Have been looking for this solution in 2007 when airtel was not having 512 unlimited plans & I think I found one but didnt try bcoz I finally got the cable connection 512 unlimited. Try this if it works then let us know how smooth it is & what kind of problem you face. Its always good to have this option.
Wingate proxy can do this.
http://www.wingate.com/products/wingate/index.php
multiple Internet connections
You can use multiple Internet connections at the same time with WinGate, thereby increasing your system throughput. On a per-proxy basis in WinGate, you can specify multiple methods of using these multiple connections as well.
for instance you could:
* Specify that the WWW Proxy uses all your available internet connections
* Specify that another proxy uses only one of the connections, but if that becomes unavailable, to fail over to the next one
WinGate monitors connections for availability, including remote gateways, so even if your Internet connections go through another router or a device such as a DSL/NAT device, you can still keep track of it.
WinGate's gateway selection features also allows you to specify on a per service basis which gateway will be used, so if you had a combination of multiple DSL/NAT devices, network gateways, modems, etc, you could still specify which connections go through which gateway, even if they are on the same physical ethernet segment.
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