Paradise Datacom 4 RU Chassis, Руководство по эксплуатации

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120 201627 REV X 4 RU SSPA Chassis Operations Manual
The main concern is the transient magnetic fields which surrounds the wires and the
magnetic fields generated externally by the other transmission lines in the cable, other
network cables, electric motors, fluorescent lights, telephone and electric lines, light-
ning, etc. This is known as noise. Magnetic fields induce their own pulses in a trans-
mission line, which may literally bury the Ethernet pulses.
The twisted-pair Ethernet employs two principle means for combating noise. The first is
the use of balanced transmitters and receivers. A signal pulse actually consists of two
simultaneous pulses relative to ground: a negative pulse on one line and a positive
pulse on the other. The receiver detects the total difference between these two pulses.
Since a pulse of noise (shown in red in the diagram) usually produces pulses of the
same polarity on both lines one pulse is essentially canceled by out the other at the
receiver. In addition, the magnetic field surrounding one wire from a signal pulse is a
mirror of the one on the other wire. At a very short distance from the two wires, the
magnetic fields are opposite and have a tendency to cancel the effect of each other.
This reduces the line's impact on the other pair of wires and the rest of the world.
The second and the primary means of reducing cross-talk between the pairs in the ca-
ble, is the double helix configuration produced by twisting the wires together. This con-
figuration produces symmetrical (identical) noise signals in each wire. Ideally, their dif-
ference, as detected at the receiver, is zero. In actuality, it is much reduced.
Pin-out diagrams of the two types of UTP Ethernet cables are shown in
Figure B-3
.
Note that the TX (transmitter) pins are connected to corresponding RX (receiver) pins,
plus to plus and minus to minus. Use a crossover cable to connect units with identical
interfaces. If you use a straight-through cable, one of the two units must, in effect, per-
form the crossover function.
Two wire color-code standards apply: EIA/TIA 568A and EIA/TIA 568B. The codes are
commonly depicted with RJ-45 jacks as shown in
Figure B-4
. If we apply the 568A
color code and show all eight wires, our pin-out looks like
Figure B-5
.
Note that pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either stan-
dard. Quite contrary to what you may read elsewhere, these pins and wires are not
used or required to implement 100BASE-TX duplexing.
Figure B-3: Ethernet Cable Pin-Outs