Transferring data from Android to MS SQL Server

We are extending a PDA enterprise application which is in Windows to Android.

The application is intended for sales persons. When a sales person visits a shop, they will put invoices from the PDA and send the information to the back end which is hosted in IIS. MS SQL Server (2008 R2) runs as the data base server.

Communication could happen through GPRS, WIFI or through connecting the PDA to a lap top via a USB cable (if the GPRS connectivity is not available at a certain location).

Currently we have the application in windows mobile and have deployed WCF (Windows communication foundation) services to connect to the back end (also we use sql server replication at certain instances).

With our new extension to Android (while obviously the back end is still being the same) what would be the best mechanism to transfer data from the client PDA to the server (at any single instance there can be around 1000 PDAs in operation with many bytes of data uploads; and the speed matters)?

I am not sure whether the question is well suited here. Do let me know if it is not.

Thanks in advance.

If you already have the WCF services published for the WM apps, I would just build an Android client for them. You could consider exposing the services with JSON so that the payloads are smaller that the XML equivalents and probably faster to parse (though you'd have to measure that). What you won't be able to use directly are the replication endpoints.

Dunno if there is a WCF analog for Android but you could use sockets to move your data and have the sock server talk to your db. SSL would make it all safe and clean and the (basic) code is pretty quick to implement.