| 2.4.4 PCS (Physical Coding Sublayer)
The PCS performs the important jobs of encoding the XGMII characters and passing the
encoded stream to the PMA (Physical Medium Attachment) on the transmit side, as well
as decoding the PMA bit stream and passing the decoded output to the XGMII on the
receive side. There are three types of PCS defined in the 10 Gigabit Ethernet Architecture:
a) 8B/10B PCS: This form of PCS is referred to as the 10GBASE-X PCS, which provides
services to the XGMII that include:
1) Encoding of 32 XGMII data bits and 4 XGMII control bits into four parallel
lanes with each lane conveying 10-bit code groups.
2) Decoding of 10-bit code groups on four parallel PMA lanes into 8 XGMII data
bits and 1 XGMII control bit per lane. Hence, this PCS is basically an 8 to 10-bit
converter and vice-versa.
3) Conversion of the XGMII idle sequence into a pseudo-random pattern so as to
reduce electromagnetic interference.
4) Synchronization of code groups on each lane to determine word boundaries.
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5) De-skew of received code groups to eliminate the lane-to-lane skew problem.
6) Clock rate compensation to compensate for difference in clock rates.
Table 2.2 below shows the relationship between XGMII and PCS-Code Groups on the
transmit side, and it also applies to the receive side except that transmission is replaced
by reception.
TXC
TXD
PCS Code Group
Description
0
00-FF
ID /
Data
Transmission
1
07
IK /
XGMII Idle
1
9C
1 0 /
Sequence
1
FB
/S I
Start
1
FD
m
Terminate
1
FE
m r
Error
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