Manchester Signal
Encoding
Introduction
The device
driver software receives a frame of IP, IPX, NetBIOS, or
other higher-layer protocol data. From this data, the
device driver constructs a frame, with appropriate
Ethernet header information and a frame check sequence
at the end.
The
circuitry on the adapter card then takes the frame and
converts it into an electrical signal. The voltage
transitions in the transmitted bit stream are in
accordance to the format called Manchester Signal
Encoding. Manchester encoding describes how a binary ONE
and ZERO are to be represented electrically. Manchester
encoding is used in all 10 Megabit per second Ethernets;
for example, 10BASE2 Thin Ethernet, 10BASE5 Thick
Ethernet and 10BASE-T Twisted-Pair Ethernet.
Here we see
an example of the signal transitions used to encode the
hexadecimal value "0E", which converts to "00001110" in
binary:
Notice that
there is a consistent transition in the middle of each
bit-time. Sometimes this transition is from low-to-high
and sometimes it's from high-to-low. This is the clock
transition. The receiving adapter circuitry 'locks on'
to this constant signal transition and, thereby,
identifies the timing to determine the beginning and end
of each bit.
To
represent a binary ONE, the first half of the bit-time
is a low voltage; the second half of a bit is always the
opposite of the first half, that's how the clock
transition is created. To represent a binary ZERO, the
first half of the bit-time is a high voltage. You see
that sometimes there is an additional transition at the
beginning of a bit-time (not drawn in in the diagram
above) where the signal is pulled either up or down in
preparation for the next bit.
Consider
what happens if an external electromagnetic field
interferes with the Manchester bit encoding. This
external field could be the result of an electric motor,
radio transmission or other source of interference. You
should be able to see that if the Manchester signal is
disrupted the bits will be destroyed - because the clock
signal will be disrupted.
It would
not be reasonably possible for electrical interference
to change a binary ONE into a binary ZERO. Since each
bit is symmetrical (second half is always opposite the
first half) the result of electrical noise would be the
destruction of the bit, not a change in bit value.
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