Magnum 6KQE Managed Edge Switch Installation and User Guide | 02/09 |
--the second of the two connected devices is powered up*, i.e., when LINK is established for a port, or
--the LINK is re-established on a port after being lost temporarily.
•NOTE – Some NIC cards only auto-negotiate when the computer system that they are in is powered. These are exceptions to the “negotiate at LINK – enabled” rule above, but may be occasionally encountered.
When operating in 100Mb half-duplex mode, cable distances and hop-counts
may be limited within that collision domain. The Path Delay Value (PDV) bit-times must account for all devices and cable lengths within that domain. For Magnum 6KQE Fast Ethernet switched ports operating at 100Mb half-duplex, the bit time delay is 50BT.
4.3Flow-control, IEEE 802.3x standard
Magnum 6KQE Switches incorporate a flow-control mechanism for Full- Duplex mode. The purpose of flow-control is to reduce the risk of data loss if a long burst of activity causes the switch to save frames until its buffer memory is full. This is most likely to occur when data is moving from a 100Mb port to a 10 Mb port and the 10Mb port is unable to keep up. It can also occur when multiple 100Mb ports are attempting to transmit to one 100Mb port, and in other protracted heavy traffic situations.
Magnum 6KQE Switches implement the 802.3x flow control (non-blocking) on Full-Duplex ports, which provides for a “PAUSE” packet to be transmitted to the sender when the packet buffer is nearly filled and there is danger of lost packets. The transmitting device is commanded to stop transmitting into the 6KQE Switch port for sufficient time to let the Switch reduce the buffer space used. When the available free- buffer queue increases, the Switch will send a “RESUME" packet to tell the transmitter to start sending the packets. Of course, the transmitting device must also support the 802.3x flow control standard in order to communicate properly during normal operation.
Note: When in Half-Duplex mode, the 6KQE Switch implements a back-pressure algorithm on 10/100 Mb ports for flow control. That is, the switch prevents frames from entering the device by forcing a collision indication on the half-duplex ports that are receiving. This temporary “collision” delay allows the available buffer space to improve as the switch catches up with the traffic flow.
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