
1000BASE-T Functional Description
2-11
The requirements for the PCS receive functionality are also defined in
the IEEE 802.3ab specification in section 40.3.1.4 “PCS Receive
Function.”
2.2.4.1 Delay Skew Compensation
This function is used to align the received data from the four PMA
receivers and to determine the correct spatial ordering of the four
incoming twisted-pairs (which twisted-pair carries A
n
, which one carries
B
n
, and so on). The deskewed and ordered symbols are then presented
to the FEC Decoder. The differential time or time delay skew is due to
the differences in length of each of the four pairs of twisted wire in the
CAT-5 cable, manufacturing variation of the insulation of the wire pairs,
and in some cases, differences in insulation materials used in the wire
pairs. Correct symbol order to the FEC is required, since the receiver
does not have prior knowledge of the order of the incoming twisted-pairs
within the CAT-5 cable.
2.2.4.2 Delay Skew Control
This function controls the delay skew compensation function by providing
the necessary controls and selects to allow for compensation in two
dimensions (time and position). The time factor is the delay skew
between the four incoming data streams from the PMA RX A, B, C,
and D. This delay skew originates back at the input to the
ADC/DAC/Timing subsystem. Since the receiver initially does not know
the ordering of the twisted-pairs, correct ordering must be determined
automatically by the receiver during start-up. Delay skew compensation
and twisted-pair ordering is part of the training function performed during
start-up.
2.2.4.3 Forward Error Correction (FEC) Decoder
This function decodes the quartet of quinary symbols from the PMA
receivers and generates the SD
n
binary values. The FEC decoder uses
a standard 8-state Trellis code operation.
The FEC decoder decodes the quartet of quinary (PAM-5) symbols and
generates the corresponding SD
n
binary words. Initially, SD
n
[3:0] may
not have the proper bit ordering, however, correct ordering is established
by the reordering algorithm at start-up.