 | This article is within the scope of WikiProject Computing, a collaborative effort to improve the coverage of Computing on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks. | |  Start | This article has been rated as Start-Class on the project's quality scale. | | High | This article has been rated as High-importance on the project's importance scale. | | | [edit] Is 56 kbit/s really the fastest? Have there been any laboratory experiments with faster rates over phone lines? - Theaveng 12:22, 27 September 2007 (UTC) - It depends what you consider to be a phone line. Copper phone pairs can carry far more than 56K and indeed to in many parts of the world (see DSL), the 56K limit comes from the structure of the modern digital phone systems. Plugwash 13:21, 29 September 2007 (UTC)
- The 56Kbps limit comes from the infrastructure. It is common for the core telco system to use Robbed Bit Signaling (RBS) - so a 64Kbps DS0 circuit 'loses' 8Kbps to signaling, leaving 56Kbps for the voice content - or the modem. I used to work for Livingston Enterprises, and later Lucent, maker of the PortMaster line of access servers. I was there for the introduction of PCM modems - Livingston supported K56flex with Lucent chips, which is a factor in why Lucent acquired us. (We tried Rockwell's chips - but they were too hot and too unstable at the time.) I've personally seen K56flex modems hit speeds over 56Kbps, and theoretically a PCM modem could hit 64Kbps, if the line is a clear 64Kbps circuit - such as an ISDN B-channel which doesn't have RBS. But when you're talking a POTS line 64Kbps is the theoretical max raw rate, that's what the DS0 carries. The copper itself can be used for other protocols - like DSL - but that's a different story entirely. (Lucent's original V.flex2 proposal was the most advanced of the three, with things that finally made it into v.92 - like PCM uplink. But for strategic reasons they partnered with Rockwell to take on USR, and Rockwell's K56Plus was the weakest - so the K56flex compromise sacrificed some features for the partnership.) MegaZone (talk) 06:59, 28 July 2009 (UTC)
[edit] Is error-correcting codes included or excluded in the bit rates for V.92 modems? Does v.92 modems have Forward error correction (FEC), a.k.a. error-correction codes? Or does it only have error detection and automatic repeat request (ARQ)? Perhaps in slower modes than the 56kbps mode? And it that case, is the FEC code included or excluded in the bit rate indicated by the operational system? I ask because I wrote the following in the bit rate article. Is it correct? - The connection speed of a network access technology or communication device is indicated by some operational systems. The connection speed of a technology that involves forward error correction typically refers to the physical layer net bit rate. ...
- However, the connection speed of maximum 56000 bit/s in the downlink of a V.92 modem refers to the gross bit rate (the line rate), while the modem data transfer rate (the average useful bit rate) sometimes can be higher higher due to data compression, and sometimes lower due to bit-errors and retransmissions.
Mange01 (talk) 19:11, 26 August 2008 (UTC) Yes, but we should remind readers that the 56K figure is an ideal or fantasy. Ignoring the overhead of v.42 error correction is only part of what makes it unreal. Jim.henderson (talk) 15:32, 1 September 2008 (UTC) - It's not unusual at all for people living a reasonable distance from the central office or a DLC to get 53K. Squidfryerchef (talk) 03:43, 2 September 2008 (UTC)
- True enough, but the section in question suffers from more problems than that. The v.42 overhead paragraph presents an unsourced calculation based on a 56K line rate, which never actually happens. This paragraph should come after one that clearly explains the various line rates, the conditions that create them, and the likelihood of achieving them. Then there is a paragraph discussing the 115.2K port rate in a way that suggests far greater relevance than actually applies. It's a very poor section and likely to mislead many who don't already know about this stuff. Jim.henderson (talk) 01:45, 8 September 2008 (UTC)
- Thnx for your response. Please check the rest of the discussion in the bit rate article about net bit rate and gross bit rate, and connectin speed, not only for phone modems. What speed does MS Windows show, i.e. what is the connection speed, when we are using 56K modems? Does it show 56K or 53K at most? Are there any forward-error correction codes in some modes of a v.42 modem? The error coding you talk about, is that error-detecion, used for ARQ? Mange01 (talk) 21:23, 24 September 2008 (UTC)
- Ideal perhaps, but I wouldn't call it fantasy. I've seen a great many solid 56Kbps K56flex & v.90 connections 'in the wild', from my time working in the industry. You certainly can't expect it all the time, but it is certainly possible in the real world if conditions are good. There is an oft repeated myth that the FCC imposed a power cap which limits PCM to 53.3K. That's marketing crap some modem vendors started back in the day because *their* modems couldn't get past 53.3 without violating the power level limits, but it was not a limitation of the protocol, just their chipset. Others were hitting 54.6 & 56 with the same protocol - or higher on a clear DS0 link, though you'd be luck to catch that in the wild. MegaZone (talk) 07:10, 28 July 2009 (UTC)
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- I guess you didn't notice but our article in fact repeated this myth. I did some searching, and it appears you are correct so I've added a poor source (the original claim was unsourced as with most of the article). If you are aware of better sources, your welcome to improve the article Nil Einne (talk) 17:47, 19 September 2009 (UTC)
[edit] Ten bits per byte Umm, this is sometimes true but not usually. It's part of a paragraph with precise calculations based on assumptions that are sometimes true but not usually. Seems to me, this article ought to discuss, not user throughput, but line rate, with links to articles about the various factors besides line rate that affect throughput. On the other hand, if someone has published a survey of actual results with 56K modeme, rather than calculations, then it would belong here. Jim.henderson (talk) 04:46, 16 December 2008 (UTC)
- Well I haven't taken a survey, only made observations of my own experience. Back in the days of 2400 bit/s modems, they did use 10 bits per byte per the settings (8N1 aka 1 start bit, 1 stop bit, and 8 data bits) with a throughput that was only 240 bytes/second. ---- In today's world the maximum download speed I can get on a 53.3 kbit/s connection is only 5.1 or 5.2 kbytes/second. If it were 8 bits/byte then it should be getting 6.6 kbytes/second but that never happens, so I can make an engineering estimate that the modem is using the functional-equivalent of 10 bits per byte just like the old 80s-era modems used. ---- Theaveng (talk) 13:40, 21 April 2009 (UTC)
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- A possible but not likely explanation. Almost all dial-up Internet connections by "56K" modems are v.42 synchronouse ones, with an overhead of about 4%, not start-stop asynch connections with overhead of about 20%. The changeover to synch mode happened about the same time, and on the same modems, as the increase in line rate from 9.6K to 14.4K. Much more likely you, like everyone else with "56k" modems, are getting not merely a lower line rate than advertised by the name, but lower than the 53k theoretically possible. Namely, some 40-45b/s. Jim.henderson (talk) 03:50, 25 April 2009 (UTC)
- Ah. A little research shows that it's worse than I thought. Yes, the calculation is incorrect, not arithmetically but in the assumption that 8N1 or similar start-stop protocol is in effect rather than Link Access Procedure for Modems. LAPM is almost always used in the past decade. Even were the calculation correct, a custom made engineering estimate would violate WP:SYNTH and have to be removed on that ground. So, no, all that calculation must be deleted as both erroneous and WP:OR. Jim.henderson (talk) 00:32, 28 April 2009 (UTC)
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- First off, when I connect it clearly states I have a 53.3 kbit/s line. So if we follow your reasoning, I should be seeing 53.3 kbit/s divided by 8 bit per byte == 6.6 kbytes/second download speed. In over ten years of using these modems, I have never seen that kind of speed. They always max-out at around 5.1 or 5.2 kbytes/second.
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- I have seen lower connections, like 44.0 kbit/s while traveling, and yes that translates to 4.1-4.2 kbyte/sec as you would expect with a 10 bit per byte protocol. Can you give us a GOOD explanation for that discrepancy between your assertion that there's 8 bits per byte, and the apparent 10 bits per byte seen in the real world? ---- Theaveng (talk) 17:17, 4 May 2009 (UTC)
- Drat; been taking too many outdoor pictures in the nice weather and will be busy integrating them into other parts of Wikipedia and won't give this bit of electronic antiquity the time it needs. Yes, 8N1 remains commonplace for the DTE connection, that is computer to modem, since bandwidth is plentiful, thus overhead and error rate aren't important in that link. LAPM is used for the modem to modem connection through the phone line, where those are important considerations. The EC feature of v.42 is constantly at work, using in most connections a significant percentage of the channel capacity and I figure those are the missing bytes you are misattributing to the excess overhead of inefficient framing.
- Anyway if you can find time to link reliable sources that say I'm right or wrong I'll be delighted to see them included in this article; otherwise the matter should be omitted. Eventually I'll photograph and upload everything of encyclopedic interest in my neighborhood and have time to devote to the joys of voiceband modemming. Meanwhile have fun and don't forget the perils of WP:OR. Jim.henderson (talk) 15:45, 7 May 2009 (UTC)
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