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←Part 4 MAC Part 6 PMD→
Physical Medium Independent (PMI) Sublayer Functions
The 100VG-AnyLAN physical-medium independent functions, summarized in
figure 4, include quartet channeling data scrambling, 5B6B encoding, and the
addition of the preamble, start frame, and end frame delimiters to prepare
the packet for transmission by the Physical Medium Dependent (PMD) sublayer.
Figure 4
Quartet Channeling is the process of sequentially dividing the MAC frame
data octets into 5-bit data quintets and sequentially distributing them
among four channels, as shown in figure 5.
Figure 5
Each of the four channels
represents a transmission pair for the 4-UTP demand priority network:
channel 0 data will be transmitted on twisted-pair wires 1 and 2, channel 1
data will be transmitted on wires 3 and 6, channel 2 data will be
transmitted on wires 4 and 5, and channel 3 data will be transmitted on
wires 7 and 8. (2-pair and fiber optic 100VG-AnyLAN networks use a
multiplexing scheme, implemented in the PMD sublayer, to convert the four
channels into either two channels or one channel, respectively.)
Data Scrambling is the process of scrambling the 5-bit data quintets-using a
different scrambling mechanism for each channel-to randomize the bit
patterns on each transmission pair. (Scrambling each channel eliminates
repetitious data patterns such as all 0's or all 1's.) Random bit patterns
reduce both radio frequency interference and signal crosstalk from one pair
to another pair in the cable.
5B6B Encoding is the process of encoding or mapping the scrambled 5-bit data
quintets into predetermined 6-bit symbols. This process creates a balanced
data pattern-containing equal numbers of 0's and 1's-to provide guaranteed
clock transition synchronization for receiver circuitry.
5B6B encoding also provides an added error-checking capability. Invalid
symbols and invalid data patterns (such as more than three 1's or three 0's
in a row) are easily detected.
Figure 6
Figure 6 shows an example of the 5-bit data quintets, scrambled and
converted to 6-bit 5B6B symbols. Because there are only sixteen balanced
symbols available, sixteen of the 5-bit data patterns require two 6- bit
symbols, used alternately to maintain DC balance.
Preamble, Start Frame Delimiter, and End Frame Delimiter are added to each
channel to package the data into a format ready for transmission on the
network.
←Part 4 MAC Part 6 PMD→
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