ATM and TDM are two types of data transfer technologies. TDM stands for Time-division multiplexing, which is a method of combining multiple data streams into one and sending it together over one signal. ATM stands for Asynchronous Transfer Mode. It is based on a fixed “cell” that is used to convey a Multimedia payload, i.e. Voice, Data, etc..
Summary: ATM uses “cells” whereas TDM uses a “nailed up” TimeSlot to convey information. Internally, HSL uses ATM “cells”.
Over 20 years ago when TDM was overloaded with the signalling load there was the move to 2Mbits/s HSL. This was an interim step before the full adoption of Sigtran (SS7overIP).