It all depends on what your ISDN counterpart supports. If it immediately begins to send frames, then you've probably reached an sync PPP machine. Usually using sync PPP works fine, and it is slightly more efficient. To take advantage of newer features of the pppd , use async PPP . It is very important to set up the authentication information properly.
Improper authentication is probably the most frequently reported problem on the mailing list.
Please, work through section pap completely yourself, before asking others for help. Next time, configure ipppd without these unused options. For everyone for whom that's to expensive and who use async PPP , there's a little trick. See this question for Sync PPP, it works the same way for pppd. This is probably due to an incorrect block size on your side.