Nah, there's no real limit. It all depends on the longest distance between a sender and a receiver (there might be more than one on your last-mile) the quality

Nah, there's no real limit. It all depends on the longest distance between a sender and a receiver (there might be more than one on your last-mile) the quality of the cable and output power on the sending end. European cities seem to have settled one 15mbps as both standard and max, American ones seem to have settled on lower. The phoneline infrastructure was probably not as good.