8137 for NetWare. Examples of IEEE SAP values are: E0 for NetWare, 04 for SNA, and AA
for SNAP.
bandwidth and autonegotiation. Autonegotiation allows an Ethernet card, hub, or switch to
determine which type of 100-Mbps Ethernet is supported by the device, hub, or switch on the
other end of the cable. Also, support for half duplex or full duplex is negotiated. If the other
device, such as a 10BaseT NIC, does not support autonegotiation, then autonegotiation will
settle for half-duplex 10BaseT.
CSMA/CD logic still exists and can be disabled for full-duplex point-to-point topologies in
which no collisions can occur. A variety of cabling options is allowed--unshielded and shielded
copper cabling, as well as multimode and single-mode fiber. Both Fast Ethernet shared hubs and
switches can be deployed. However, because Fast Ethernet gained market acceptance around
the same time that LAN switching became popular, most Fast Ethernet cards either are
connected to a switch or are directly cabled to another device.
still used and can still be disabled for full-duplex support. Although gigabit hubs are allowed,
it is more likely that Gigabit Ethernet switch ports will be the most popular use for Gigabit
Ethernet, along with use as a trunk between high-throughput switches and routers.
difference and the use of a different physical layer. The physical layer differences are truly
beyond the scope of the CCNA exam, but the history is interesting. To improve speed to market,
the specifications call for the use of a slightly changed FibreChannel physical layer (ANSI
X3T11) to operate at 1.25 gigabaud. An 8B/10B encoding scheme, which transmits a 10-bit
energy signal (baud) that represents an 8-bit value, is used. (The concept of the 8B/10B
encoding is not terribly different from FDDI's 4B/5B encoding.) The net result is that 8 bits are
exchanged per baud; at 1.25 gigabaud, 1 Gbps is achieved.
obvious ways. Collisions and wait time are decreased when compared to 10-Mbps Ethernet,
simply because it takes 90 percent (FE) or 99 percent (GE) less time to transmit the same
frame on the faster LANs. Capacity is greatly increased as well: If all frames were 1250 bytes
long, a 10,000 frames per second theoretical maximum could be reached on Fast Ethernet, and
a 100,000 frames per second theoretical maximum could be reached on Gigabit Ethernet. (Of
course, this little math problem ignores such details as interframe gaps and the unlikely case
of identical length frames; they're listed here for perspective.)