Figure 7-13 Transition-Based Manchester Binary Encoding
Unfortunately, Manchester encoding introduces some
difficult frequency-related problems that make it unsuitable for use at higher
data rates. Ethernet versions subsequent to 10Base-T all use different encoding
procedures that include some or all of the following techniques:
� Using
data scrambling�A procedure that scrambles the bits in each byte in an
orderly (and recoverable) manner. Some 0s are changed to 1s, some 1s are changed
to 0s, and some bits are left the same. The result is reduced run-length of
same-value bits, increased transition density, and easier clock recovery.
�Expanding
the code space�A technique that allows assignment of separate codes for
data and control symbols (such as start-of-stream and end-of-stream delimiters,
extension bits, and so on) and that assists in transmission error detection.
�Using
forward error-correcting codes�An encoding in which redundant information
is added to the transmitted data stream so that some types of transmission
errors can be corrected during frame reception.
The 802.3 Physical Layer Relationship to the ISO Reference
Model
Although the specific logical model of the physical layer
may vary from version to version, all Ethernet NICs generally conform to the
generic model shown in Figure 7-14.
Figure 7-14 The Generic Ethernet Physical Layer
Reference Model
The physical layer for each transmission rate is divided
into sublayers that are independent of the particular media type and sublayers
that are specific to the media type or signal encoding.
�The
reconciliation sublayer and the optional media-independent interface (MII in
10-Mbps and 100-Mbps Ethernet, GMII in Gigabit Ethernet) provide the logical
connection between the MAC and the different sets of media-dependent layers. The
MII and GMII are defined with separate transmit and receive data paths that are
bit-serial for 10-Mbps implementations, nibble-serial (4 bits wide) for 100-Mbps
implementations, and byte-serial (8 bits wide) for 1000-Mbps implementations.
The media-independent interfaces and the reconciliation sublayer are common for
their respective transmission rates and are configured for full-duplex operation
in 10Base-T and all subsequent Ethernet versions.
�The
media-dependent physical coding sublayer (PCS) provides the logic for encoding,
multiplexing, and synchronization of the outgoing symbol streams as well symbol
code alignment, demultiplexing, and decoding of the incoming data.
�The
physical medium attachment (PMA) sublayer contains the signal transmitters and
receivers (transceivers), as well as the clock recovery logic for the received
data streams.
�The
medium-dependent interface (MDI) is the cable connector between the signal
transceivers and the link.
�The
Auto-negotiation sublayer allows the NICs at each end of the link to exchange
information about their individual capabilities, and then to negotiate and
select
the most favorable operational mode that they both are capable of supporting.
Auto-negotiation is optional in early Ethernet implementations and is mandatory
in later versions.
Depending on which type of signal encoding is used and how
the links are configured, the PCS and PMA may or may not be capable of
supporting full-duplex operation.
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