When connecting, modems do not always negotiate their fastest built-in modulation rate. Depending on the quality of the phone line connection and the quality of the modem hardware, two modems may negotiate a certain modulation standard, but may not use the maximum speed defined in that standard. For example, two V.34 modems, made by the same or different manufacturers, may negotiate a modulation rate in 2400-bps increments somewhere between 2400 bps and 28800 bps. They may agree to use 2400 bps because of noise in the telephone line.
Even though the line speed may be only 2400 bps, these V.34 modems are still using the V.34 modulation scheme. V.34 modulation also enables the modems to change the line speed dynamically during a call, in response to changes in the phone line quality. Other modulation standards may also enable the modems to connect at slower speeds, but may not enable the modems to dynamically adjust the line speed during a connection.
To change to a different modulation scheme, as opposed to just changing the speed within a modulation scheme while the modems have a connection, the modems need to support an error control protocol that supports this functionality. The non-standard MNP10 error control protocol is an example. However, the MNP10 protocol is needed only if the modem is not a V.34 modem and only if the modem needs to downshift from 4800 bps V.32/V.32 bis to 2400 bps V.22 bis (or upshift, accordingly).
RAS is usually oblivious to the dynamic speed changes. However, MNP10 modulation does change, and very slow connections that may occur with cellular modems or with low quality telephone lines may cause time-out problems in the RAS network protocols. Only Windows 95 supports cellular modem connections.
Table E.2 displays the fastest possible modulation mode likely to be negotiated when modems configured with the same or different modulation modes attempt to make a connection. This table assumes that modems with different modulation configurations and capabilities are configured to enable them to negotiate up or down to the highest modulation standard that the other modem supports. If both modems are not configured to negotiate to a different modulation, and their modulation settings differ, they cannot establish a connection.
For example, if a V.22 bis (2400 bps) modem and a V.32 bis (14400 bps) modem try to establish a connection and the V.32 bis modem is not configured to negotiate down to V.22 bis, the modems are unable to establish a connection. If the V.22 bis modem is not configured to negotiate down, but the V.32 bis modem is configured to negotiate down, then the modems can connect at V.22 bis (but not at V.22).
Table E.2 Modulation Modes
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V.22 | V.22 | V.22 | V.22 | V.22 | V.22 | ||||||
V.22 bis | V.22 bis | V.22 bis | V.32 bis | V.22 bis | V.22 | ||||||
V.32 | V.32 | V.32 | V.32 | V.22 bis | V.22 | ||||||
V.32 bis | V.32 bis | V.32 bis | V.32 | V.22 bis | V.22 | ||||||
V.fc/V.fast (V.32 bis if V.fc/V.fast is not supported by answering modem) | V.fc/V.fast | V.32 bis | V.32 | V.22 bis | V.22 | ||||||
V.34 | V.fc/V.fast (V.32 bis if V.fc/V.fast is not supported by calling modem) | V.32 bis | V.32 | V.22 bis | V.22 | ||||||
Note
V.fc and V.fast are proprietary modulations that use modulation technology similar to V.34. Therefore, two V.fc or two V.fast modems from different manufacturers may not be able to connect at V.fc or V.fast, respectively, and may have to negotiate down to V.32 bis. Also, the use of the V.fc and V.fast names may be inconsistent between manufacturers, making it difficult for a buyer to determine compatibility between these modems. The V.34 standard does not include support for V.fc and V.fast. Therefore, some V.34 modems that do not support V.fc or V.fast fall back to V.32 bis when connecting with a V.fc or V.fast modem.