MAC Control Frames

￿Additional functionality easily integrated

￿Currently only Pause-Framesupported

Always 64 bytes

 

8 bytes

6

 

6

2

2

44

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

preamble

DA

 

SA

8808h

MAC-ctrl opcode

MAC-ctrl parameters

 

FCS

 

 

 

 

 

 

 

 

 

 

 

 

 

MAC-ctrl opcode

...........

Defines function of control frame

 

 

 

MAC-ctrl parameters ....

control parameter data (always filled up to 44 bytes)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(C) Herbert Haas

2005/03/11

 

 

 

 

 

 

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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.

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