Principles of Synchronous Digital Hierarchy

200 Principles of Synchronous Digital Hierarchy

These justication bits are added before multiplexing in the transmitter, and

are removed after demultiplexing in the receiver to get the E1 back into its

original shape.

But how does the receiver know that the incoming E2 stream contains jus-

tication bits or not?

This information is conveyed to the receiver by means of “justication control

bits.” The justication control bits are carried in the E2 bit stream at xed loca-

tions in each frame. To understand the concept, let us have a look at Figure 8.14,

which shows an arbitrary bit stream (for illustration only), showing one justica-

tion bit ...