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and smooth pattern flow enables MTL-3 to operate over bandwidth less than that of
the original rate, without the excessive susceptibility to EMI and RFI that comes with
Manchester.
Block Encoding Schemes
For high-speed operation, however, Manchester's serial bitstream transmission
mechanism inefficiencies and its EMI and RFI vulnerabilities were unacceptable. So,
a different encoding method was used: block encoding. Block encoding works by
taking small bites or nibbles of bit data of a defined size and using a bit code that
encodes in accordance with a set of defined symbols. The advantage of using block
encoding is that it provides a way to send both data and control data, such as clock
or error correction information. There are three popular block encoding methods
used for high-speed data transmission: 4B/5B, 8B/6T, and 8B/10B.
4B/5B uses a 5-byte code to represent 4 bytes of data. There are 16 5-bit codes
used to represent data and 5 codes used to send control information. 8B/6T uses
6-bit ternary codes (each bit can be VDC +1, -1, or 0) to represent the 256 possible
8-bit words. 8B/10B encoding works by using the same approach as 4B/5B, except
that an 8-bit word is encoded using a 10-bit symbol. The actual encoding is
accomplished with two different block code sets, 5B/6B and 3B/4B, in combination.
The use of symbols to represent data has some advantages over variable voltage
line signaling. First, the symbols can be formed to minimize signal transitions, which
makes them more efficient and less susceptible to misinterpretation. Second, the
use of the additional bits provides a mechanism to send control and timing
information independent of the transmission method.
The use of block encoding methods such as 4B/5B and 8B/10B also has the
advantage of being independent of DC voltage transitions for symbol representation
that results from symbol construction. This makes them suitable for use with fiber
optic media, which can only transmit signals using an on or off signal state.
Physical Cabling Basics
In Chapter 1, we briefly discussed the types of transmission media used to transmit
data signals, such as coaxial, shielded and unshielded twisted-pair, and fiber optic
cabling. During that discussion, the idea of cable specifications or categories was
also introduced. These categories and their operational specifications are defined in
North America by the Telecommunications Industry Association (TIA) and
Electronics Industries Association (EIA) in their 568 standards document. The
TIA/EIA 568 standard describes the operational characteristics of telephone and