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(C) Herbert Haas
2005/03/11
MAC Control Frames
Additional functionality easily integrated
Currently only Pause-Frame supported
preamble FCSMAC-ctrl parametersMAC-ctrl opcode8808hSADA
8 bytes 6 6 2 2 44 4
Always 64 bytes
MAC-ctrl opcode ........... Defines function of control frame
MAC-ctrl parameters .... control parameter data (always filled up to 44 bytes)
Different data rates between switches (and different performance levels) often
lead to congestion conditions, full buffers, and frame drops. Traditional Ethernet
flow control was only supported on half-duplex links by enforcing collisions to
occur and hereby triggering the truncated exponential backoff algorithm. Just let
a collision occur and the aggressive sender will be silent for a while.
A much finer method is to send some dummy frames just before the backoff
timer allows sending. This way the other station never comes to send again.
Both methods are considered as ugly and only work on half duplex lines.
Therefore the MAC Control frames were specified, allowing for active flow
control. Now the receiver sends this special frame, notifying the sender to be
silent for N slot times.
The MAC Control frame originates in a new Ethernet layer—the MAC Control
Layer—and will support also other functionalities, but currently only the "Pause"
frame has been specified.