Allied Telesis AT-S21 - Manual (Page 46)

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Menu: System configuration>No backpressure
Backpressure is disabled as a default.
Note
Although listed in the software, this option is not available and should
remain disabled.
In half-duplex mode, the switch implements the backpressure
methodology when its input buffer is running out of resources; for
example, outbound packets are traversing a single uplink port. When
backpressure is enabled, the switch simulates a collision when its
input buffers are filled up so that sending devices will defer
transmissions. These sending devices will retry transmissions
according to the Ethernet back-off algorithm. Once switch resources
are available again, it stops sending the collision signals and devices
may freely transmit again.