The sellers make lots of claims for “HIGH SPEED.” For the most part, it would be a mistake to believe any of them. Note the very small printed up to.
With very minor exceptions, there are really only two choices. One uses the twisted pair wires of the telephone infrastructure — this is referred to commonly as DSL or ADSL. There are many resellers, particularly of ADSL. The second uses the coaxial cable of the cable TV infrastructure.
Both infrastructures are gradually being upgraded by replacing parts of their systems with fiber optic cables. Fiber optic transmission is dramatically faster and more reliable. The extent to which these fiber optic cables reach your computer modem will have a huge effect on what kind of performance you can get. The closer the fiber to your modem, the better. If you are considering DSL, ideally, the fiber should be no more than 300 meters wire length away. That effectively means on the pole outside your home/office.
The modem is the device you rent or buy from your Interent provider that connects your computers to their network.
Upgrading these infrastructures to fiber optic cable is taking place but at a slow pace and it has not reached most homes and very few businesses. So most have to choose which of these systems to use. Interestingly, pricing is about equal, regardless of performance. Most people do not seem to be aware of the difference, although in many cases, the difference is quite substantial. Effectively, you can get much more for the same money if you choose correctly.
Coaxial cable is capable of much greater bandwidth (speed) than twisted pair. ADSL (twisted pair) is much more vulnerable to less than perfect connections. The fact that coaxial is faster than twisted pair is physics, not quality of management. In an older building, twisted pair connections are frequently not perfect and there can be 10 to 100 in a typical home. Many of these connections are buried in inaccessible places behind walls. If any one of them is not perfect, ADSL performance will downgrade quite significantly. Most of these telephone wires were installed for telephones, not computers, when quality of these connections was not nearly as critical.
The bottom line is that unless the telephone network has fiber optic cable outside your window, it will not perform nearly as well as cable. In my neighbourhood, coaxial cable delivers at least twice the performance at the same cost as the telephone-based system, AND is equivalent in price.
If you would like to test your current connection: www.speedtest.net will give you real world actual performance to compare to the claim from your provider. You may be surprised to see the difference.