Magnum 4K24 Switches Installation and User Guide (10/04)
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negotiation (for example, if it is a 10 Mb repeater or a standard 100Mb hub or older
Switch w/ Fixed 100 Full duplex), the 4K24-Switch’s RJ-45 ports will default to operate
at half-duplex after the negotiation.
4.6
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 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 switched 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 4K24-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.
4.7
Power Budget Calculations for 4K-Series with Fiber Media
Receiver Sensitivity and Transmitter Power are the parameters necessary to
compute the power budget. To calculate the power budget of different fiber media
installations using Magnum products, the following equations should be used:
OPB (Optical Power Budget) = P
T
(min) - P
R
(min)
where P
T
= Transmitter Output Power, and P
R
= Receiver Sensitivity
Worst case OPB = OPB - 1dB(for LED aging) - 1dB(for insertion loss)
Worst case distance = {Worst case OPB, in dB} / [Cable Loss, in dB/Km]
