Difference between DS1 and T1

Hello mates,
DS1 and T1 signals are exactly the same signals except for a small technical difference. T1 carrier carries 24 speech channels and DS1 also has 24 speech channels with a difference that all T1 carriers have DS1 signals in them whereas all DS1 need not go only through T1 carrier.
In short, DS1 is a digital signal and T1 carrier is a physical carrier carrying DS1 signal in it.