My PC is connected to the Internet via ADSL. I have a 3G USB dongle, so if I connect both, will the Internet speed be the sum of both the speeds?
If the answer is "no":
Is it possible via alternative methods?
How do I know which connection my software is using? Can I control (by example) if my Firefox will use 3G and FreeDownloadManager will use ADSL?
This is possible through link aggregation or bandwidth bonding, but it's very difficult to achieve with low-end hardware, and also probably expensive.
The DD-WRT router firmware has some support for bonding here:
http://www.dd-wrt.com/wiki/index.php/Bonding
You would need to buy something like a CradlePoint device to convert your 3G dongle to ethernet, so it could connect to the router as one of the bandwidth sources, and have a DD-WRT compatible router.
Here's a way to do it on a PC with certain Intel network cards, but again, not easy and probably not inexpensive: http://www.intel.com/support/network/sb/cs-009747.htm
UPDATE : Here's a router than can do this out of the box. You'll probably still need a CradlePoint (not cheap) to convert the 3G to ethernet.
http://edimax.com/en/produce_detail.php?pd_id=213&pl1_id=3&pl2_id=65
What you are really looking for is for something that implements MPTCP -- Also known as MultiPath TCP.
It seems that there are few routers on the market that do support it at this moment but the situation will change quite fast in the next 12-24 months because the big players already adopted it: Apple (iOS and MacOS), Citrix (NetScaler), F5.
No. Unless you are an Autonomous System with your own address range, you cannot aggregate two lines to double the speed of a single host with a single address.
However, you can have two addresses with two separate lines (that will happen by default). The two interfaces will be exposed as different hosts, so (for instance) each tcp connection will have to use either one or the other. But depending on your operating system, you may be able to set up some rules to make some connections go trough adsl and others trough 3g. You can typically control this by changing the "bind address" of programs that use networking. The way to do it is program-specific.
It is probably more trouble than it is worth, however.
Another possibility, if you control some server outside on a fast connection, would be to put yourself on a VPN, make two separate connections to the vpn hub, and load-balance the routes between the two. Again this is not for the faint of the heart.
Dray Tek has routers affordable for SOHO usage which can do load balancing. See http://www.draytek.com/user/PdInfoDetail.php?Id=35
