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Flow Control 1-3
When the receiver’s input buffers can store packets again, it can either
send another pause frame to tell the transmitter to resume transmission,
or wait for transmission to resume.
If the Hub 100 SX receives a pause frame, it does not remove the frame
from the network. Instead, it corrupts the frame’s CRC field and
retransmits the frame to all other ports.
Asymmetric
Flow Control
With asymmetric flow control, only one of two linked devices can use
incoming flow control to receive pause frames. Most full-duplex repeaters
default to asymmetric flow control. The Hub 1000 SX downlinks support
asymmetric, outgoing flow control only. The downlinks can send pause
frames to tell attached devices to stop transmission. They ignore any
pause frames that they receive, because acting upon them would stop
transmission to all ports, bringing hub operation to a halt.
Symmetric
Flow Control
With symmetric flow control, both linked devices can use incoming
and outgoing flow control to send and receive pause frames. The
Hub 1000 SX uplink defaults to symmetric flow control—also known
as bidirectional flow control—which prevents congestion at the core of
the network when the hub is connected to the network backbone.
The uplink can be reconfigured for asymmetric, outgoing flow control to
support a connection to a server or 10/100 Mbps switch.
Auto-Negotiation Related to flow control is the auto-negotiation capability, in which the
Hub 1000 SX ports and linked devices advertise their flow control
capabilities and automatically select the best common mode of
communication. During an auto-negotiation sequence, the Hub 1000 SX
downlinks advertise their capability for outgoing flow control and the
uplink advertises its capability for incoming and outgoing flow control.
For example, the result of an auto-negotiation sequence between a
Hub 1000 SX downlink and a SuperStack II Switch 3000 port is that the
downlink uses outgoing flow control and the switch port uses incoming
flow control.
Forced Link The Hub 1000 SX ports default to auto-negotiation. Rare cases (for
example, linking to a device that does not support auto-negotiation) may
require that auto-negotiation be disabled on a port, thereby enabling
forced link on that port. When forced link is enabled, linked devices must
have matching flow control capabilities. For example, a port that is set for
forced link and outgoing flow control can connect successfully only with
a port that is set for forced link and incoming flow control.