Xref: utzoo news.newusers.questions:2629 comp.dcom.modems:6594 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!aplcen!uunet!saxony!dgil From: dgil@pa.reuter.COM (Dave Gillett) Newsgroups: news.newusers.questions,comp.dcom.modems Subject: Re: Modem Speed Message-ID: <358@saxony.pa.reuter.COM> Date: 27 Aug 90 21:02:37 GMT References: Organization: Reuter:file Inc (A Reuter Company) Palo Alto, CA Lines: 46 In genesis@kk4fs.UUCP (Genesis) writes: >Is it possible to push a 2400 baud modem to 4800 without doing any damage?If >so,what must i do to do this?THANKS! There are three areas where you might wish to change the bps rate of your transmissions. Each has its own problems and pitfalls. 1. CPU -> modem This is the digital link from the computer to the modem. (This may not be relevant if you have an internal modem.) If you send characters too fast here, the modem will not be able to handle each one before the next arrives. A Hayes-type modem uses the "AT" at the front of a command to determine the rate at which you are sending to it--in the process, it may not be able to do anything else with them. In order to detect the speed you are using, it must sample the line at some high frequency; the exact frequency used imposes a physical maximum detectable rate. The firmware in the modem imposes a logical maximum configurable rate--which is probably 2400 in this case... 2. modem <-> modem This is the analog link between your modem and the other modem, probably over a telephone line. At higher baud rates, signals are more likely to be lost due to noise. Most 1200, 2400 and above modems actually use 600 baud in each direction, and use the phase of the signal (okay; 9600 gets more complicated...) to encode multiple bits per baud. Since you probably don't want to tinker with the phase encoding/decoding logic in the modems, your alternative is to switch states more often. 2400 bps modems are already at about 80% of the reliable baud rate for voice-grade lines, so switching states twice as often means that you're likely to encounter noise bursts big enough to wipe out entire groups of bits. No good. 3. modem -> CPU This is the digital link back from the modem to your CPU. At some low level, your code that talks to the serial port has to be able to grab characters as fast as the modem can supply them. This is subject to a lot of variables, but it *can* be a non-trivial problem, especially above 2400 bps, which you should be prepared to tackle. Again, the modem components (especially firmware) may impose a maximum here. Conclusion: Assuming that you can get around logical conditions imposed by the modem firmware, you're likely to be running up against constraints imposed by the physical universe. It's not likely to be worth the effort; it's definitely not going to stand comparison against buy 9600 bps modems. Dave