Information Technology Reference
In-Depth Information
The 4B/5B coding takes 4 bits of data and expands it into a 5-bit code for transmission on the
physical channel. Because of the 20 percent overhead, pulses run at 125 MHz on the wire to
achieve 100 Mbps. Table 4-4 shows how some data numbers are converted to 4B/5B code.
FE 4B/5B Code
Table 4-4
Data
Binary
4B/5B Code
0
0000
11110
1
0001
01001
2
0010
10100
...
...
...
D
1101
11011
E
1110
11100
F
1111
11101
The following specifications are covered in this section:
•
100BaseTX
•
100BaseT4
•
100BaseFX
100BaseTX FE
The 100BaseTX specification uses Category 5 UTP wiring. Similar to 10BaseT, FE uses only
two pairs of the 4-pair UTP wiring. If Category 5 cabling is already in place, upgrading to FE
only requires a hub or switch and NIC upgrades. Because of the low cost, most of today's
installations use switches. The specifications are as follows:
•
Transmission over Cat 5 UTP or Cat 1 STP wire.
•
RJ-45 connector (same as in 10BaseT).
•
Punchdown blocks in the wiring closet must be Category 5 certified.
•
4B5B coding.
100BaseT4 FE
The 100BaseT4 specification was developed to support UTP wiring at the Category 3 level.
This specification takes advantage of higher speed Ethernet without recabling to Category 5
UTP. This implementation is not widely deployed. The specifications are as follows:
•
Transmission over Cat 3, 4, or 5 UTP wiring.
•
Three pairs are for transmission, and the fourth pair is for collision detection.
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