Understanding Flow Control

On a network, there needs to be some way of controlling the flow of data between devices. Otherwise it is possible for a network interface card to overrun a network segment resulting in data loss.

On an Ethernet network operating in half-duplex mode, there is built in flow control. Each device must wait until the channel is clear before sending any data. This is done through Carrier Sense/Multiple Access with Collision Detection (CSMA/CD).

With CSMA/CD, each device will listen on the wire to determine if another device is in the process of transmitting data. A device will only send data if it senses that no other device is currently transmitting. As a result there is no chance of overloading the Ethernet segment.

CSMA/CD does not operate in full-duplex mode. So switches that are operating in full-duplex mode must use another method to avoid traffic congestion. Flow control is the mechanism used to control the flow of data between two devices that are operating in full duplex mode.

The 802.3x standard defines that a “Pause” packet be sent to slow down the transmission of data to eliminate traffic congestion. A device will send a pause message to another device to temporarily reduce the amount of data being transmitted. One of the advantages of flow control is that it enables lower speed devices to communicate with higher speed devices without being experiencing buffer overflow.

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