14 years ago
22.2.3 says, "Data frames transmitted through the MII shall
have the frame format shown in Figure 22-10."
Figure 22-10 looks a lot like this:
Paragraph 184.108.40.206.1, "Transmit case", tells us
"The preamble begins a frame transmission. The
bit value of the preamble field at the MII is unchanged
from that specified in 220.127.116.11 and shall consist of 7
octets with the following bit values:
"10101010 10101010 10101010 10101010 10101010 10101010
This is intended to represent hex values 55,55...
My question is, how many nibbles of value 5 are required?
Here we are told 7 octets, which is 14 nibbles. (15 if you
include the first nibble of the SFD.)
And Table 22-3 shows 15 nibbles of value 5.
But in 18.104.22.168.2, "Receive case", we read
"Table 22-4 depicts the case where no preamble nibbles
are conveyed across the MII, and Table 22-5 depicts
the case where the entire preamble is conveyed across the
And these two tables indeed show two different size
preambles: one has 15 nibbles of value 5, the other has
just one (the SFD first half.)
How could you have a short preamble if the transmitting
party is following 22.214.171.124.1, "Transmit case" ???
I'm designing a PHY, does it need to handle a random
size preamble or should it reject packets that don't have
the exact size specified for the transmit case?
I've read the previous discussion back in 2003 and 2001,
still not confident what the right answer should be.