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 With asymmetric flow control, only one of two linked devices can use Flow Control 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 With symmetric flow control, both linked devices can use incoming Flow Control 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-NegotiationRelated 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.
