GarrettCom 4K-Series - Manual (Page 32)

Magnum 4K8 Switches Installation and User Guide (10/04)
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Fast Ethernet switched ports operating at 100Mb half-duplex, the bit time delay is 50BT.
4.6 Auto-negotiation for 10/100Mb ports,
The Magnum 4K-Series Switches perform half- or full-duplex mode auto-
negotiation independently on all switched ports. If the device or node on the other end of
a port’s attached cable supports auto-negotiation, the Magnum 4K8 Switch will
negotiate to run full-duplex. If the attached device or node doesn’t support auto-
negotiation (for example, if it is a 10 Mb repeater or a standard 10 Mb hub), the K-
Switch’s RJ-45 ports will default to operate at half-duplex.
4.7 Flow-control, IEEE 802.3x standard
Magnum 4K- Series 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
speed difference makes the 10Mb port 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 4K-Series Fiber 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 4K8-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 indicate 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 4K8-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.