MOTOROLA
Chapter 3. Receiver
3-21
Ethernet Compliant Applications Modes (COMPAT = High)
tolerates deletion of /I2/ as described above. Deleting /I2/ raises concerns about IPG
shrinkage. The IPG on transmit is required to be at least 12 code groups in duration,
including the end of packet delimiter (EPD). The received IPG is only required to be at least
eight code groups in duration, leaving four code groups available to remove per IPG.
NOTE
The MC92603 does not verify that the IPG meets minimum
length requirements after removal of the /I2/ code groups. It
assumes that the IPG has at least 12 code groups in length when
received.
On detection of an imminent underflow, the MC92603 searches for an /I2/ ordered set and
repeats an /I2/ in the packet stream, adding a total of two code groups. The packet stream
tolerates additional /I2/ ordered sets because maximum IPG length is not limited.
Special consideration is given to IPG in the data context to accommodate Jumbo frames and
frame bursting. These are described in the next section.
3.7.2.4
Data Context
The data context is when the transceivers are transmitting data frames encapsulated into
code group packets. The code groups in the packet cannot be disturbed, therefore, rate
adaption is accomplished in the IPG as described above.
On detection of an imminent overflow, the MC92603 searches for and deletes an /I2/
ordered set, removing a total of two or four code groups from the IPG.
On detection of an imminent underflow, the MC92603 searches for a /I2/ ordered set and
inserts an /I2/ adjacent to it into the IPG, adding a total of two code groups.
A special case that must be considered in the data context is Jumbo frames. Jumbo frames
are not supported in the standard but are rather a defacto-standard. Jumbo frames violate
the untagged maximum frame size of 1518 code groups and increases the size to 14K code
groups. Given a maximum total frequency offset of 200 ppm, a Jumbo frame could lead to
a surplus or deficit of 3.0 code groups for which rate adaption must account.
The depth of the receivers elastic buffers may be increased by asserting JPACK in order to
ensure against starvation in the presence of Jumbo frames. This increase will lead to longer
receiver latency.
3.7.3
Error Handling
The receiver interface error reporting mechanisms as described in Section 3.7.1.2, “TBI
Operation,” and Table 3-11 are used in the compatibility mode with one further definition;
overrun/underrun errors are processed specially.
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