Few years ago the main business case for low latency was telephony. And that is the reason why there is/was so much inertia with replacing various frame based T

Few years ago the main business case for low latency was telephony. And that is the reason why there is/was so much inertia with replacing various frame based TDM/SDH/SoNET systems with packet based solutions (this was whole reason why ATM was developed as it is sort of in between of these extremes but still does not have latency and QoS characteristics that were originally sought). These systems are designed to either induce constant latency (TDM, one frame period per switch) or induce lowest possible latency (SoNET/SDH with very clever mechanism to construct frames without knowing their full content and decoding them incrementally, which gives best case latency of few bit times + latency of any fiber interface, worst case comparable with traditional TDM). The problem with latency and telephony is two fold: echo cancellation for single pair analog phones (phone itself only substracts outgoing signal from incoming so anything that is echoed from other side of line goes through) and maximal latency for comfortable conversation (which is surprisingly small). First problem can be solved with smarter interfaces that do echo-cancellation in some DSP (some kind of license for doing that is major part of cost of analog interface cards for open source PC based softswitches), second not so much. So in all you can reasonably jam remotely (on distances of say less than 2000km) using this infrastructure and using it is probably few orders of magnitude cheaper (renting channels in SDH/SoNET optical networks is actually surprisingly cheap) than laying special fibres.