Technical data

144 Theoretical and general applications www.westermo.com
Ethernet on the cable
10 Mbit/s Ethernet
Signals sent over all 10 Mbit/s media systems uses Manchester encoding. Manchester
encoding combines data and clock into bit symbols, which provide a clock transition
in the middle of each bit. A logical zero (0) is defined as a signal that is high for the
first half of the bit period and low for the second half, i.e. a negative signal transition.
A logical (1) is defined as a positive signal transition in the middle of the bit period.
The signal transition makes it easy for a receiver to synchronise with the incoming
signal and to extract data from it. A drawback is that the worst case signalling rate is
twice the data rate. A link test signal is transmitted when there is no data to send.
Fast Ethernet
100Base-T media systems uses 4B/5B block encoding. Blocks of 4-bit data are trans-
lated into 5-bit code symbols for transmission over the media system.The 5-bit
encoding system allows for transmission of 32 5-bit symbols, including 16 symbols
that carry the 4-bit data and 16 symbols used for control.The IDLE control symbol is
continually sent when no other data is present. For this reason Fast Ethernet is con-
tinually active, sending 5-bit IDLE symbols at 125 Mbit/s if there is nothing else to
send. Each 100 Mbit/s (Fast Ethernet) system uses different media signalling.
100Base-TX uses scrambling and multilevel threshold-3 (MLT-3) signalling.The signal,
on the cable, can have one of three levels. A change from one level to the next
marks a logical one (1). Constant single level indicates a logical zero.
To reduce (spread out) the electromagnetic emission a scrambling process is applied
before the signal is MLT-3 modulated.The scrambler produces a non-repetitive bit
sequence of the bits to be transmitted.
A 100Base-FX fibre media system uses NRZI encoding.This system makes no
change in the signal level when sending a logical zero, but inverts the level at logical
ones.
Gigabit Ethernet
1000Base-T (copper) uses 4D-PAM5 encoding.The system transmits and receives
data on four wire pairs simultaneously (4D), using five voltage levels (PAM5) at each
twisted pair.
100Base-T (fibre) uses 8B/10B encoding. Data and control symbols are transmitted
at a rate of 1250 Mbit/s.The high signalling rate requires use of laser transceivers.
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