Cabletron Systems 100BASE-FX, Руководство по кабельной продукции

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Token Ring Media
9-2 Cabling Types
Figure 9-1. STP Cable Pair Association
Twisting the pairs together throughout the cable helps to reduce the effects of
externally-induced electrical noise on the signals that pass through the cable. In
each pair, one wire carries the normal network signal, while its associated wire
carries a copy of the transmission that has been inverted.
The twisting of associated pairs helps to reduce the interference of the other
strands of wire throughout the cable. This is due to the method of transmission
used with twisted pair transmissions.
In any transceiver or Desktop Network Interface Card (DNI or NIC), the network
protocol signals to be transmitted are in the form of changes of electrical state. The
means by which the ones and zeroes of network communications are turned into
these signals is called encoding. In a twisted pair environment, once a transceiver
has been given an encoded signal to transmit, it copies the signal and inverts the
voltage (see Figure 9-2). The result of this inverted signal is a mirror opposite of
the original signal.
Both the original and the inverted signal are then transmitted, the original signal
over the TX + transmit wire, the inverted signal over the TX - wire. As these wires
are the same length, the signal travels at the same rate (propagates) through the
cable. Since the pairs are twisted together, any outside electrical interference that
affects one member of the pair will have the same effect on the other member of
that pair.
The transmission travels through the cable, eventually reaching a destination
transceiver. At this location, both signals are read in. The original signal is
unchanged, but the signal that had previously been inverted is reverted to the
original state. When this is done, it returns the encoded transmission to its
original state, but reverses the polarity of any signal interference that the encoded
transmission may have suffered.
Once the inverted transmission has been returned to the normal encoded state,
the transceiver adds the two signals together. As the encoded transmissions are
now identical, there is no change to the data content. Line noise spikes, however,
are combined with noise spikes of their exact opposite polarity, causing them to
cancel one another out.
Tx+
Tx-
Rx-
Rx+
Outer Jacket
Pair Shield
Overall Shield
1845n21