BINARY TRANSMISSION AND THE CONCEPT OF TIME
received. Although such periods have no transitions, they carry meaningful information.
Likewise, the memory must maintain timing for reasonable periods in case of circuit out-
age. Note that synchronism pertains to both frequency and phase and that the usual error
in high-stability systems is a phase error (i.e., the leading edges of the received pulses are
slightly advanced or retarded from the equivalent clock pulses of the receiving device).
Once synchronized, high-stability systems need only a small amount of correction in tim-
ing (phase). Modem internal timing systems may have a long-term stability of 1
or better at both the transmitter and receiver. At 2400 bps, before a significant timing
error can build up, the accumulated time difference between transmitter and receiver must
exceed approximately 2
sec. Whenever the circuit of a synchronized transmitter
and receiver is shut down, their clocks must differ by at least 2
sec before signif-
icant errors take place once the clocks start back up again. This means that the leading
edge of the receiverclock equivalent timing pulse is 2
in advance or retarded
from the leading edge of the pulse received from the distant end. Often an idling signal
is sent on synchronous data circuits during periods of no traffic to maintain the timing.
Some high-stability systems need resynchronization only once a day.
Note that thus far in our discussion we have considered dedicated data circuits only.
With switched (dial-up) synchronous circuits, the following problems exist:
No two master clocks are in perfect phase synchronization.
The propagation time on any two paths may not be the same.
Thus such circuits will need a time interval for synchronization for each call setup before
traffic can be passed.
To summarize, synchronous data systems use high-stability clocks, and the clock at the
receiving device is undergoing constant but minuscule corrections to maintain an in-step
condition with the received pulse train from the distant transmitter, which is accom-
plished by responding to mark-to-space and space-to-mark transitions. The important
considerations of digital network timing were also discussed in Chapter 6.
Bits, Bauds, and Symbols
There is much confusion among professionals in the telecommunication industry over
terminology, especially in differentiating, bits, bauds, and symbols. The bit, a binary
digit, has been defined previously.
The baud is a unit of transmission rate or modulation rate. It is a measure of transitions
per second. A transition is a change of state. In binary systems, bauds and bits per second
(bps) are synonymous. In higher-level systems, typically
m-ary systems, bits and bauds
have different meanings. For example, we will be talking about a type of modulation
called QPSK. In this case, every transition carries two bits. Thus the modulation rate in
bauds is half the bit rate.
The industry often uses symbols per second and bauds interchangeably. It would be
preferable, in our opinion, to use "symbols" for the output of a coder or other conditioning
device. For the case of a channel coder (or encoder), bits go in and symbols come out.
There are more symbols per second in the output than bits per second in the input. They
differ by the coding rate. For example, a 1/2 rate coder (used in FEC) may have 4800
bps at the input and then would have 9600 symbols per second at the output.
Period of a Bit, Symbol, or Baud
. The period of a bit is the time duration of
a bit pulse. When we use NRZ (nonreturn-to-zero) coding (discussed in Section 10.7.5),