2.9 USB Modems
USB = Universal Serial Bus. Most USB modems are winmodems, so many will not work with Linux. Linux has support for modems that conform to the USB Communication Device Class Abstract Control Model (= USB CDC ACM). There's a module for ACM named acm.o. See the /usb/acm.txt document in the kernel documentation directory (/usr/share/doc/kernel-doc-2.6.x in Debian, perhaps /usr/doc/kernel... in some distributions). The ACM "serial port" for the first (0th) such modem is: /dev/usb/acm/0 or possibly /dev/usb/ttyACM0. This should be the case regardless of whether or not you use the new "device file system". It's not really a serial port, but the driver makes it look like a serial port to software which uses the modem.
Since the bandwidth on the USB is high it's possible to send a lot more that just data to a USB modem. This means that it's feasible to create a USB winmodem where the driver does most of the modem's work on the CPU and sends the results to the modem. So beware of USB winmodems (unless they have Linux support).
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