ppp server

I am trying to build a ppp server on RedHat 7.2 with mgetty. I had follow all the intruction on the web about how to setup a ppp server.
After I tested, it can successfully auto answer the call, and check the password and user name. But once the checking process is done, the client will disconnect from the host. it occurs everytime. I found there is 2 different error msg on win98 and win2k client. On wi98, it says something like disconnecting by the host. On win2k, it is something like access denid, pls check user and password. I have no idea which part i go wrong. I alread setup a ppp account on host. pls help