As is the case with level 1 of the plesiochronous digital hierarchy (2
Mbit/s), the higher levels also use a frame
structure that begins with a frame alignment signal (FAS),
with the difference that, at these levels, multiplexing is carried out
bit-by-bit (unlike the multiplexing
of 64 kbit/s channels in a 2 Mbit/s signal, which is byte-by-byte), thus
making it impossible to identify the lower level frames inside a higher
level frame. Recovering the tributary frames requires the signal to be
demultiplexed. 
The higher hierarchical levels (8.448, 34.368 and 139.264 Mbit/s etc - for the sake of simplicity, referred to as 2, 34 and 140 Mbit/s) are obtained by multiplexing 4 lower level frames within a frame whose nominal transmission rate is more than 4 times that of the lower level in order to leave room for the permitted variations in rate (justification bits), as well as the corresponding FAS and alarm and spare bits.
