GarrettCom Quad - Manual (Page 40)

Magnum Quad-Series Fiber Switches Installation and User Guide (06/09)
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4.5 Auto-negotiation for 10 Mb ports, half- or full-duplex mode
Full-duplex Ethernet provides separate Transmit and Receive data paths,
enabling simultaneous bi-directional collision-free data movements on a port. The
network topology must be a “star” type, not a “bus” type. With full-duplex mode, the
cable distance is only limited by the physical layer line driver and cable attenuation.
There are no collision-domain restrictions or limitations.
The Magnum Quad-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 F/H mode auto-negotiation or is set to operate as full-
duplex, the Magnum Quad-Port Switch will negotiate to run full-duplex. If the attached
device or node doesn’t support F/H mode auto-negotiation (for example, if it is a 10 Mb
repeater or a standard 10 Mb hub), the Quad-Switch’s RJ-45 ports will default to operate
at half-duplex.
4.6 Flow-control, IEEE 802.3x standard
Magnum Q- 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 Q-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 Quad-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 quad-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.