Flow Control
Flow control concept
The switch can become overloaded if incoming frames arrive faster than the switch can process them. This results in the frames being discarded until the overload condition passes. The flow control mechanism overcomes this problem and eliminates the risk of lost frames.
Overload
If a potential overload situation occurs, the switch simply generates a “pseudo collision,” which forces all transmitting stations to immediately stop transmitting and wait a random amount of time before trying to retransmit. Following a simulated collision, any buffered frames are sent to their destination, clearing the switch’s buffers and allowing it to receive future frames.
When to use flow control
The flow control mechanism is ideal for situations where only one station is attached to one switch port. Do not use flow control on a port connected to a hub. However, consider the case where there is more than one station attached to a port as the figure below shows:
If station C tries to send data to station D via the overloaded switch, station C is “invited” to stop transmitting and wait a random time before trying again.
Because stations A and B are on the same cable segment as station C, they also detect the collision and are therefore also prevented from sending data to each other (not just to station D), for as long as the switch is overloaded.
Flow control influences all ports that have flow control enabled regardless of which port(s) is/are responsible for the overload situation.
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