There would be shared bandwidth somewhere in the path between your DSL circuit and your ISP but that does not mean that it would be limiting. More likely you would approach double the aggregate bandwidth with two DSL lines if you could keep both occupied. The real challenge might be balancing the load so you get significant utilization of both circuits. If you have enough users that bitmask or source-hash distribution would work, then that would be a good solution. Round-robin has the potential benefit of providing both WAN connections to a single user, but in practice some applications (FTP, some HTTP, others ?) will break with some hosts. Sorry I am not familiar with the Sonicwall product and its options for load- balancing. The terms I used are the ones that would apply if you set it up using pf. The concepts should be valid either way.
Your understanding is incorrect. Both DSL and a T1 provide you with your own personal circuit back to the central office.
It is true that a DSL might provide you shared access to a port to your Internet provider while a T1 would likely provide you dedicated access a bit further down the line. However, that port is likely to be 155Mbps or higher, so it's absurd to think that one DSL line to it is not better than two.
It's like saying you can't drink more water from the ocean with two straws than one because there's only so much water in the ocean.
Well, you have 2x to/fro you, so you should see considerable improvement on multiple simultaneous connections. You will not see it on a single connection.
"Shared bandwidth" is a very difficult term to define. Other posters have tried, and I may do no better:
The entire internet is based on shared bandwidth. That's what makes it so cheap. This isn't voice where a dedicated 64kb/s channel is made available end-to-end and it will literally carry silence. Your ISP takes your packets, mixes them with all the other customer's then shoves them all out one or more OC-3s (or higher). The real question is whether they have enough upstream capacity for their customers.
The main parts of the internet that are not shared are only the end links. They carry exclusively your traffic (on your T1, DSL or modem line) or the site's (on it's T1+).
Not bad. They have different IPs (interfaces have IPs, _not_ computers) so without very special link aggregation software (at both ends VPN?) why cannot be used for one datastream at 2x.
It could be 100% better. 2x bandwidth. But only the same for single TCP connections. So any given webpage will not load faster (nor any iso download faster). But multiple users at your place will experience better service when they otherwise would have collided.
Sorry to be late to the thread, but I haven't seen any answers I thought directly addressed your question.
never heard of a load balancing in DSL connection --->
but if your using two circuits, one might be for the internet and the second one could be for something else---->> as a matter of fact indeed this could only be the reason why you are getting what you are !!
In India MTNL is gearing up to provide video on a second virtual circuit !! And thats really cool :-)