I have read that this is due to the absence of an A/D or D/A conversion (only) in the down stream which would normally introduce some quantization error. This conversion is present in the upload, so the speed is then 33k. I can understand that some analog to digital conversion is needed only for voice transmission. Does this mean that this speed is only for voice transmission?
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1\$\begingroup\$ Voice normally uses 64k, i.e. 8k samples per second at 8 bits each. There was a time I used to dream of having a 56k modem... But the speed difference is the way they were designed to work, nothing to do with A/D/A conversion. \$\endgroup\$– FinbarrCommented Jan 6, 2020 at 17:35
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3\$\begingroup\$ You'll find some information on Wikipedia's page for V.92. It at least explains why V.90 and its predecessors were limited to 33.6 kbit/s upstream (use of analog rather than PCM like on the downstream). It does not explain why even with PCM in both direction the upstream is slightly slower. Probably related to the position of the PCM segment in the communication. \$\endgroup\$– jcaronCommented Jan 6, 2020 at 18:00
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\$\begingroup\$ I also read that the downstream uses PCM. Does this mean that we need to replace the analog telephone line with a digital line if we wish to enjoy this speed? \$\endgroup\$– nikos chatziathanasiouCommented Jan 6, 2020 at 18:27
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3 Answers
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56kbps (PCM) became possible once most dial-up ISPs were using digital services (e.g., T1 lines) for their incoming telephone connections. This meant that the entire path between a network server and your computer was digital, EXCEPT for the local loop between your modem and your central office.
That local loop has codecs (A/D and D/A converters) at each end, one in your central office, and one in your modem. The difference in data throughput is based on the relative performance of those codecs.
The phone company uses standard 8-bit µ-law codecs for their local loops. However, your high-speed modem has the flexibility to use a much higher-resolution (12-bit or more) and higher-speed (oversampling) ADC. With enough "training", the DSP in the modem can actually figure out exactly which PCM code the ISP sent for each audio sample, which gives you 7 bits1 × 8000 samples/second = 56 kbps downlink speed. The ADC in the central office can't do the same thing for the uplink direction, so you're limited to the 33.6 kbps of a purely analog connection in that direction.
1 You don't get 8 bits per sample for a number of complex reasons, including things like robbed-bit signaling and FCC spectral power limits.
answered Jan 7, 2020 at 0:55
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\$\begingroup\$ Ok, so for voice the download speed is 56k. What would the speed be for other types of incoming data? Whould PCM still be used for incoming data transmission? \$\endgroup\$ Commented Jan 7, 2020 at 7:12
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\$\begingroup\$ It's 56k for ALL data. And PCM is the name of the encoding method used by the codecs on the local loop. As far as the telephone company is concerned, the data just represents uncompressed audio. \$\endgroup\$ Commented Jan 7, 2020 at 12:30
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\$\begingroup\$ Where was the modem which transmitted at 56k? If it was outside the CO, the ADC in the CO would also limit that uplink speed, thereby limiting the end user to 33.6k download. Were these connected on purely digital lines? \$\endgroup\$– TassCommented Nov 25, 2023 at 15:34
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\$\begingroup\$ @Tass: Yes. Reread my first paragraph. \$\endgroup\$ Commented Nov 25, 2023 at 23:47
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The bandwith for the transmission is limited to view kHz. This bandwidth was splitted into upstream and downstream. The required math to use even noise to transmit date was applied in later standards. Most of the users of a modem demand more downstream than upstream. Therefore downstream speed is higher.
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4\$\begingroup\$ If I am not mistaken, the sharing of availlable bandwidth between download and upload applies to ADSL and not to dial-up modems. \$\endgroup\$ Commented Jan 6, 2020 at 18:54
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\$\begingroup\$ @nikoschatziathanasiou The split can be in bandwidth or time. As far as I remember, the modem were able to switch between both modes, depending on the quality of the connection. DSL was providing a split between the bandwith of voice and data. \$\endgroup\$ Commented Jan 6, 2020 at 19:21
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1\$\begingroup\$ I believe Nikos is correct. I spent a LOT of time studying old trellis 56k but also ADSL, HDSL, etc. Never saw a single 56k modem case with asymmetric up and down speeds and I'm not sure how a trade-off would have been achieved with their trellis design. But I can't speak for the entire world. Just my perception of it. (I didn't ever design a 56k. My only design was an older FSK 600 bps using 6-pole transmit and 10-pole receive filters. So it's just theoretical knowledge I have here.) Any example cases you can propose to look at? \$\endgroup\$– jonkCommented Jan 6, 2020 at 19:23
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1\$\begingroup\$ As I recall, 56k was reduced from 64k because of SNR issues dictated by FCC, also the LSB of the last of 24 channels was dedicated for DS0 signalling which could not be heard but technically is 64k -1 bit. The asymmetry in bit rates achieved 40kbps upstream with improved compression, but the number of delays in the network that could be equalized for flat spectrum and group delay was limited to 1 for software modems and 2 for Rockwell DSP's. A compromise was made in the constellation for good BER with 20 bps/Hz BW. A faulty connect was rarely an SNR issue but more than 1 or 2 buffer delays \$\endgroup\$– D.A.S.Commented Jan 6, 2020 at 19:35
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1\$\begingroup\$ Actually, all modems from 9600 bps on up used adaptive echo cancelling, which allowed them to use the full audio bandwidth in BOTH directions. \$\endgroup\$ Commented Jan 7, 2020 at 1:09
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Most telephone networks these days are fully digital between the exchanges. Voice - and hence the signal between dial up modems - is transmitted at quite a low bit rate. This may be only 64kbps. That's why modems can never match the speed of ADSL.
If the connection is modem-to-modem, then the modem creates an analog signal. That goes to the exchange, where it hits an ADC. the signal goes over the digital network to the destination exchange, where a DAC converts it back to analog again. the remote modem receives this analog signal. Note that there is both an ADC and a DAC in this arrangement. Since the DAC will never perfectly digitize the signal, you won't be able to achieve anything near 64kbps.
But the ISP can rent a line directly from the phone company, and install their own equipment. They can digitally convert the binary data from the internet to the binary protocol of the phone network, missing out the ADC entirely. This allows them to use more of the theoretical 64kbps of the phone line. And so you get a faster downstream speed than the upstream one.
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\$\begingroup\$ There's still an analog link in both directions, so this explanation doesn't quite cover the whole story. \$\endgroup\$ Commented Jan 7, 2020 at 1:10
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\$\begingroup\$ @DaveTweed There is. But the ADC in the modem can be optimized as part of the chipset for receiving the data. The ADC in the exchange is optimized for voice calls, and not under the control of the ISP or modem manufacturer. \$\endgroup\$– Simon BCommented Jan 7, 2020 at 12:35
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\$\begingroup\$ Yes. I made that point explicit in my answer. You need to do the same thing. \$\endgroup\$ Commented Jan 7, 2020 at 12:36