines. It need not be the case that there is a 64kb/s digital uphaul somewhere in the circuit between you and the modem at the other end. Indeed your link to the local exchange (the local loop) will be analog anyway. In the exchange is a converter that converts the analog signal to a digital data stream - and there is the converse at the receiving exchange, and another analog local loop to get you to the modem at the other end. These converters first priority is to move audio about - not data. However they do tend to be designed to be compatible with the needs of the advanced (i.e. 56kb/s) protocols. It would be little short of a miracle of the these converters could manage to capture the signal with total fidelity within the constraints of their operation. Indeed, capturing a 56kb/s analog modem protocol on a digital link running at 64kb/s is itself a minor miracle. I would be asking what was done to get so close, not why they didn’t manage to close the gap.

One nit pick. There are not, in general, 28.8kBaud modems. There are 28.8kb/s modems, but they are 9600 Baud modems. Baud rate is the rate of signalling changes in the modulation. As described above, newer protocols provided for multiple bits per signalling change, and thus you get more bits per second than signalling changes per second. Notice that 28.8k is three times 9600.

Interestingly, an ADSL link is essentially just a large number of these modem links, each modulated by a different carrier and all stacked next to one another. The ADSL modem doesn’t use the frequency slot right at the bottom, leaving it free for voice communication.

But the old things that were called 9600 baud modems really were 9600 bits per second, some of those being signalling bits, so the data throughput was less. It would be unnecessarily confusing to refer to a 28.8 kbps modem as “9600 baud,” because “baud” just isn’t used anymore.

As you say, a standard phone line is 64K. This is due to the sampling rate set by the phone company long ago. It was determined that this rate would be adequate for decent voice quality. The 56K limit is because a byte is 8 bits. One of those bits is a check digit to ensure accuracy of transmission, so only 7 of the bits are actual data. 7/8 of 64K is 56K.

I thought it might be something like this, that the bandwidth value didn’t count overhead, or in this case, error checking.


And here’s where it gets even more complicated. There are quite a few variables in the equation. When configuring RS/232 communications, you can specify parity or no parity, for example. Transmitting 8-bit characters may take 11 bits (8 data, 1 parity, 1 stop, 1 start). Changing to 7-bit characters and dropping the parity would drop you to 9 bits, increasing the number of characters per second you transmit by 11/9, but not changing the baud rate.

As a side note, just in case anyone’s interested, I put an old article I wrote for Blacklisted! 411 (a hacker magazine) about phone phreaking and how DTMF works on my Web site a while ago.

Also, the limit isn’t actually 56k, but somewhere around 53. The source I found has a lot more information on the topic, if you are interested.

I will take a look at it. Thank you.

When I got a DSL line (nine years ago) suddenly I was getting several hundred bps on the same wiring and at suddenly could make voice calls without interfering with the data. True I had added something called Z-filters to the phone lines but I suspect that was to avoid interference. Actually, I don’t know what they did.

It seems to me that the main thing the phone company did was change their equipmment at the central office to allow high frequency to pass. They also provided things like a name server and an email account. A year later, they had to change the wire into the house, but we were getting neither voice nor data since squirrels had nibbled off the insulation.

The bottom line is that it is not the copper wire so much as the Telco equipment that limits the data rate. At least that is the case if you are within a certain distance of the central office. I still have friends living in rural areas for whom DSL service is simply not available.

That was a very interesting article. Thanks for posting that!