how to two way sync multi-location openerp (postgre) databases

I am interested to know whether you have a solution for my problem.

I want to have a setup where we have multiple shops in
remote locations. Head office has central server and each local shop
has its own openerp server and database.
The scenario is as follows:

Head office: - Main OpenERP server & Postgre server (installed on same 

machine like All in one) 
Branch Office1 - BranchServer1 (OpenERP server & Postgre server installed 

on same machine like All in one) 
Branch Office2 - BranchServer2 (OpenERP server & Postgre server installed 

on same machine like All in one) 
Branch Office3 - BranchServer3 (OpenERP server & Postgre server installed 

on same machine like All in one)
.. 
.. 
.. 
Branch OfficeN - BranchServerN (OpenERP server & Postgre server installed 

on same machine like All in one) 

Branch Servers will maintain their data and will sync with the main server 

only at decided time/interval. This will help in not requiring 100% 

connectivity to the main server, and still head office will have all the 

data (surely there will be lag due to frequency lag). So headoffice can 

pull corporate level report, and then drill down to branch level and 

further department level and employee level. All branch sales will synch to corresponding shop on central server etc. New products on central server will also get updated on local server. Basically a two way sync.
The internet connectivity is not reliable so we cannot host these online centrally. therefore if the internet went down the synch would resume after.

Do you have a solution for this? or suggest something

Thanks....

Peter