Huh ?
A brief explanation, from an older Electrical Engineer.
We always try and squeeze more data down a link.
There are 2 approaches.
Providers ( Like Telstra ) may use one or both.
1
Coding.
It is possible to have a logical coder at the transmitter end, and a logical decoder at the receiver end.
A CO(der) + DEC(oder) is often referred to as a codec.
Using a codec, it is possible to stuff more information down a link than plain vanilla transmission can achieve.
2
Electronics.
By deploying some smart electronics at the transmitter end, and (usually) even smarter electronics at the receiver end, it is possible to stuff even more data down a link.
By using both techniques concurrently, an even greater amount of data can be stuffed down a link.
This stuff was really important in the past, when limited capability links, like copper telephone lines prevailed.
In this modern era, with optic fibres and such like, it is less important.
More ?