Intel 82559 用户手册

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The 82558 supports the PHY based flow control scheme known as the “Bay Flow Control” 

scheme. This scheme is supported only when the 82558 is operating using its internal PHY TX 

unit. It is not supported when an external PHY is used through the MII. The Bay Flow Control 
scheme is activated by setting bit 15 in MII register 16 in the 82558 PHY TX unit.

The 82558 generates flow control pause indications according to the number of bytes in its receive 

FIFO. This pause indication tells the 82558’s link partner to pause (or delay) future transmissions 

to allow the 82558 some time to empty its receive FIFO. The threshold at which the 82558 triggers 
the pause indication is determined by the flow control threshold register. PHY based flow control 

should not be used at the same time as frame based flow control.

Since frame based flow control is an IEEE 802.3x standard, it is recommended that software use 

the frame based FC method instead of PHY based FC.

The 82558 and later generation devices support the frame based flow control scheme as specified 

in the IEEE 802.3x standard. Frame based flow control operates independently of the PHY. Thus, 
this method is supported with either the 8255x (except the 82557) internal PHY or an external 

PHY.

These devices also support a priority aware flow control scheme, which is a non-standard variation 

of frame based flow control and is described later in 

Frame based flow control should be negotiated with a device’s link partner. This is accomplished 

through the N-Way auto-negotiation algorithm. To advertise frame based FC capability, the device 

should have their pause operation for full duplex (FDX) links bit set in the auto-negotiation 
advertisement register (MDI register 4, bit 10 [

]). A device should enable the pause functionality only if both the local device 

and the link partner advertise this capability and the negotiated link utilizes a FDX technology 
(regardless of data rate).

The 82558 defaults to advertising that it is not capable of FDX FC. The 82559 defaults to 

advertising that it is capable of FDX FC. To enable this feature on the 82558, software needs to 
configure the corresponding bit in the ability advertisement register and force re-negotiation.

IEEE 802.3x flow control (FC) is defined as point to point flow control. This means that it is 
relevant only for two devices connected by a dedicated link. This form of flow control is 

implemented in full duplex only. It includes a set of commands (and indications) for stopping and 

re-starting frame transportation between the two connected devices. The commands are encoded 
into special frames.

Flow control frames are the minimum Ethernet frame length (64 bytes). The flow control frame 

includes the following fields:

Special DA (6 bytes), SA (6 bytes), Type (2 bytes), Command (2 bytes), Parameters (2 bytes), 
Pad (42 bytes), CRC (4 bytes).