| Not all building infrastructures use
Category 5 cable; some use Category 3. Category 3 cable was
installed in many locations to support voice transmission, and it is
frequently referred to as voice-grade cable. It is tested for voice
and low-speed data applications up to 16 megahertz (MHz). Category 5
cable, on the other hand, is intended for data applications, and is
tested up to 100 MHz. Because Category 3 cable exists in so many
installations, and because many 10BASE-T installations are on
Category 3 cable, the IEEE 802.3u committee included this as an
option.
As with 10BASE-T, 100BASE-T4 links work
up to 100 meters. To support the higher data rates, 100BASE-T4 uses more cable pairs. Three pairs support transmission
and one pair supports collision detection. Another technology aspect
to support the high data rates over a lower bandwidth cable comes
from the encoding technique used for 100BASE-T4. 100BASE-T4 uses an
encoding method of 8B/6T (8 bits/6 ternary signals), thus
significantly lowering the signaling frequency and making it
suitable for voice-grade wire.
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