DSL vs T-1?

Why can't a DSL line that gets speeds of 1.5Mbps be used instead of a T-1 line? Can you please explain using an example of connecting a branch office to headquarters 20 miles away? Thanks for your help.

It can be, in some cases.

A DSL line is a consumer based product which provides a 256k to ~5mb connection (or even more) depending on the distance from the CO (Carrier Office) and the type of DSL used. DSL is typically sold in a layer 3 (OSI Stack) service providing a direct connection to the internet. If you had two connections via DSL that is connected to the public internet then you would need to create a VPN between the two.

A T1 line is a 1.544mb connection using time division multiplexing from the phone company. These can be sold as a layer three service with a direct connection to the internet, or as a layer two service which could privately connect two offices without touching the public internet. This is a business grade service and T1 outages are typically repaired quicker than DSL outages. Most T1 service from major carriers will also come with a service level agreement to ensure a quality connection around areas such as jitter (frequency between packets), latency (delay of packets, lag), and uptime.

The benefit of DSL is that it is cheap. The benefit of the T1 is the quality.

T1 lines are a much more sturdy delivery "pipe" than DSL and is much more capable of supplying the necessary connectivity to multiple computers than a single DSL line (even if your network has good switches and routers).

well, soemtimes you actually can, it all depends on the type of connection you need. But typically a point-to-point T1 is what is used for a main/branch office style scenario. 2 reasons, a 1.5mbps DSL is not really 1.5mbps. It has a burst rate of 1.5mbps and an upstream that is likely a fraction of that (unless you are looking at SDSL, which is much less commonand much more expensive). a T1 is truly a 1.5mbps pipeline, both ways. A T1 can also be set up to connect the two site almost completely directly to each other. The DSL connection would be a connection to the internet and then you could set up a VPN or tunneling to simulate the direct link between the 2 sites. There is considerable bandwidth overhead on a VPN setup.

I am definatley can tell you way but i can tell you that t-1 is better than normal DSL.

The DSL Line can do 1.5 mbps but would go thru the internet to the computer at the home office (using your example) unless you had a static IP address which costs more. Then, you'd get 1.5 mbps for one pc in that office, and less for every pc you add to the router in that office, but you could do it and probably get away with it if it's a small office. With T-1 you'd get 1.5mbps across every connection in the building, if I understand it correctly.