IEEE 802.3 clause 22 talks about the Media Independent Interface (MII).
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 22.2.3.2.1, "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 7.2.3.2 and shall consist of 7 octets with the following bit values:
"10101010 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 22.2.3.2.2, "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 MII."
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 22.2.3.2.1, "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.
Wood