The T1 and E1 carriers are the firs

The T1 and E1 carriers are the first level of multiplexing in the Plesiochronous Digital Hierarchy (PDH).
Four first level carriers are multiplexed together to create a higher capacity second level carrier.
Each subsequent level in this digital hierarchy multiplexes together four or more carriers from the level below (the only exception is the intermediate level DS1C between DS1 and DS2 in the North American system).
The bit rates for each level of the North American, European and Japanese Plesiochronous digital hierarchies are shown in the table below.