1490 Chapter 39
100 m. Longer runs up to 2000 m were possible using a
fiber version called 10BaseF. Cat3 cable was more
durable and less expensive than the coax formerly
required. Of greatest importance, a problem with a cable
only affected a single station, and would not bring down
the entire network.
Since then, 100BaseT or Fast Ethernet has been
introduced. It runs at ten times the data rate of
10Base-T, or 100 MBit/s. Since the data rate is ten times
as high as 10Base-T, and the minimum packet size is the
same, the maximum network diameter had to be
reduced to 200 m. This is the most common form of
Ethernet today although Gigabit Ethernet is catching up
quickly. CobraNet® uses Fast Ethernet ports but can be
transported over Gigabit Ethernet between switches.
Within Fast Ethernet there are several varieties.
100Base-T4 uses all 4 pairs of a Cat3 UTP cable.
100Base-TX uses 2 pairs of a Cat5 cable. This is the
most common variety of Fast Ethernet. Both of these
varieties allow single cable runs of 100 m. 100Base-FX
uses multimode fiber, and allows single runs of up to
2000 m. A version of Fast Ethernet to run over
single-mode fiber has not been Standardized, but many
manufacturers sell their own versions, which allow
distances of as much as 100,000 m in a single run.
Many Fast Ethernet devices sold today not only
support 100Base-TX, but also 10Base-T. Such a dual
speed port is commonly called a 10/100 Ethernet port. It
will negotiate automatically with any Ethernet device
hooked to it and connect at the highest speed both ends
of the link support. The technique for this negotiation is
described below.
Gigabit Ethernet is now available, and the price has
dropped so much it is more and more replacing Fast
Ethernet. As you might have guessed it runs at a rate ten
times as fast as 100BaseT, or 1000 MBit/s. The first
versions ran over optical fiber, but now a version that
runs over Cat5 UTP cabling is available. It does,
however, use all four pairs in the cable. Gigabit Ethernet
increases the minimum packet size from 64 bytes to 512
bytes in order to allow the network diameter to stay at
200 m. Ethernet ports that support 10/100/1000 MBit/s
speeds and auto-negotiate to match the highest speed
the connected device can support are now common.
Within Gigabit Ethernet there are also several vari-
eties. 1000Base-LX (L for long wavelength) can be
used with either multimode or single-mode optical fiber.
1000Base-SX (S for short wavelength) is used with
multimode fiber only. 1000Base-SX is less expensive
than 1000Base-LX. 1000Base-LH (LH for long haul) is
not an IEEE standard, but is supported by many manu-
facturers. Manufacturers make different versions
depending on the distance to be covered. 1000Base-T
runs over Cat5 cable using all four pairs. The maximum
single cable run is 100 m.
A version of Ethernet that will run at ten times the
speed of Gigabit Ethernet is available and the price has
been dropping.
Several manufacturers power their products over
Ethernet cabling. There is now an IEEE Standard for
Power over Ethernet (PoE) and most manufacturers
sending power to their products over the Ethernet
cabling have gone to this standard.
Wireless Ethernet to the IEEE 802.11 Standard has
become vary popular and inexpensive. It provides a
variable data rate based on distance and environmental
conditions. The best case data rate for 802.11n (the
latest as of this writing) is 300 MBit/s, but typical data
rates are closer to 74 MBit/s.39.11.5 Ethernet TopologyThe Ethernet topology shown in Fig. 39-25 as a
collapsed backbone is commonly called a star topology,
since every station connects back to the common hub. It
is also permissible to tie multiple stars together in a star
of stars, Figs. 39-26 and 39-27.
Using fiber to interconnect the stars can increase the
distance between clusters of stars, Fig. 39-28.39.11.6 Ethernet EquipmentThis will become clearer when we examine the internal
functions of the repeater hubs we have been talkingFigure 39-25. Transforming the Ethernet backbone into a
repeater hub.
TTnode node node nodenode node node nodenodenode nodenodeOriginal Ethernet BackboneBackbone in the Process of Being CollapsedEthernet Backbone Collapsed into a Hub
hub