Flow Control

AT-FS709FC Series Installation Guide

In order for an Ethernet switch to maintain the orderly movement of data between the end nodes that are connected to its ports, it will occasionally need to signal an end node to stop sending data. This can occur under several situations. For example, when two end nodes are operating at different speeds, the switch, while transferring data between the nodes, might need to instruct the faster end node to stop transmitting data to allow the slower end node to catch up. An example of this situation would be when a server operating at 100 Mbps is sending data to a workstation operating at only 10 Mbps.

Another situation when a switch might need to stop the transmission of data from an end node is if two end nodes are vying for the same switch port at the same time. An example of this would be if two workstations were attempting to send data to the same network printer simultaneously. The switch could allow only one workstation to send data out the port to the printer, and it would need to instruct the other workstation to delay data transmission.

To some degree, these conflicts are minimized by the switch’s port buffers. These buffers are used to store data when a port is either already busy transmitting other data or when a transmitting port on a switch is operating at a slower speed than the port on the switch receiving data.

How a switch signals an end node to stop transmitting data differs depending on the speed and duplex mode of the end node and switch port. A twisted pair port operating at 100 Mbps port and half-duplex mode stops an end node from transmitting data by forcing a collision. A collision on an Ethernet network occurs when two nodes attempt to transmit data using the same data link at the same time. A collision causes end nodes to stop sending data. When the switch needs to stop a 100 Mbps, half-duplex end node from transmitting data, it forces a collision on the data link, which stops the end node. Once the switch is ready to receive data again, the switch stops forcing collisions.

A twisted pair port or fiber optic port operating at 100 Mbps and full-duplex mode uses PAUSE frames, as specified in the IEEE 802.3x standard. Whenever the switch wants an end node to stop transmitting data, it issues a this frame. The frame simply instructs the end node to cease transmission. The switch continues to issue PAUSE frames until it is ready to again receive data from the end node.