UbuntuHelp:DialupModemHowto/Modems
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This page gives you some information on modems. See also: DialupModemHowto FaxHowto
目录
Modems and Fax Modems
Basics
Modulator / Demodulator. The kind of Modem we are talking about are devices that are used to transfer data over a plain old telephone line. These can be the most reliable/available form of long distance networking, it works even when DSL and cable are out, however speeds are limited to about 53 Kbps down and 33.6 up if connected to an ISP or other network provider. The fasted one can transfer data between two regular modems is 33.6 kbps (v90). maybe 48 kbps (v92) Converting binary data into sound, transmitting it and listening on the same line while using the full allowed capacity of a phone line is a complicated process, Modems must negotiate what protocols to use, cancel out echos, test and compensate for line noise, and perform error correction. Newer (v92) modems can even pause the internet connection to allow for a phone call. If you only want news, and email, turn off flash and images in your browser, and a 56k modem and a cheep prepaid ISP will likely work fine for you and be the cheapest way to get online. You may live in an area where there is no broadband, and this is all you have! It may also be possible to use two phone lines and two modems to double your speed.
Recommendations
- take a look at http://xmodem.org It's a great list, if you go to the pci list (or pcmcia if you have a laptop) and
- search for cards with the green chip icon, you'll probably find one that works.
- If you might want to have new features, or are really cheap, try clicking on the the ones with the penguin. You will have to follow the links because support has been dropped from some. at 2.4 and some for 2.6.8 (2.6.19 kernel for Feisty)
- If you can find one an internal hardware modem would provide less clutter and work with least hassle. PCI hardware modems are very hard to come by. ISA modems are easy to find, but ISA slots hard to come by on anything newer than a 1gz PIII computer.
- You probably already have a software modem. You might be able to selectively purchase one that works, but it is hard to tell what the chipset is from the package. In general, you a probably best off with an intel dq....836 or 837 chipset modem. it may take work, but you won't have a tangle of wires. Security is questionable, because the closed source drivers often run in kernel space.
- If you don't mind a tangle of cables, and have an old PC with the right ports, external modems are good.
- Current Mainstream modem advertised specs: v.92 56k, -- this has lots of nice features, If you can't find one, you should be able get by with one labeled v.90 56k.
- Remember, with modems, newer isn't always better. the 836 intel chipset has a DSP, the 837 does not. It's a trade off. 837's will be cheaper but will use more cpu. the 837 might be faster, but the software may need more configuration.
Software modems
You most likely have a software modem. (unless you bought before 2000... if if it's much earlier than that then you could even have one that is slower than 28.8) Interfaces: PCI, AMR, Mini-PCI pcmcia, or USB, or built into motherboard. Most of these are not supported under Linux, yet quite a few work after extensive tweaking or purchasing drivers. These use software on the host computer to do the majority of the work. This software is often guarded by the manufacturer and is typically not supported, not open source, and very complicated and hardware dependant. In practice the software is extremely complex and guarded by the manufacturers, so support is very limited. These companies go out of business or drop support so that they can sell new products. A Linmodem is a software modem that works thanks to the Linmodems.org project. Most software modems have closed source drivers only released and or officially supported only with windows, so they are known as winmodems and you should avoid them. The ultimate modem would be software or DSP based(so it's upgradable, and has additional features) (GPLed driver so that actually happens), and be officially supported. It is unclear if any exist! It may provide more flexibility, including upgradability and additional features some even can provide answering machine functionality and theoretical lower latency. There are a few varients of these, Digital Signal Processor (DSP) modems do the work on the modem, but need a special driver,
Hardware Modems
There are very few v92 56k ISA modems, v90 56k is common, but you'll run into a lot of 33.6 and 28.8 ones. 14.4 and 9600 are also out there, and you probably don't want them! speeds are typically not listed on the cards.
PCI Modems
A PCI modem card is one which inserts into a PCI-bus slot on the motherboard of a PC. While many PCI winmodems will not work under Linux (no driver available) other PCI modems will work under Linux. The Linux serial driver has been modified to support certain PCI hardware modem cards (but not winmodems/linmodems). If it's a linmodem, it will work only if you install a certain linmodem driver. If the Linux serial driver supports your hardware modem then the driver will set up the PnP configuration for you.
RS232 Serial
Advantage: Works! Disadvantages: Requires "legacy" ports, desk space, rats nest of cables, rare, may be slow(most are not upgradeable to new standards), and expensive (v92 $30+!!!) External Modems that connect to a serial port (RS232 - 9 pin, 25 pin or maybe db8) don't need the complicated software to do the modulation/demodulation. If you have an old style serial port, an extra plug for a wall wort, and the appropriate serial cable and port(for PC's many need 25 pin to 9 pin adapter/ and cable), These work extermely well. These modems don't need a special driver. If your modem connects to the serial port and you know the COM port, the device name is /dev/ttySx, where x is one less than the COM port number. For example, if your modem is on COM2, the device is /dev/ttyS1, or if your modem is on COM1, the device is /dev/ttyS0. This is because computers start counting at 0. You can use this device name to configure your dial-up connection. If you only have USB ports, you may find a USB to RS232 adapter, but that will require a driver... it should probably just work, but this author hasn't tried one.
