At least from the point of view of a telecommunications carrier, when we say "leased line", we mean a physical, point-to-point connection billed per air mile. If you order a leased line from San Francisco to New York, we will obtain a leased circuit from the local telco to a long-distance inter-exchange carrier, and then another local loop as we call it, from the IXC to the New York telco destination. You pay for all it, at so many dollars per airmile. Note that the line by not always be a wire -- we may transform your "circuit" into a digital link along the way.
Serial lines are lower-speed leased lines, typically less than the standard 1.544Mb/2Mb/s lines. We don't use those much any more as few people want 128Kb/sec circuits.
A dedicated line is a leased line where your "line" may be virtualized along the way. You have a dedicated virtual pipe at a given capacity. Typically, you'd buy for example, a dedicated 10Mb and we'd create a local link to you, perhaps metro ethernet, turn that ethernet into a long-haul shared ethernet circuit (2-10Gb), and then back into ethernet at your end.
It comes down to this:
- If you need low-speed connectivity point-to-point, that's a serial
line, but I'd recommend going straight to T-1 dedicated.
- If you need higher-speed dedicated links without any possibility of
sharing the pipe along the way (perhaps for compliance reasons),
that's a leased circuit.
- If you need high-bandwidth point-to-point, but you can tolerate
"commingling the pipe", that's a dedicated connection.