100BASE-T2, 100BASE-T4
Two Fast Ethernet physical
layers that are mentioned are 100BASE-T4 and 100BASE-T2. Both were developed
to utilize lower speed cabling that may exist in an installation. 100BASE-T4
requires four pairs and is incapable of full-duplex operation. 100BASE-T2
only needs two pairs but uses a sophisticated signaling scheme that is
difficult and expensive to implement. The industry seems to have rewired
for the higher speed Category 5 cable so these two interfaces are not
popular.
100BASE-X
Like 100BASE-T, 100BASE-X is
not a unique physical layer but details the encoding for the two most popular
physical layers —100BASE-TX and 100BASE-FX. One physical layer is
for copper and the other for fiber optics, yet the standard applies to both.
Much of the 100BASE-X standard comes from the FDDI standard including the
4B/5B encoding.
4B/5B
Data transfers over the MII are
done with 4-bit nibbles that represent actual data. With 10BASE-T, Manchester
encoding is used which guarantees a transition within every bit cell regardless
of logic state. This effectively creates a 20 Mbaud signal for a 10 Mbps
data rate. If the same encoding were used for Fast Ethernet, a 200 Mbaud
signal would result making it difficult to maintain the same 100m maximum
segment length due to high frequency attenuation. If we could use a code
such as non-return to zero (NRZ), we could match the data rate with the
baud rate. The problem with NRZ is that it has a DC component, which transformers
do not like, and it provides little information about clocking. A compromise
is the 4B/5B code where the 4-bit nibbles being transferred over the MII
are actually encoded as five-bit symbols sent over the medium. The encoding
efficiency is 80% and the baud rate increases to 125 Mbaud. This is still
fast but not as fast as 200 Mbaud. The actual codes used are chosen so that
sufficient transitions occur in the resulting bit pattern so as not to lose
clocking. With twice as many codes as necessary, there are many that are
left unused and some that are defined for control purposes. The 4B/5B scheme
is used for both the 100BASE-TX and 100BASE-FX physical layers.
100BASE-TX
The 100BASE-TX twisted-pair physical
layer retains the same MDI (RJ-45) connector and pinout as 10BASE-T so that
auto-negotiation is possible. Two twisted-pairs with separate receive and
transmit paths allow for full-duplex operation. Cabling requires a higher
performance Category 5. This cable type is an unshielded twisted-pair characterized
as 100 ohms. The standard mentions a 150 ohm shielded twisted-pair (STP)
option with a DB-9 connector that can be used as well. A unique feature
of 100BASE-X encoding is that the link is always active even without data
since a unique symbol called IDLE is sent continuously during inactivity.
Reception of IDLE serves as a link integrity function. Maximum segment length
for 100BASE-TX is 100m just like 10BASE-T. Signaling on the twisted-pair
incorporates a three level multi-level technique called MLT-3. The benefit
of this code is to further reduce the electromagnetic emissions (EMI) and
the bandwidth requirements of the medium.
100BASE-FX
The 100BASE-FX fiber optic physical
layer is very similar in performance to 10BASE-FL. Maximum segment length
is 2 km for both technologies; however, for 100BASE-FX this is only achieved
on full-duplex links. On half-duplex links the segment length cannot exceed
412m. Either SC or ST fiber optic connectors can be used, but SC is recommended.
Multimode fiber optic cable (62.5/125 mm) is what is normally used; however,
it is possible to use single mode fiber optics for greater distances on
full-duplex links. The signaling on fiber optics is NRZI (non-return to
zero inverted) since there is no concern for EMI on fiber optic links. With
NRZI, the state of signaling inverts on logic 1s, thereby providing some
clocking information unlike NRZ.
Table 3 summarizes the
three most important Ethernet physical layers and Figure 2 shows the various
encoding schemes.
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Figure 2 — Various encodings for the same bit pattern.
Notice that MLT-3 approximates a sine wave with a much lower fundamental
frequency than the data rate.
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