Planex UE-100TX, User Manual

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The USB Fast Ethernet Adapter supports an advanced feature called “Flow 

Control” to prevent packets from overflowing the packet buffer of switches.  

The flow control technology used in half-duplex mode is called “Back 

Pressure”, while the flow control used in full-duplex operation is called 

“IEEE 802.3x”.  In Back Pressure, a collision signal is sent out whenever 

the packet buffer becomes full.  On the other hand, a switching hub with 

IEEE 802.3x support transmits a “pause” command to the source node to 

stop the data flow itself.  To utilize IEEE802.3x flow control, the attached 

network card must also support IEEE802.3x flow control (as of now, most 

of the NICs on the market do not support this standard.  The UE-100TX is 

the first USB Ethernet adapter equipped with this advanced feature).

If either the switch or network interface card does not support flow control, 

the source device (PC) will continue to send packets to the switch even 

when its packet buffer is full.  As a result, packets received by the switch 

will overflow and those overflowed packets are lost.   Upper layer protocols 

handle the lost packets, and TCP/IP (as one of the protocols) requests the 

PC to resend the packets.  Since packet overflow itself at the switch persists, 

PCs will be forced to consume extra system resources by sending packets 

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UE-100TX

Appendix B

About Flow Control