In baseband transmission, the frame information is directly
impressed upon the link as a sequence of pulses or data symbols that are
typically attenuated (reduced in size) and distorted (changed in shape) before
they reach the other end of the link. The receiver's task is to detect each
pulse as it arrives and then to extract its correct value before transferring
the reconstructed information to the receiving MAC.
Filters and pulse-shaping circuits can help restore the
size and shape of the received waveforms, but additional measures must be taken
to ensure that the received signals are sampled at the correct time in the pulse
period and at same rate as the transmit clock:
� The
receive clock must be recovered from the incoming data stream to allow the
receiving physical layer to synchronize with the incoming pulses.
�Compensating
measures must be taken for a transmission effect known as baseline wander.
Clock recovery requires level transitions in the incoming
signal to identify and synchronize on pulse boundaries. The alternating 1s and
0s of the frame preamble were designed both to indicate that a frame was
arriving and to aid in clock recovery. However, recovered clocks can drift and
possibly lose synchronization if pulse levels remain constant and there are no
transitions to detect (for example, during long strings of 0s).
Baseline wander results because Ethernet links are
AC-coupled to the transceivers and because AC coupling is incapable of
maintaining voltage levels for more than a short time. As a result, transmitted
pulses are distorted by a droop effect similar to the exaggerated example shown
in Figure 7-12. In long strings of either 1s or 0s, the droop can become so
severe that the voltage level passes through the decision threshold, resulting
in erroneous sampled values for the affected pulses.
Figure 7-12 A Concept Example of Baseline Wander
Fortunately, encoding the outgoing signal before
transmission can significantly reduce the effect of both these problems, as well
as reduce the possibility of transmission errors. Early Ethernet
implementations, up to and including 10Base-T, all used the Manchester encoding
method, shown in Figure 7-13. Each pulse is clearly identified by the direction
of the midpulse transition rather than by its sampled level value.
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