Reference Manual

HOW TO configure NAT32 for USB DSL Modems.

HOW TO choose a DSL Modem.

Several vendors supply low-cost DSL modems that connect to your PC via a USB Port. Such devices are very convenient because they are easily attached to your machine, require no external power supply, and are handled by NAT32 in the same way as any conventional Dial-Up Networking connection. No special configuration within NAT32 is needed, and no MTU problems arise.

However, before purchasing such a device, you should consider the following factors:

  1. Most of these devices support only USB Version 1.1, which means that the maximum bandwidth available on any USB adapter is 12 Mbps, half-duplex.
  2. Many of the low-level DSL-related protocols are performed on your PC, resulting in a major increase in CPU load.
    Drivers for the following Adapters and Protocols are typically installed on Windows:
    ADSL USB (Native ATM Mode)
    ATM Call Manager -> ADSL USB (Native ATM Mode)
    NDISWAN -> PPP over ATM (miniport)
    NDISWAN -> PPP over ATM (miniport)
    PPP over ATM (miniport) -> ADSL USB (Native ATM Mode)
    PPP over ATM (miniport) -> PPP over ATM (protocol)
    PPP over ATM (protocol) -> ADSL USB (Native ATM Mode)
    In addition, a Control and Status application is loaded at startup and usually places an icon in your System Tray.
    Given that ATM transfers data in 53-byte cells, your CPU will be kept very busy handling this traffic over the USB bus. In addition, your private LAN adapter will require CPU cycles, as will any other USB devices you might be using at the time, such as printers and external disk drives.
  3. While many vendors claim that their devices require only a Pentium 2 CPU, tests have shown that a 400 MHz Celeron is a more realistic lower limit for a 256K DSL connection, and then only if no other USB devices or private LAN adapters are used at the same time. For faster DSL connections, or if you are also using other high-bandwidth USB devices, at least an 800 MHz CPU is required for reasonable perfomance. If you are running Windows 7 or higher, a 1000 Mhz CPU (or better) is required.
  4. Many USB DSL modem drivers do not allow any additional Dial-Up Networking connections to be established at the same time. This can be a problem if you subscribe to two or more DSL services, or if you need to use a conventional 56K modem connection to supplement the DSL connection, or if you need to use a VPN connection "on top of" the DSL connection. It is unclear whether this limitation is caused by the OS, the DSL drivers or simply insufficient CPU performance.
  5. You cannot connect more than one USB DSL modem to your computer at a time. Again, the reasons for this limitation are unclear.


Unless you are on a very tight budget, avoid USB DSL modems and purchase an Ethernet-connected DSL modem instead. Use the free RASPPPoE driver (on pre-Windows 7 systems), and never install the bloated software most ISPs give you on CD when you subscribe to their service.


Configuring NAT32 for PPPoE
Speedtouch FAQ
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