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[EFM] Half-duplex deferral for MAC-PHY rate matching and compatibility with existing silicon

The copper track have been looking at using "false carrier sense"
from the PHY to throttle back MAC transmissions to match the
MAC's intrinsically faster data-rate with the PHY's slower

The MAC would be configured for half-duplex and 100Mbit/s.

This has raised two questions about how MAC's in the field
have been actually implemented.

1) According to clause it is permissable for a MAC
to transmit frames back to back in half-duplex ignoring the
state of carrier sense; although it is suggested that the MAC
defer if it sees CRS active in the first two thirds of the
interframe gap.

For half-duplex deferral to work as a means of rate-matching
the MAC would have to defer if it saw CRS high immediately
after transmitting a frame.

The question is does the group have a feeling for the
proportion of MAC's in the field that ignore CRS when
transmitting frames back to back in half-duplex mode?

2) 802.3 allows reception during transmission in half-duplex

However it may be that MAC's in the field discard anything
they receive while transmitting in half-duplex mode.

Does the group have a feeling for whether MAC's in the
field are able to receive while they are transmitting
in half duplex mode?

If they cannot then the EFM PHY would have to buffer
receive frames until the MAC halted transmission.

Thanks and regards,


Arthur Marris
Chief Consulting Engineer
Phone: +44 (0)1506 595104
Fax: +44 (0)1506 595959
Email: arthurm@xxxxxxxxxx