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First of all, Gigabit Ethernet is a true Ethernet standard
and is compatible with both 10- and 100-Mbps Ethernet.
Because it uses the same frame formats and flow-control methods
as earlier Ethernet versions, networks readily recognize it.
The primary difference is that Gigabit Ethernet almost always
runs in full-duplex mode, rather than the half-duplex mode
commonly found in 10- and 100-Mbps Ethernet.
Full-duplex means a lot more bandwidth: A single
Gigabit Ethernet switch port can handle traffic from as many
as 20 half-duplex fast Ethernet links or up to 200 half-duplex
Ethernet ports.
When Gigabit Ethernet first appeared, fiber was crucial
to running Gigabit Ethernet effectively. Since then, the
IEEE 802.3ab standard for Gigabit over Category 5 cable
has been approved, enabling short stretches of Gigabit speed
over existing copper cable.
The First Gigabit Standards:
1000BASE-X.
1000BASE-X refers collectively to the 1000BASE-LX,
1000BASE-SX, and 1000BASE-CX Gigabit Ethernet standards,
ratified in 1998. 1000BASE-LX and 1000BASE-SX are fiber
standards; 1000BASE-CX is an early copper standard that enabled
Gigabit Ethernet to run over twinax cable.
1000BASE-LX.
The “L” stands for “long” and it uses long wavelength lasers
operating in the range of 1270 to 1355 nanometers to transmit
data over fiber optic cable. 1000BASE-LX supports both
single-mode and multimode fiber.
The “L” could just as well stand for “long distance.”
1000BASE-LX supports longer distances than shorter wavelength
standards—up to 550 meters (1804 ft.) full-duplex over
multimode fiber and up to 5000 meters (16,404 ft.)
full-duplex over single-mode fiber. At half-duplex, 1000BASE-LX
goes up to 316 meters.
1000BASE-SX.
The “S” in 1000BASE-SX stands for “short.” 1000BASE-SX
uses short wavelength lasers, operating in the 770- to 860-
nanometer range, to transmit data over fiber. It has the advantage
of being less expensive than 1000BASE-LX but operates only over
multimode fiber at distances up to 316 meters, half duplex or 550
meters, full duplex.
1000BASE-CX.
The “C” stands for “copper” and 1000BASE-CX operates
over copper twinax cable at distances of up to 25 meters.
This standard never caught on, largely because of the short
distance it covers. In addition, twinax cable is only slightly
less expensive than fiber optic cable and is difficult to install.
Gigabit over CAT5—1000BASE-T.
The relatively new IEEE 802.3ab standard, approved
in June 1999, defines running Gigabit Ethernet over Category 5
copper wiring. The importance of this is that now you can run
IEEE-compliant Gigabit Ethernet over existing copper cable
at distances of up to 100 meters. This results in a significant
savings over the cost of fiber, because not only is copper twisted
pair less expensive to install, it’s also often already installed.
The 802.3ab specification works with any existing Category 5
cable, as long as it’s been tested and certified. However, Gigabit
Ethernet uses all four twisted pairs within the cable unlike
10BASE-T and 100BASE-TX, which only use two of the four pairs.
1000BASE-T works by transmitting 250 Mbps over each
of the four pairs in 4-pair Category 5 UTP cable.
And although the 100-meter maximum defined by 802.3ab
might not sound like much compared to fiber, it’s the same
distance limit as Fast Ethernet on Category 5 cabling.
But what can I do with it?
At a billion bits per second, Gigabit Ethernet has the potential
to blow through virtually any bandwidth bottleneck.
(continued on page 6)
Technically Speaking
(56 pages)
(2 pages)
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