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SONET 59

However, the signals are multiplexed one bit at a time into a DS-3
signal using a DS-3 multiplexer (the M13). Because each DS-1 signal
operates with its own clock, phase and frequency deviations may
exist among the individual bit streams. This means that the bits can
arrive early or late, in contrast to the other inputs. This is shown in
Figure 3-3, which presents a conceptual view of the arrival sequence.

Bit Stuffing

To compensate for the potential problems and delays in the data
arrival, the equipment must be capable of inserting a bit into an
empty slot. This is called bit stuffing. The bits will be inserted into
blank slots so that if a DS-1 line is slow to deliver the necessary bits,
substitutes will be put in their place. This creates a scenario where
the data can be aligned properly. Unfortunately, when we stuff these
bits into a DS-3 data stream, the equipment at the receiving end can-
not tell the difference between real data and fake data placed there
to keep the slot occupied.Thus, the multiplexer does not know where
to look for its reference point to find the DS-1 signals inside the DS-
3 stream.To get back to the DS-1 signals, the entire DS-3 stream (all
28 DS-1 signals) must be demultiplexed.This is time-consuming and
expensive. Figure 3-4 illustrates the bit-stuffing process.

Direction of data flow
Figure 3-4
Bits are “stuffed”
because the time
slots were empty.
Timing Expected
arrival of bit at center 1 1
line 0 0
10 1 0 1 0 1 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 0
S S S S