Understanding the network.pdf - Back to Home

There are several baseband line signaling techniques. The most common is
Non-Return-to-Zero (NRZ). NRZ uses the presence and absence of voltage to
represent ones and zeros. Non-Return-to-Zero Invert on Ones (NRZI) is another
popular baseband line signaling technique. NRZI uses the shift in line level to
indicate a one and the absence of a line level shift to represent a zero. In other
words, when there is no line signal, it's a zero, and when there is a signal, it's a one.
For NRZ, a signal on the line represents a one, but the line starts off with no signal,
whereas with NRZI there is always a line signal. Hence, the transition from an active
state to passive state can indicate a zero. However, neither of these methods is
easily implemented for LAN use, because a timing element is needed to distinguish
the bit periods. Without timing, the sending and receiving stations will not be able to
discern how many bits are actually being transmitted when a stream of consecutive
ones or zeros are sent. To use these signaling systems, the protocol must specify a
timing mechanism.
Manchester and MTL-3 Line Encoding
Because LAN transmissions are intermittent in nature and cannot be used to provide
a constant source of clocking, a variation on NRZ was developed, Manchester
encoding. Manchester encoding is used by 10Mbps Ethernet and Token Ring PHY
implementations. Manchester encoding transmits bit data in a bit-by-bit serial
stream across the transmission path. Positive and negative voltage is used to
represent binary ones and zeros. Timing is derived through the voltage transitions
in the middle of each bit. High to low transitions represent a zero bit and low to high
represent a one bit. Manchester encoding works well, but is not very efficient
because it requires a lot of signal bandwidth to accommodate the positive and
negative voltage transitions. NRZ, by comparison, only requires half as much, as it
utilizes just two voltage states (none and positive).
Manchester encoding also utilizes fairly high voltage states (+/- 3VDC). This high
signal load becomes problematic at high frequencies (required for high-speed
transmission) because it becomes susceptible to electromagnetic interference (EMI)
and radio frequency interference (RFI). For 10Mbps Ethernet, Manchester's
serial-bit stream and self-timing attributes make it attractive and its limitations go
unnoticed.
For high-speed line encoding over copper transmission media, a different encoding
scheme is used: Multilevel 3 (MLT-3). MLT-3 is used by 100Mbps Ethernet over UTP
and Copper Distributed Data Interface (CDDI), which is an implementation of FDDI
over copper cabling. MLT-3, like Manchester, is a bitstream "line" encoding
mechanism, but it uses a low bandwidth circular ternary signaling system where bit
changes are indicated with voltage transitions 0 to +1, to 0 to -1. MLT-3's efficient