RAS via Euro ISDN - High latency problems

[E1UK]

Hi All,
Scenario:

ISDN Dial-up connection into a RAS from User X.

Part of the route is over a high round trip latency of >=500mS, unknown latency on the terrestrial ISDN network.

Time for log in and IP assignment is around 13 seconds.
Anyone know of a way to speed this up?

Z-Modem induced modem to modem call takes 2 seconds or so to establish hardware linking.

Appreciate anyones input who may have been in similarly head scratching position.
Cheers
E1