Telephone line requirements vs modem speed.

300bps (Bell 103 & ITU-T V.21 protocol)
2400 to 12,000bps (ITU-T V.32, V.32bis and V.34 protocols)
Connections limited to this range indicates severe channel impairment. Usually noise and bandwidth restrictions cause connect speeds this low. Long cable runs of over 5 or 6 miles can cause it, as can very old analog carrier systems.

14.4Kbps (ITU-T V.32bis and V.34 protocols)
This is the minimum speed that a V.34 modem should be able to connect at *if* the telephone line is just able to meet specs. If this speed cannot be obtained either the modem is defective or the line is out of spec in some way.

16.8Kbps (ITU-T V.34 protocol)
This is the minimum speed that most V.34 modems will actually be able to connect at on a minimally specified line. That means if the modem _can_ get this speed, the line *must* be within specs.

This is also the upper limit for connections through various types of digital carrier systems that use ADPCM or 32K bits per channel instead of standard 64Kbps PCM encoding. Due to lower sampling rate and fewer bits per sample more quantization noise is generated by the analog-to-digital/digital-to-analog conversions in these systems, thus reducing the Signal to Noise Ration (SNR) limiting speed.

19.2Kbps to 26.4Kbps (ITU-T V.34 protocol)
Speed in this range generally indicates a bandwidth limited channel. Some older carrier systems and some digital system that packetize and compress data are limited to 19.2 or 21.6Kbps connections. Long cable runs are also reasons for connect rates at 26.4K or below.

26.4Kbps, perhaps with occasional 24K and 28.8K (ITU v.34 protocol)
Usually indicates a SLC that has what is called a "universal" interface to the Telco switch. Such an interface adds an extra DA/AD conversion, which prohibits a V.90/92 connection, and also adds at least 3 dB of quantization noise and a small amount of bandwidth restriction, all of which combine to usually disallow 28.8K connections. Note that for modems which can measure Signal-to-Noise Ratios, 37 dB is the *best* that can be obtained on a connection through any form of 64Kbps/channel PCM digital carrier. The specification for a voice grade telephone line is only 24 dB.

28.8Kbps to 33.6Kbps (ITU-T V.34 protocol)
Indicates the line is so close to perfect that it would be difficult for the Telco to actually measure any change that would improve speed. This means SNR is better than about 32 dB, the bandwidth of the channels being used is at maximum, and the cable must not be very long. 33.6Kbps simply means the line is about as good as it can get.

28.0Kbps to 53.3Kbps (ITU-T V.92 & V.90 protocols)
Means there are no sharp bends in your cable, no goblins camped in any splice boxes, and you've been blessed by a supernatural entity. These connections amount to pure FM (Freaking Magic, if you will). If you breathe funny tomorrow your speed may drop significantly! Brand new, perfect line cards may or may not work. Cables that look good might work, or might not etcetera. It all depends on parameters that have nothing to do with a voice grade telephone line. If you cannot get a V.90/92 connection the Telco is not obligated to change anything to fix it.

The minimum requirements to get a V.90/92 connection are a virtually noise free local loop (modems which measure SNR for v.90 connections will show values from 45 dB for lower speeds all the way up to 55-60 dB with higher speed connections), plus a digital connection to the ISP which has exactly one Analog-to-Digital conversion (the codec in the line card for your line at the Telco switch) between you and the ISP.

V.90/92 modems preferentially connect in V.90 mode if they cannot they automatically fallback to V.34.

High-end connection speed of V.34 and low end of V.90/92 overlap. To determine if the modem is in V.90/92 mode external modems typically include a mode indicator. For modems without this feature connect speed can be used to determine mode. If the modem reports 28,000, 29,333, 30,667, 32,000, 33,333, 34,667, 36,000 or higher it is in V.90/92 mode. If the report is 33,600, 31,200, 28,800 or lower it is in V.34 mode.

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