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58 Chapter 3

Figure 3-2 DS-1 DS-1 DS-1 DS-1 DS-1 DS-1 DS-1
DS-3 mapping and DS-1
DS-1 DS-1 DS-1 DS-1 DS-1 DS-1 DS-1
multiplexing uses DS-1
28 DS-1 signals DS-1 DS-1
inside.

the information, although using synchronous DS-1 signals inside, is
transmitted in an asynchronous manner. Asynchronous means that
the timing of signals carried on the network is random.

Asynchronous Transmission

Just what does it mean to say that a signal is asynchronously trans-
mitted, especially that DS-1 signals are synchronous? SONET is
synchronous. DS-1 is also synchronous. However, DS-3 is asynchro-
nous! Are you confused?
When using a copper- , radio- , or fiber-based network, asynchro-
nous refers to digital signals generated on equipment that operates
in one of two conditions—using its own internal clock (timing source)
or using separate timing sources (master clocks). Conceptually,
again, we have to understand what is happening. DS-1 signals
are synchronous unto themselves but not unto others. This means
that a DS-1 signal is internally timed and operated synchronously.

Direction of data flow
Figure 3-3
Because some data
bits arrive later
than other bits,
Timing Expected
timing can be a arrival of bit at center 1 1
line 0 0
problem. 1 1 1
10 1 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0 0 0
1