ns2 does not have built-in multi-rate support for wireless links, not even in the latest 2.28 release. There is an RBAR (Receiver Based Auto Rate) and OAR (Opportunistic Auto Rate) extension for ns2 available at Rice Networks Group. Unfortunately it applies to ns2.1b7a only. I’ve created a patch and tried to apply it to the recent 2.28 version with no success, too much code has changed since then. The patch is still useful: it’s easier to apply to 2.1b7a than the Rice Networks Group release.