Broadband data is sent down telephone lines as lots of oscillating electrical signals layered on top of each other. Your modem divides the total available frequency range into channels of 4.3125kHz each and analyses the amount of interference in each one. The more interference in a given channel, the less data per second that can be sent at that frequency, without introducing errors.
Living more than 4km from your telephone exchange, old telephone wires in your house, having more than five phones connected to the same landline and long telephone extension cords will all introduce electrical interference. Fibre-optic broadband is much faster because it uses light pulses in special glass wires that don’t suffer from interference in the same way.
Answered by Luis Villazon