Cabletron Systems 100BASE-FX, Cabling Manual

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FDDI Media
11-6 Cabling Types
In any transceiver or Network Interface Card (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 11-5, below). 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 one transmit wire, the inverted signal over the other. 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.
Figure 11-5. Twisted Pair Signal Equalization
STP cable is made up of four or more wires, and each wire within the cable has a
specific insulator color. These colors are part of the industry specifications to
which the cable construction process must be held. Each color identifies a
particular usage for the cable. The four colors are black, red, green, and orange.
Table 11-1, below, identifies the type of signal that each wire carries.
Inverted
Transmission
Normal
Transmission
Induced
Noise Spike
Noise spikes
cancel out
Resulting Signal
Reversion of Inverted
Transmission
Original Signal
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