In a standard Ethernet installation, where several nodes transmit to a common destination, such as a file server, the total traffic on the destination port can exceed Ethernet's standard 10-Mbps wire speed. When this happens, overloaded ports are bound to drop packets. In addition to outfitting each port with hefty buffers, switch vendors are using back pressure and/or software flow control to prevent traffic jams. These techniques force the source addresses to slow down or hold a steady pace until the overloaded destination ports catch up with the forwarding.
Back pressure generates collision-detection signals in the CSMA/CD MAC-layer (media access control) protocol of Ethernet
when a port is overloaded. These collision-detection packets from the congested port would "fake" the sending port into thinking that the collision is about to occur and therefore back off. This in turn forces a throttling back of the LAN segments that are experiencing congestion. When the sending ports try to transmit after the standard Ethernet delay time, the congested port may again send another collision-detection packet or accept the incoming frames if ready.
The figure illustrates
the advantages and drawbacks of back pressure. (These scenarios are hypothetical, because they don't factor in normal collisions that delay throughput.) For the first scenario, let's assume that device A is a workstation and that each of the five workstations attached to the repeater hub transmits at 2 Mbps, for an aggregate transfer rate of 10 Mbps going into the switch port. If just the five workstations are transmitting to the file server, the aggregate at the switch port connecting the file
server is within the legal Ethernet limit of 10 Mbps.
But if the two workstations directly attached to the switch each send data to the file server at 10 Mbps, this will create an aggregate rate of 30 Mbps at the switch port. This exceeds the Ethernet legal limit of a 10-Mbps wire rate. Without flow control, the file-server port would be forced to drop 67 percent ((30-10)/30*100) of the packets. In this case, back-pressure flow control would help, especially if it can be applied on a port-by-port basis.
But in the second scenario, assume that only two of the five systems on segment B send data through the repeater hub to the switch, and the two workstations connected directly to the switch are both sending data at 10 Mbps. Applying back pressure would slow down all five systems in segment B, because a repeater cannot transmit to selective ports.
illustration_link (7 Kbytes)
Back-pressure flow control can prevent dropped packets when Ethernet's 10-Mbps limit is exceeded.
Putting The Brakes On Runaway Frames
