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EVDO over OSX shared internet connection howto: natd and ipfw configuration

By Gnordic ·
Hello All,
This seems the most appropriate forum for a discussion by knowledgeable people on how to get EVDO sharing right. Many people out there are now using EVDO as a primary internet connection; its time we understood all the dimensions. Perhaps I'll find some guidance as well.

I use a Franklin CDU-680 EVDO USB dongle to connect via EVDO rev A. I want a simple solution, so was not willing to invest in a ready-made box that may or may not work such as the D-link or Kyocera routers with USB modem support.

I use a macbook pro as the modem server; the CDU-680 connects to the macbook pro, which connects to the internet. That connection is shared via Mac OS X's Internet sharing tool, which is a simple front-end to NATD and ipfw. The connection is shared over ethernet to an Airport Extreme Base Station (I've tried the Express with exactly the same results) which creates its own network and serves as my company's main net server.

Configuration specifics
When using the standard connection options on the Extreme (DHCP to the Pro server, bridged connection rather than creating another NAT and DHCP server) the connection is relatively reliable for light usage, but under heavy usage I can get around 30 MB of download before the connection drops (dialog comes up on the server Pro saying the PPP server terminated the connection). This always happens. By the way, this connection is nice and fast: 2.6 Mbps down speed.

The connection gets much more reliable (150 MB of data downloaded before connection termination) when the Extreme router is set up to use a static IP address. No change if the connection is changed to creating a NAT and DHCP server on the router. This implies that the problem may lie in packet collision due to subnet overlap, an error by the Internet Sharing-initiated DHCP and NAT server. However, since creating another NAT and DHCP server on the Extreme router doesn't change things, it seems to be another issue. My suspicions are:

MTU difference causing some sort of problem?
packet collisions?
NATD or ipfw setup problem?

I have ruled out wireless interference by switching around 2.4Ghz and 5 Ghz frequencies and channels.

The most likely culprit is Apple's implementation of NATD and ipfw in Internet Sharing. So I'm working on modifying the connection via the command line--but I don't know what the likely culprit is. Any suggestions on what NATD and ipfw settings to play with would be appreciated--I haven't delved into this before. Thanks for any hints or discussion on this topic.

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