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/E/s and /T/s across the XGMII

Bob Grow, Pat Thaler,

Let's take a look at the following (XAUI less) case:

The transmit RS chooses to drive the "Transmit Error
Propagation" encoding across the XGMII during a packet,
as shown in figure 46-4, page 107.

The transmit PCS replaces that and all remaining words
in the packet, including the word with the Terminate,
with some number of 66-bit frames using the 0x1e Type
Field and 8 Cx values of /E/. This is due to the state
transition from the D state to the E state when
TYPE(tx_tobe_coded) equals neither D nor T. When the
RS/XGMII returns to sending Idles, the Transmit PCS
will return to the Z state and send frames with the
0x1e Type Field and Cx values of /I/.

The receive PCS follows the data coming from the
transmitter and pretty much tracks states, transitioning
from state D to state E when the transmitter does. When
it does this, it stops sending Data across the XGMII
and starts sending Receive Error. It does not do this
in just one byte location as depicted in Figure 46-5,
page 109. In fact, this errored packet won't even have
a Terminate value across the XGMII.

I don't think we can fix this by modifying the PCS,
given the limited code space allowed by the 64B/66B
encoding. I think we need to better describe the EOP
over the XGMII in the case of errored packets.

I haven't looked at XAUI/10GBASE-X PCS so I don't
know if the problem also exists there.



Benjamin Brown
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