Regarding a backup scenario

Hi all,

I want some information regarding backups.

The scenario is

I have got a master server which has Linux as OS and postgres as Database
Other than this server i have got 5 client servers(same configuration as
All the transactions are done at different client locations and the
data is inserted into respective client servers. I would like to
maintain a backup of all client servers at 1 place ie at master
location.

My Idea:
I am thinking to take incremental backup for every 30 min and send the
.bak file to master server place so that the file is inserted into
database like the same i want to do the same at every client place for
30 min send the backup to master server.
but, as from the documentation there are no incremental backups at
database level.
I cant go for replication because the distance between each client and
master server is in order of around 60 Kilometer and there is problem
with network connectivity.

Please give me some idea so that this setting can be done

Thanks in Advance
Nandan