Information Technology Reference
In-Depth Information
100-Mbps Fast Ethernet Design Rules
IEEE introduced the IEEE 802.3u-1995 standard to provide Ethernet speeds of 100 Mbps
over UTP and fiber cabling. The 100BASE-T standard is similar to 10-Mbps Ethernet in
that it uses carrier sense multiple access collision detect (CSMA/CD); runs on Category
(CAT) 3, 4, and 5 UTP cable; and preserves the frame formats. Connectivity still uses
hubs, repeaters, bridges, and switches.
100-Mbps Ethernet, or Fast Ethernet, topologies present some distinct constraints on the
network design because of their speed. The combined latency due to cable lengths and re-
peaters must conform to the specifications for the network to work properly. This section
discusses these issues and provides sample calculations.
The overriding design rule for 100-Mbps Ethernet networks is that the round-trip collision
delay must not exceed 512-bit times. However, the bit time on a 100-Mbps Ethernet net-
work is 0.01 microseconds, as opposed to 0.1 microseconds on a 10-Mbps Ethernet net-
work. Therefore, the maximum round-trip delay for a 100-Mbps Ethernet network is 5.12
microseconds, as opposed to the more lenient 51.2 microseconds in a 10-Mbps Ethernet
network.
The following are specifications for Fast Ethernet, each of which is described in the fol-
lowing sections:
100BASE-TX
■
100BASE-T4
■
100BASE-FX
■
100BASE-TX Fast Ethernet
The 100BASE-TX specification uses CAT 5 UTP wiring.
Like 10BASE-T, Fast Ethernet uses only two pairs of the four-pair UTP wiring. If CAT 5
cabling is already in place, upgrading to Fast Ethernet requires only a hub or switch and
network interface card (NIC) upgrades. Because of the low cost, most of today's
installations use switches. The specifications are as follows:
Tran s mis s ion over CAT 5 UTP w ire
■
RJ-45 connector (the same as in 10BASE-T)
■
Punchdown blocks in the wiring closet must be CAT 5 certified
■
4B5B coding
■
100BASE-T4 Fast Ethernet
The 100BASE-T4 specification was developed to
support UTP wiring at the CAT 3 level. This specification takes advantage of higher-speed
