Database Replication over Slow Link ???

We are trying to develop a distributed system over a slow network. The system is consisted of three sites ( more than hundreds of miles away),
each site is linked by a slow link, probably 56k bits/s. The system should allow a user to query and UPDATE data from the database at any site.

To reduce the network traffic, we are considering to have replicated copies of
the database at the three sites. This raises the multiple copy update problem.
Issues need to be addressed are: concurrency control-- two users may access
the same data at different site at the same time. If we use lock mechanism, what would happen if the link is down after the data is locked by a remote user? What will the performance be? Is there any database supporting this database distribution functionality? Any experience
with the performance? How much work will implementing such database replication
would involved?

By the way, we are using HP Unix workstation, HP-UX 9.01/9.05, if that matters.

Any comments/references will be appreciated.

Tao