Talk:Modem

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ridiculous statement[edit]

from the article: "Without the discovery and eventual application of trellis modulation, maximum telephone rates would have been limited to 3429 baud * 4 bits/symbol == approximately 14 kilobits per second using traditional QAM".

This is statement is speculative, and on top of that, incorrect. There are many other ways to improve the modulation - for example, OFDM would give the same spectral effiency, as witnessed in ADSL protocols (which use OFDM). please remove statement.

[By a different editor:] The statement is very likely correct, if one confines data bandwidth to the audio analog voice telephony band. ADSL protocols do not pass through channels that are limited to analog voice bandwidth. In a C.O. that provides ADSL, the subscriber's wire pair connects to a DSLAM (DSL access multiplexer), which splits off DSL data to a separate wideband link. Nikevich (talk) 07:14, 1 February 2011 (UTC)

[Yet another editor] Telebit had a modem based on the same principles as OFDM that ran at 19.2 kbps within the voice band. See John A. C. Bingham's book "The Theory and Practice of Modem Design" ISBN 0-471-85108-6 — Preceding unsigned comment added by Dspmandavid (talkcontribs) 20:21, 25 July 2013 (UTC)

UTC does improve speeds?[edit]

europrobe 07:15, 2005 May 19 (UTCdoes improve speeds by using a compression algorithm optimized for internet content (html etc). 'appears to be' should be rewritten with a technical sourced explanation.

The reason why we're having this conversation is robbed bit signaling. This "steals" the least-significant bit in every 6th byte transmitted in each DS0. (That is: less than one control bit per byte.) On average, only half of these bits will have the "wrong" value from the point of view of the bearer channel. The actual theoretical bitrate on channels subject to robbed bit signalling is therefore 64 - (8 / 6 / 2) kbit/s = 63.333 kbit/s. This assumes either only one T-1 circuit subject to robbed-bit signalling being in the end-to-end communications channel, or superframe synchronisation between all concatenated T-1 circuits. For this bitrate to be available to modems, these modems would have to be able to achieve superframe synchronisation themselves (i.e. not just bit synchronisation with the communications channel). A very tall order indeed. JanCeuleers 19:38, 24 July 2006 (UTC)

FCC Speed Myth[edit]

'Current FCC regulations (Part 68) limit the top speed of these modems to 53Kbps.' There is no such regulation on speed limits. The limitation is on power output which limited the old X2 to 53.3K. Newer modems can acheive 54K and under perfect conditions 56K. Someone please rewrite and source. see http://www.google.com/search?hl=en&lr=&q=fcc+modem+speed+myth+56k

AT&T history[edit]

I have not found much information on AT&T modems which preceded the 202. Modem designers were active in the 1950s and did publish several papers which I have not examined. SAGE is covered by several papers. http://www.att.com/history/milestone_1958.html provides a different version of the modem shown in the wiki article. The device is labelled RECORDED CARRIER SUBSET. Another device (not in wiki image) is labelled DIGITAL SUBSET.

AT&T documentation[edit]

Most of my claims regarding dating of AT&T 202, 201 and 103 came from Bell Telephone Magazine issues in the 1960 to 1963 era. BTM Autumn 1963 suggests 103A is a 1963 development but other sources claim 1962. I will stick with 62 until evidence is available.

1200bps history[edit]

Almost all of this information came from: Data Communications A User's Handbook published by Racal-Vadic sometime after 1978. This sentence: "According to Vadic, this made the 212 protocol incompatible with acoustic coupling." is a personal recollection.

Modulation technique history[edit]

Quite a bit could be written about abandoned modulation schemes. There were some problems with CCITT's recommendation of V.29 which was covered by Codex patents. There was a belief that V.29 modulation with multiple circles in constellation provided better protection against phase hits than did the Bell 209 scheme with a square constellation. Perhaps I should create and import and some drawings. Rdmoore6 05:38, 19 January 2006 (UTC)

See the entry on modulation first (unless you wrote it :)
I think some corrections are also called for. To my knowledge, 24 64Kbps lines make a T-1 since forever (DS-0 was standardized around 1957, AT&T ESS-1A computerized phone switches began deployment around 1969-1973 I think), and since about 8kbps gets used for control, the theoretical limit for a US analog phone line has _always_ been 56kbps (though many people didn't think so before modulation schemes past 2400bps were thought of). Digital switches helped reach that max by having a cleaner signal path, but it was always at least theoretically possible.
(In the "older tech had certain advantages" department, if you made a local call on a step-switch phone system, you basically had a clean piece of copper from one handset to the other and could probably run a megabit at least (with modern ICs, which of course didn't exist at the same time as step-switches were in public use).
Most interesting! I wonder whether anyone has measured the bandwidth of such a connection. Last I read, there's a functioning crossbar system set up as a working museum in the Pacific Northwest; pretty sure it's not connected to the phone network. Returning to the original author... Regards, Nikevich (talk) 07:31, 1 February 2011 (UTC)
On a related note, I never heard of echo cancellation as being a motivation for telebit's multiple carriers. though I'm not saying it wasn't. They were promoted as being useful because a very common form of signal loss on phone lines was loss within a narrow frequency band; interference might nuke 1580-1700 Hz, for example, and that would totally hose a modulation scheme that needed the whole bandwidth relatively clean. Plus, the multiple carrier scheme allowed for gracefull fallback, whereas earlier schemes would lose a minimum 25% capacity (9600 to 7200).
The inaccuracies noted above make me have concern for the accuracy of other parts of the article; I don't remember Hayes being the only folks to allow a computer to control answering and calling out, they were just the ones who managed to become the defacto standard, especially in the microcomputer market. DEC, IBM, and AT&T/Teletype all made non-Hayes compatibles, but I don't know if any of them were introduced before 1981.

SAGE Modems[edit]

I'm also not sure SAGE units would be the first modems. Other than being connected to "computers", is there much difference between the boxes that hooked a SAGE unit to a copper line and what wire services and teleprinters used?
Akb4 22:31, 11 May 2006 (UTC)

The argument for SAGE "Data Terminals" being the first modern modems is based on three papers presented at the AIEE Fall General Meeting in Pittsburgh October 26-31, 1958. These were subsequently reprinted in the January 1959 issue of AIEE Transactions, vol 77, pt I, 1958 (January 1959 issue). The article entitled "SAGE Data Terminals" by R.O. Soffel and E.G. Spack is of particular interest.

A SAGE DDT was effectively a simple modem which transmitted at either 1200 bps or 1600 bps. A 2 kHz carrier was used. "Since some of the channels which carry the line signal have narrow bandwidths, vestigial side-band transmission is employed; the usable sideband is limited to approximately half the bit rate." Data clock was supplied by the data source and passed to the data sink. Unlike RS-232 modems the data source also supplied word marks which were passed to the sink. Unlike RS-232 the data source could be located about a mile from the modulator (DDT).

An important aspect of the SAGE DDT/DDR scheme is the modulation technique. Prior to WWII FSK was used by telegraph companies. Reichspost had a particular efficient FDM system for slow telex channels. I suspect the significance of SAGE system was that it was the first instance of synchronous data transmission.--Rdmoore6 02:56, 7 August 2006 (UTC)

It might be of some interest that in 1960 there was a digital link between the BMEWS NORAD COC (above ground) and SAC HQ in Omaha, Neb. iirc. It had a very low data rate, probably lower than 30 bits/sec. IIrc, zeros were no signal, and ones were "dibits", one complete 360-degree cycle of the carrier frequency; these could be AC-coupled without significant degradation. Consecutive ones were akin to a tone burst. Nikevich (talk) 07:38, 1 February 2011 (UTC)

Um, what?[edit]

I find the following:

Because signals travelling through the air must be analog (digital signals cannot be transmitted without a guided transmission medium such as wire)

to be very suspect. Can anyone explain to me what this lines is trying to say? Because I'm pretty sure what it actually says isn't true... User:Glenn Willen (Talk) 18:04, 28 March 2006 (UTC)


This can get a bit philosophical, but you could say "digital signals" don't even really exist. Signals are analog, though they can represent digital values. Note that even a signal which consists only of two levels of 0V and 1V "looks binary", but IS analog (with an ASK modulation). That's why on reception one has to deal with the degraded levels and decide wether a, say, 0.4V signal was 0V or 1V in origin.
Digital signals are an abstraction. They have a finite number of levels, which are not measured in Volts and can't degrade from one level into another. You would have a 0 level and a 1 lever, for example, with nothing in between.
Even IF the original sentence meant digital modulations instead of digital signals, it would still be wrong. Digital modulations can of course be transmitted by radio - look at WiFi or GSM, which are digital systems using digital modulations over the air.
--Ermija 10:43, 23 April 2006 (UTC)
I'd say a "signal" could be digital. But to move that signal around via an analog medium, such as a radio wave or a piece of wire, you would have to encode it via an analog means, such as modulation. At the very least whoever wrote the original phrase was unclear; it's true that digital signals travelling thru the air must do it by piggybacking on something analog, but that's true of signals on wires, too! Anyway, I'm rewriting it. Akb4 23:11, 11 May 2006 (UTC)
oh heck, out of time. the whole radio modem section needs lots of work. I'd recommend making it clear that radio modems are not really different than other kinds. Akb4 23:20, 11 May 2006 (UTC)
If anyone does rework this, then please be careful about the terminology. For example, encoding is different from modulation, and modulation is not a form of encoding (nor vice versa). In general, a data communications transmission device can contain both an encoder and a modulator (and the receiver obviously a demodulator and decoder). Recommended reading: Bernard Sklar's excellent book on digital communications. JanCeuleers 12:50, 25 July 2006 (UTC)
It's not just the radio modem section that needs work: the introduction says that voiceband modems "converts those sounds back into 1s and 0s"!
Unless you want to talk about encoding schemes, or explain the difference between a 'digital' and a 'binary' signal, those signals on the analog lines are 1s and 0s all the way. The digital signal is represented by an electrical analog on both sides of the modem. A voiceband modem converts signals between a modulated signal and a baseband signal, or between a coded signal and an uncoded signal, not between 'analog' and 'digital'.
A 'modem' is not an 'analog to digital converter'. A modem would have some kind of 'analog to digital converter' on both sides. If you were looking at a simple RS232 interface, the adc would be a single bit adc, perhaps formed by a clock recovery circuit and a comparator - just like the single bit ADC on the 'analog' side of a 300 baud modem.
Anyway, those 'analog' phone lines are called that because they carry analog voice conversations. When you connect a modem to them, they are just as 'digital' as -the most digital thing you can think of -. Using the analog phone line as a definition of a 'modem', an ADSL modem is not a modem because it does not connect to an 'analog' phone line. The problem is, that leads to the difficulty explaining what a radio modem is: clearly it doesn't connect to an analog phone line, but it is a modulator/demodulator, so what is it, and why?
Perhaps it's time to go through the whole article and delete references to 'analog' (and analogue). (david) 218.214.148.10 07:08, 20 November 2006 (UTC)
Our anonymous correspondent makes valid criticisms but poor recommendations. Yes, the distinctions among "Digital", "Discrete" and "Baseband" are not make clear in the article. They could be either discussed or referenced.
On the other hand, the ability radio lacks is baseband, not digital. If a DSL modem is not wired into an analog line, to what kind does it connect? And most modems have a ADC and DAC inside them. Otherwise the DSP couldn't do its job. Jim.henderson 14:30, 20 November 2006 (UTC)
The data is digital on both sides. Implying that the data is not digital on the 'analog' side is a source of confusion. Can the (obvious) confusion caused by the misuse of these terms be corrected?
BTW, a DSL modem connects to a Digital Subscriber Line :~) (david) 218.214.148.10


All things are analog. Even inside a basic computer device, a 3.3 volt level (for example) represents a '1', and 0 volt level represents a '0' state. But it doesn't have to be exactly those values.... a '1' state is actually a range from 2.0 upto 3.7. Hence the signal is truly an analog signal, and when we high-speed engineers model our circuits via simulation, we use analog to catch any flaws in the design (like indeterminate states or bad timing).
Back to the original statement: Whether you are using radiowaves, or wires, or two cans with a string tied between them, makes no difference. They are ALL analog mediums. They all have a wide range of varying voltages, currents, or frequencies. What makes a wire "digital", or a radio station "Hybrid Digital", is not the medium. The medium is still analog. What changes is the conscious decision to define 2.0-to-3.7 volts (for example) as '1' and values below 1.0 volts as '0'.
It's not the medium that changed; it's our interpretation of the incoming voltages that has changed.
theaveng 12:10, 20 July 2007 (UTC)



Earlier this month I wired two DSL modems to phone lines. Plain old telephone lines. The phones still ring; all analog jobs are still done. Yeah, the same wires are carrying voiceband signals, also called baseband and analog, while they carry complex, frequency divided partial response highband signals. And yes, those complex signals are Discrete signals and Digital signals. Perhaps the article should go into detail on how these words, as well as Analog, apply to modems, but it's already a long article with a very long history of equipment no longer used. Is it time to split off a "Modem History" article and add more theoretical material to the "Modem" article? Jim.henderson 02:10, 28 November 2006 (UTC)

Echo cancellation[edit]

The discussion about side tone does not seem correct to me. It is true to say that telephone sets create some feedback from the microphone to the earpiece, and if a modem were acoustically coupled to a telephone set then this would be a source of echo. The real problem, it seems to me, is that the outgoing and incoming signals are transmitted and received on the same physical twisted pair. They are combined in the modem by a 2/4-wire transformer circuit, and again on the analog line circuit in the central office, usually embedded on a hybrid circuit (i.e. the H in BORSCHT). Each one of these conversions causes reflections, which means that the echo canceller needs multiple taps. JanCeuleers 15:07, 6 June 2006 (UTC)

Yes, there is an Echo cancellation article, but it is sparse and dry, and that's where a more detailed discussion belongs. The link should replace, not supplement, any mention of "sidetone." Jim.henderson 02:10, 28 November 2006 (UTC)

Questions regarding the v.90 section[edit]

Reading the v.90 section brought several question and concerns to mind regarding accuracy and omission of important data. The following statement may be inaccurate in some ways: "Given that v.90 only works when the circuit is digital end-to-end, there is little reason to take the digital data from the computer, convert it to analog sounds, and then have the phone system convert it back to digital data so the process can be repeated at the far end of the phone line." It is my understanding that home modem users could benefit v.90 if the following criteria where met:

  • The ISP used v.90 modems connected the telephone network digitally.
  • The digital signal from the ISP could only be converted to analog once at the telephone central office (or central switch) before going to the ISP customer's house. Thus the connection does not need to be digital end-to-end, only between the ISP and the caller's CO. Multiple analog to digital or digital to analog conversions would limit speeds to 33.6kbps. This also limits direct connection between two home users of v.90 modems to 33.6kbps since they both have analog connections their CO's.

The os also two issues with v.90 this section seems to omit. The first is the v.90 allows only higher speeds for downloading. Uploading was still limited to 33.6kbs. The second issue (at least in the U.S.) was that while full 56kbps was theoretically possible, FCC regs limit the speed 54kbps. I am indeed remembering things correctly? If so, then this section needs to be rewritten to reflect this info. --Cab88 23:10, 13 June 2006 (UTC)

I found the answers to my questions myself using the book Upgrading and Repairing PCs (16th Edition) by Scott Mueller. The book backed ups my understanding and as such I decided to rewrite the section to conform to info in the book. --Cab88 11:03, 16 June 2006 (UTC)

Just putting in my two cents[edit]

Don't forget, that a carrier wave is not produced with analog modems on speeds lower than 1200 baud.

Um, yes, there is a carrier wave. (A baseband carrier wave). It is modulated at the signal frequency (the baud rate). If you turn the speaker on, you can hear the carrier wave, and the modulation. For V21 (asynchronous transmission at rates up to 300 baud), the carriers were 1080 Hz and 1750 Hz. Bell 103 used a similar system, but it was normally quoted giving the min/max frequencies, rather than the carrier frequency: conceptually, you could think of it as modulating 4 frequencies, rather than 2. 218.214.148.10

Analog modem[edit]

This phrase is used in the article to refer to a voiceband modem. All modems are analog modems; that's implicit in its job description. Some, such as microwave modems and DSL modems are wideband; that is they exploit bandwidth beyond the 3.6 Khz dial-up telephone voiceband, but the majority of 20th Century modems were voiceband dial-up modems. The only literal sense in which some modems are "analog" and some are not is, all modern ones use DSPs inside, while some 20th Century ones used only LC filters, op amps and other analog parts inside to generate and discriminate signals. Should "voiceband" be 1) explained, and 2) used in all parts of the article that now say "analog"? Jim.henderson 14:33, 29 October 2006 (UTC)

Cab88: DSL, cable and satellite modems are actually real modems, as they are not baseband tranceivers. All three convert digital data into analog signals for transmission with a (one or more) modulated carrier(s).

A modem is not an Analog to Digital Converter. Cable Modems do not 'covert digital data into analog signals'. A Cable Modem has analog signals on both sides, and has digital data on both sides. The digital data is carried by an electrical analog. A modem modulates and demodulates. DSL, cable and satallite modems are actually real modems, not baseband tranceivers: but the analog side of a voice modem is called 'analog' for historical reasons, because it was used to carry an electrical analog of an air pressure wave, not because one side of the modem was 'digital' and the other 'analog' 218.214.148.10
So, if a voiceband modem is called "analog" because it is voiceband, why not simply cut to the chase and tell it like it is? That is: Voiceband modem. History is lovely but not when it gets in the way of straight talk.
Jim.henderson 01:40, 28 November 2006 (UTC)

What OSI layer are modems working at? Why is PPP over modem not tunneling?[edit]

The article should discuss the OSI layers of the modems.

Have I understood it correctly?

The oldest modems were only working at the physical layer, since their function was pure modulation and demodulation over a circuit switched channel. Data were either transmitted asynchronously, using start and stop bits, or synchronously at constant bit-rate, using self-synchronizing line code (?).

In 1981 the Hayes smartmodem supported computer controlled number dialling and hang-up, which could be considered as Medium Access Control adressing. (???)

Modern modems also include Logical Link Control, since they support error correction and deal with frames.

Modern modems also include data compression. In the OSI model, this is part of the presentation layer. However the presentation layer is a end-to-end layer, while modems are point-to-point links, so in this case it should be considered as data link protocol.

From higher layer protocols point of view, a modem behaves as if it were a circuit switched physical link (an asynchronous serial link), since the interface to a computer typically is an RS232 asynchronous serial link.

Question: Why is PPP over a POTS or ISDN modems not considered as tunneling, while PPPoE, PPPoA and [[[PPPoX]]] belongs to Category:Tunneling protocols? To my understading, PPP over modem is an LLC protocol encapsulated in another. Mange01 15:03, 22 November 2006 (UTC)

Shannon and Nyquist limit in PCM systems? Why different uplink and downlink speed in V.92 and V.90 modems?[edit]

User:134.58.253.130 added some discussion on the Shannon limit for modems. I have a few questions related to this:

  • You claim that the SNR is 50dB. Today almost all telephone switches in the world are digital, using PCM. Can the maximum possible SNR be calculated for PCM, if there is no noise? (I have made an attempt below, by relating it to the Nyquist capacity.)
  • A V.92 downlink can handle 56kbps in the PCM based downlink but 48kbps in the uplink from an analog user. Why? Can this difference be explained by different Shannon limits in the PCM case as in the analog case. Is it impossible to increase the uplink rate because of this Shannon limit? Perhaps the inter-symbol-interference may be higher in the uplink, since the modem symbols are not synchronized with the PCM sample instants? Perhaps it is not possible to use all 256 levels, since it may be hard to identify the maximum possible amplitude?

I think it is more appropriate to use the Nyquist capacity limit instead of the Shannon limit in the PCM case. The Shannon capacity is the maximum net bitrate if an ideal error correction code is used (which is not possible in the real world). Nyquist gives the gross bitrate if we now the maximum number of levels. We may assume that that we could use all N=256 levels, resulting in a gross bit rate capacity of 2*B*log2256 = 16*B bit/s, where B is the analog bandwidth in Hertz. If we assume that the bandwidth is B=4,000 Hz (i.e. ideal filtering), the modulation would result in according to Nyquist we could get 16*4,000 = 64,000 bit/s. If we instead assume only 3,400-300 = 3,100 Hertz Bandwidth, we would get 49,600 bit/s. In practice a frequencies outside the passband may also be utilized. The V.92 maximum downlink speed corresponds to 56,000/16 = 3500 Hertz Bandwidth.

N = 256 levels, without any noise, gives the same bit rate as the Shannon limit would give, if the SNR was 20 log10 256 = 48 decibel. However, note that Shannon gives the ideal net bit rate, and Nyquist the gross bit rate

Mange01 14:49, 30 November 2006 (UTC)

How is spectrum shaping achieved in V.92 PCM downlink?[edit]

How is the POTS channel filter (with passband of at least300 to 3,400 Hertz, in practice a little bit wider) compensated for in the 56kbps V.90/V.92 downlink? Somehow the spectrum must be shaped to avoid energy outside the passband.

My guess:

  • Alternative 1: The modem generates 8,000 symbols per second, but only utize 7 bit per sample (128 levels), resulting in 56000 bit/s. One bit per sample is used to generate a signal that compensates for the spectrum content that is outside the passband, resulting in very low spectral energy in that area. The probllem is that the energy outside the passband can not be completely ellminated in this way. I am not clear on which bit that is optimal to use.
  • Alternative 2: The modem generates 7,000 symbols per second, but utilize 8 bit per sample (256 levels), resulting in 56000 bit/s. The sample rate is than changed to 8,000 samples per second using upsampling with a factor of 8 (inserting 7 zero value samples between each symbol), digital interpolation filtering and downsampling with a factor of 7 (only keeping every 7:th sample)? The problem is that this upsampling process may cause quantization error.
  • Alternative 3: Only 14 of 16 samples are utilized for information. 256 levels per sample are utilized. The last two samples are utlized to compensate for energy outside the passband. No quantization errors are added, since filtering is not added to the useful samples. The spectrum of the compensation signal (the last two samples in every block of 16 samples) would mainly affect the spectrum below 250 Hertz and above 3750 Hertz).

Mange01 16:58, 30 November 2006 (UTC)

Bonding modems[edit]

I started a section on bonding modems (aka inverse multiplexing modem). Their is more info I did not add in the link I provided in the references section if anyone wants to add to or improve my wording in the section. --Cab88 13:34, 6 March 2007 (UTC)

V.92[edit]

This section states that bandwidth seems to be limited to 56k for voiceband modems, but I remember reading somewhere that there the limit has been formally proven.


DSL "modems"[edit]

Um, DSL connection devices operate on a different principle and everything. Shouldn't that immediately be made clear in the subsection intro, rather than buried in the body text of the subsection? <looks confused>

--Kim Bruning 14:29, 4 June 2007 (UTC)

How different principle? They modulate; they transmit; they receive; they demodulate. They adapt between the baseband signals of the computer and the frequency dependent characteristics of a longer transmission line. The differences are outlined in the DSL modem article but none of them are differences between a real modem and something that isn't really a modem. Jim.henderson 14:36, 4 June 2007 (UTC)

Merge[edit]

Not at all. Modem is already very large and covers all kinds, voiceband, broadband, interplanetary, antique, still under manufacture, etc. 56k modem is about the only kind of voiceband modem still made, and should be the article people use when they want to find out how the modem in their home computer works. It should have all the details and none of the ancient historical background. Alas, as it stands, 56k modem is a crummy article and should be somewhat expanded and greatly rewritten, but it should remain a separate article. Jim.henderson 03:14, 13 July 2007 (UTC)

Rewrite[edit]

This article is hard to read and certain sections repeat eg 'V90' and 'Using digital lines and PCM'. The article gives greater coverage to voiceband modems than other modem types such as DSL modems, due to the many developments over the last 40 years. The descriptions of the variants of voiceband modems is now largely of historic interest. The article does not descibe the relationship between modems and networks, such as todays internet. For example a voiceband modem network typically used to be two identical modems over an analogue voice circuit but nowadays is typically a wide range of different 56k modems connecting over a digital PSTN to a shared central modem server. The section on 'Popularity' refers to using 56k voiceband modems to access the internet, obviously DSL modems are gaining in popularity. A better structure for this article is needed.John a s (talk) 23:23, 12 January 2008 (UTC)


I agree the article is repetitive. I have thought about eliminating most of the first section and merging it with the next section (about breaking the 9.6k barrier, data compression, et cetera). The only reason I did not do it sooner was because of time; maybe I will tackle it this weekend. ----- As for newer modems like DSL, it's probably worth adding a section immediately following the 56k section, but I would not eliminate the earlier stuff. POTS modems dominated for over 30 years; DSL is still just a baby in comparison. (And POTS modems still have uses; like in my hotel. A 56k is my only option.)
"For example a voiceband modem network typically used to be two identical modems over an analogue voice circuit but nowadays is typically a wide range of different 56k modems connecting over a digital PSTN" <---- The article does discuss this in regards to the transition from 33.8k to 56k. —Preceding unsigned comment added by Theaveng (talkcontribs) 20:43, 14 January 2008 (UTC)

Correction[s?] needed[edit]

>> For example, a 56k modem can transfer data at up to 56,000 bits (7kB) per second over the phone line.

I suspect that "56K" really means "56.25K" and K means 1024.

But in any case, this I know: 56K really means 57600 (which happens to be 56.25*1024.) I've designed embedded RS232 timing generators and it is 57600 bits/second.

Check any modem manual or see this faq here: http://www.urisp.net/urisp_glossary.html

Thanks, -Jesse@64.146.180.232 (talk) —Preceding undated comment was added at 22:45, 8 January 2009 (UTC).

No. K means 1,000. In datacommunications kilobit/s has always ment 1,000 bit/s, also before 1999 when IEC redefined kilobyte to mean 1,000 byte instead of 1,024 kbyte.

Vocal modulating-demodulating[edit]

There are people out there who are so unbelievably (for lack of a better term) nerdy, that they have learned to speak 'modem.' I do not believe I remember where I read this, but my question is: what would be the correct term (for searching) of this, and does an article cover this? ~九尾の氷狐~ (「Sumimasen!」 「Dochira samaka?」) 19:39, 9 January 2009 (UTC)

To refine, by 'speak modem' I mean screech and beep at the exact frequencies needed to perform the same functions that the real modem screeches and beeps are. I.e. connecting the modem-line directly into a phone and creating the correct noises with ones one vocal cords. ~九尾の氷狐~ (「Sumimasen!」 「Dochira samaka?」) 19:42, 9 January 2009 (UTC)
I highly doubt it. Imitating TCM at tens of ksymbols per second? Unlikely. Incidentally, the "screeches and beeps" aren't how the modem normally communicates information, they're the channel-estimation phase at startup. Oli Filth(talk|contribs) 01:44, 10 January 2009 (UTC)
Oh, sorry, I'm more on the 'software' side of computers, as opposed to hardware. At any rate, as unlikely as it sounds, I'm almost positive I saw an article on it somewhere, I just can't get the terminology right to look for it... ~九尾の氷狐~ (「Sumimasen!」 「Dochira samaka?」) 07:59, 10 January 2009 (UTC)
People use to be able to whistle at Telephones, using the signalling protocol understood by telephone exchanges. I think that is probably what you read about. It would also be possible to emulate the handshake at the start of a modem or fax conversation, but not to any particular purpose. After the handshake, fax and modem data just sounds like static - you can make that kind of noise, but not with any meaning.
In that case we talk DTMF modulation, which sounds reasonible to imitate. But to mimic even the slowest modems would require you to sing or wistle a melody of 110 tones per second, which I am convinced is impossible. Mange01 (talk) 12:46, 11 March 2009 (UTC)
Weren't there a number of science fiction stories surrounding this sort of thing? No wonder this kind of mythical matter has been raised to the level of an Urban legend. 198.177.27.17 (talk) 21:24, 14 March 2009 (UTC)
Yes, this seems to be a popular urban legend. I think it is popular enough to deserve a mention in this article, clearly labeled as WP:FICTION, like the common misconceptions discussed in many other encyclopedia articles. I'm pretty sure this comes from confusion between (a) the sounds produced by fax and other modems (which I agree with Mange01 is probably impossible to imitate -- however, see War Stories: Modem-Speak) and (b) the DTMF and 2600 Hz sounds that some humans actually can -- and did -- sing or whistle well enough to command the machine at the other end. Contributing to the confusion is the strong overlap between early phone phreakers and early computer hackers -- see phreaker#Computer hacking and blue box. --DavidCary (talk) 17:49, 29 April 2013 (UTC)

Building your own 300 baud modem[edit]

I think the main article would be improved considerably if there were a link to an article about building your own 300 baud modems. Surely one exists somewhere in Wikipedia already? 198.177.27.32 (talk) 03:15, 14 March 2009 (UTC)

History section is bunk[edit]

I've placed a disputed-section tag on the History section. For starters, while NORAD's SAGE air defense system was in fact responsible for the Bell 101 standard used in 110 baud subscriber line modems, SABRE had absolutely nothing to do with the development of any modems. Furthermore, to suggest that there were any 2400 baud modems in the 1950s is preposterous. 75.36.151.235 (talk) 09:47, 28 May 2009 (UTC)

While we're at it, consider the BBN Timeline which states that BBN "designed and demonstrated the first voice modem, called DataDial", in 1963, "to enable remote communication with computers by telephone." Maybe some ex-BBN person can chime in. Lupinelawyer (talk) 16:06, 3 September 2009 (UTC)

Apple Geoport paragraph[edit]

Somewhere along the line, this paragraph about the Apple Geoport got lost, ending up under the heading of 'softmodem' but with no context. I've removed it temporarily as I don't know where it belongs and seems to refer to concepts introduced previously which no longer appear in the article (like running programs in a modem?). See following:

Apple's GeoPort modems from the second half of the 1990s were similar (to what??). Although a clever idea in theory, enabling the creation of more-powerful telephony applications, in practice the only programs created were simple answering-machine and fax software, hardly more advanced than their physical-world counterparts, and certainly more error-prone and cumbersome. The software was finicky and ate up significant processor time, and no longer functions in current operating system versions.

Bold section added by me. 136.186.1.187 (talk) 03:51, 25 August 2009 (UTC)

Energy use modem[edit]

Nothing in this article about the fact, that a (broadband) modem uses energy (between 15 and 20 watt an hour). Because many seem to be not aware of this fact, it might be important information; especcially when is added, that one can save most of this energy (about 150 kWh a year) by simply shutting down the modem together with the PC. When all internetters would do so. that would lead to an essential global reduction of greenhouse gasses.--VKing (talk) 02:33, 21 April 2010 (UTC)

Of course, you mean "15 and 20 watts". Indeed, I have FiOS, and the FiOS handware and my router are on continuously, but I'm not keen about that. Compared to the power drain of an electrically-heated clothes dryer (3 kW?) (or, for that matter, an electric stove with an oven), this is not a lot, although a those appliances are not in continuous use. (I do advocate drying clothes on clotheslines, but I'm getting far off topic.)Nikevich (talk) 08:00, 1 February 2011 (UTC)

IT'S NOT ANALOG DATA[edit]

PLEASE DO NOT CHANGE THE ARTICLE TO SAY THAT THERE IS ANALOG DATA.

A modem is not a Digital to Analog (D to A) converter. The name "analog" for a telephone connection refers to the way that information is transmitted when you are talking on the telephone, not when you are using a modem. Even when you use a Modem, digital signals are not analogs of voice signals.

The second paragraph of the introduction is badly written, and was badly written before, but at least it is not wrong at this momement.

If some one could have a go a re-writting the second paragraph so that it reads well, perhaps well-meaning but technically ignorant contributers would be less likely to alter it.

This is a well known misunderstanding, but the problem is that the article is already too long to include a tutorial on Analog Data and Analog Communications. I don't see an easy solution to that either.203.206.162.148 (talk) 07:50, 8 September 2010 (UTC)

Sorry, but you're simply wrong about this. An analogue signal is one that can take a continuous range, rather than just a small number of values (usually two if it's binary, sometimes three-trinary or more). In general, analogue signals are not only voice they can be all kinds of things, sound, video, compass directions and many, many, many other things. Concorde was flown using analogue signals between the pilot and the flaps. The line side of a modem is always analogue.87.113.241.201 (talk) 05:39, 9 September 2010 (UTC)
Let's see. RS 232 signals go in *here* at this modem - that's digital data, no debate. At the other end, RS 232 signals come out *over there* out of that modem - also digital data. In between the modems we have these wires, which a 'scope shows are carrying all kinds of interesting voltages. If we chop those wires, the data stops coming out *over there*. Something is going over those analog wires that comes out the modem as digital data. Does it really confuse anyone to call it "analog data" ? Sure, it's not like a current loop where 7 mA represents 40 degrees and 17 mA represents 120 degrees, but is it not equally analogous to say 1200 Hz represents a "1" and 2400 Hz represents a "0"? Or the combinations of phase and amplitude ( or I and Q) used in higher-speed modems? A continously-varying quantity can represent a discretely quantized set of states, can it not?
Of course, when you realize a TTL "0" is anywhere from -0.7 V to +0.7 V, and a TTL "1" is anywhere from +2.4 V to +5.0 V, you realize that, too, is an "analog" signal. I'm much less happy with that realization. --Wtshymanski (talk) 13:24, 21 September 2010 (UTC)

Leased-line, synchronous, and line adapters[edit]

This article omits three important historical areas.

Many modems in the 60s-80s were used on leased (permanent) telco lines, often four-wire, at speeds of 2400-9600 bps. AT&T also offered broadband services by ganging together multiple lines.

Dial-up synchronous modems as early as the late 60s were capable of 2000 bps when used with synchronous protocols such as STR, BiSync, and SDLC.

IBM (and others, I suppose) sold "Line Adapters", which were physically small (often under-the-cover) and inexpensive modem-like devices for use on privately-owned copper (in-building or on-campus).

I'm not equipped to discuss these things in detail, but anyone who worked with IBM mainframes in the period knows about them to some greater or lesser extent. —Preceding unsigned comment added by 68.193.248.73 (talk) 18:19, 11 December 2010 (UTC)

8 or 10 bit per byte?[edit]

Which modem standard or speed was the last to use asynchronous communication, offering a gross bit rate of 10 or 11 bits per transfered word (7 or 8 bits of useful data)? I ask because in the List of device bit rates, 1 byte is assumed to be 10 bits, which is incorrect in modern modems. Which was the last to use half-duplex communications? Mange01 (talk) 18:21, 2 January 2011 (UTC)

I'm far from sure, but I recall reading that more-advanced modems (faster ones) operated asynchronously to establish a connection, and then switched to synchronous mode for the rest of the session.Nikevich (talk) 08:06, 1 February 2011 (UTC)

The differences between async and sync apply only to link between the modem and the local terminal device. The link between the two modems does not distinguish between data bits or start/stop bits, has no idea whether there are 5, 6, 7 or 8 data bits, 1 or 2 start/stop bits per word. They only signal 0 or 1. The V.21 modems didn't even know about the bit rate - the phone line signal merely followed that on the serial link at any speed up to 300bps. So the whole issue of the number of data/start/stop bits per byte is a concern for the serial interfaces (UARTs) rather than the modems. As for async modems switching to sync - that started with the MNP protocols - MNP3 specifically. MNP 1 and 2 introduced error detection codes and automatic resend protocols. This meant that the modems were no longer simply relaying the bits off the serial link anymore - the bits were gettng bundled into larger frames with headers, CRC code and framing bits etc. The start/stop bits had become redundant and useless overhead. The MNP3 completed the transformation and used a completely synchronous protocol, eliminating the 20% overhead. Another consequence of these smartmodem's buffering was that the speed between the terminal and the modem no longer needed to match the speed between the two modems over the phone line. This difference became even greater when data compression was introduced in later protocols. Later modem standards such as V.34 also auto-selected the line speed, so there was no longer any relationship between the line and port speed. Most modems communicated at the highest serial port speed 115kbps, much faster than the phone line speed.

While the old rule of 10 bits per byte is no longer strictly true now that start/stop bits are not longer used, the protocols that replace them have a similar overhead (PPP, TCP/IP headers) so it is still a reasonable guide. The later modem speeds in List of device bit rates would include the headers, but no start/stop bits.

http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.commadmn/doc/commadmndita/asynch_modems_microcom.htm


220.244.97.1 (talk) 07:07, 19 November 2013 (UTC)

Traditional TTY/TDD modem frequencies and format? Also, v.18[edit]

This topic somehow seems to have been omitted from the article. Some time ago, I read about "Wybrecht" and "Weinbrecht" modems; both were misspellings of the name of a born-deaf engineer, Robert Weitbrecht, who worked to establish TTY/TDD technology. (By all means, Google on his name!)

Apparently, TTY/TDDs were developed quite early compared to modems for use with computers, and their technical specifications differ in many respects from those of computer modems, although they operate on the same principles (FSK). There's more at {typical header}911broadcast.com/tech-emergency_technology_54.htm (a blacklisted Web site, unfortunately; do be careful!) for the details. There's a table there, although it doesn't specify start and stop bit timings.

Fortunately, modern technology makes compatibility between TTY/TDD traditional protocols and modern modem protocols affordable to many; the v.18 standard [1] specifies this. This standard also permits successful international use of TTY/TDDs.

I hope somebody else will pick up this thread and incorporate it into the article; I'm out of energy, for now. Regards, Nikevich (talk) 09:28, 1 February 2011 (UTC) Best regards,

The Carterfone decision[edit]

The decision from the FCC in the US that all Ma Bell connected devices be provided by Ma Bell, forcing the use of acoustic modems until the Ma Bell breakup should be noted. e.g. Once the monopoly was broken, the Carterfone decision no longer was in place, allowing end users of the telco to use equipment from any manufacturer, which opened the way for direct line connected POTS modems in 1987. This will eliminate citation needed in the blurb about the expense of Ma Bell / AT&T provided modems, I do not have a source for that statement, however.

8r455 (talk) 08:33, 25 May 2011 (UTC)

A computer in NY in 1940?[edit]

This doesn't look right:
"George Stibitz connected a New Hampshire teletype to a computer in New York City by a subscriber telephone line in 1940.[citation needed]"
http://en.wikipedia.org/wiki/Timeline_of_computing
I don't think there was anything that could be called "Computer" in 1940 NY.
http://en.wikipedia.org/wiki/George_Stibitz
This shows that it was a calculator. Maybe we should go ahead and replace "Computer" with "Electronic calculator"?

Diego bf109 (talk) 18:38, 15 July 2011 (UTC)

It must have been a teletype circuit; these didn't use modems. --Wtshymanski (talk) 19:01, 15 July 2011 (UTC)

Please source or remove content.[edit]

There's many statements that are unsourced and are possibly speculative and/or original research (see above for a good examples), I ask somebody to provide sources or delete that content.

Also, this article should also focuses on other type of modems. — Preceding unsigned comment added by 200.77.123.244 (talk) 18:50, 22 March 2012 (UTC)

Added citation for Shannon channel capacity calculation[edit]

I added a citation for the Shannon channel capacity calculation and removed the original research tag. The values for bandwidth and S/N ratio used by this source are different than the unreferenced values before this edit. Tomtrunnel (talk) 06:11, 28 January 2013 (UTC)

"TV" modems[edit]

I was watching an archived episode of "Net Cafe" from 1996 and they streamed data to the user by showing essentially a series of QR codes at 24 frames per second. The host claimed they were streaming at about 2Mb/s. I would love to learn more about this but can't find information anywhere. This seems like the appropriate place for that information to live. The thumbnails for the episode show what it looks like here: https://archive.org/movies/thumbnails.php?identifier=nc101_hackers — Preceding unsigned comment added by Zeroday (talkcontribs) 00:07, 5 February 2014 (UTC)