Magnum 6K25 Managed Fiber Switch Installation and User Guide (04/06)
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4.5
Flow-control, IEEE 802.3x standard
Magnum 6K25 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 6K25 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 6K25 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 6K25 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.
4.6
Power Budget Calculations for Magnum 6K25 PM’s 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]
where the “Cable Loss” for 62.5/125 and 50/125
µ
m
(M.m) is 2.8 dB/km,
and the “Cable Loss” for 100/140 (Multi-mode) is 3.3 dB/km,
and the “Cable Loss” for 9/125 (Single-mode) is 0.5 dB/km
and the “Cable Loss” for 9/125 (Single-mode) is 0.4 dB/km (LXSC25)
and the “Cable Loss” for 9/125 (Single-mode) is 0.25 dB/km
(LXSC40)
and the “Cable Loss” for 9/125 (Single-mode) is 0.2 dB/km (LXSC70)
