async PPP is a character-based protocol which is usually used over
analog serial lines (async = asynchronous). You have to use the program
pppd for it, and use it with the ttyI* devices.
In contrast, Sync PPP is a bit-oriented protocol (sync = synchronous),
for which the original pppd cannot be used. Michael Hipp has written an
adapted version called ipppd which will use ipppd* net devices.
With i4l you can have both. 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. If you can log in via same terminal screen, and then can start ´pppd´, this can be an indication of async PPP.
Usually using sync PPP works fine, and it is slightly more efficient. To
take advantage of newer features of the pppd, use async PPP.
You can write out a login session with ("Debug-Log"), and see which options the other computer is refusing. Next time, configure ipppd without these unused options. A further side effect is that such unused options increase the redundance (e.g. when the other computer has bugs and refuses the options incorrectly). To create a log file, see "How to I create a log for ipppd".
You can add more channels with MPPP (see the appropriate section). For everyone for whom that's to expensive and who use async PPP, there's a little trick. With the option "asyncmap 0" you can avoid escaping all control characters (ASCII32). If the other side goes along with this, you can increase the transfer rate by about 12%.
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. Initialize your
ttyI* device with AT&B512 or even smaller block sizes.