VPN over dsl/gprs

[iiiiss]

Hi !

I have a serious problem . I can establish a VPN connection to the PIX over a cable modem without any problems but when I try to establish the connection over gprs... the remote peer is not responding .. debugging just response many lines but they don´t help to solve the problem ..I tried mtu size 1500, 1400 and 1300(default)

When I try to establish the vpn connection over dsl, the tunnel is beeing built but I can´t ping anything ... for the test with dsl I set the mtu size of the outside interface to 1400....

Any suggestions ..

I don´t get it..

Many thanks in advance...