Information Technology Reference
In-Depth Information
exact throughput. There is another interface that is normally internal to equipment,
which splits the data into four 2.5 Gbps signals that are combined by an interface
adapter into the 10 Gb “stream.” ATM's OC-192 is very close, but not exactly 10
Gbps. The “optical carrier” OC rates are exact multiples of 51.84 Mbps (essentially
OC-1). Thus, 192
9953.28 Mbps, or 9.95328 Gbps.
This is close enough for our purposes, and certainly close enough to be called
10 Gb.
51.84
Gigabit Ethernet
is formally specified in the supplement to the base 802.3 standard
known as IEEE 802.3z, which includes the fiber implementations, and in IEEE
802.3ab for twisted-pair copper. It is sometimes referred to as 1000Base-X. The
“X” signifies a physical layer based on the ANSI X3.230 Fibre Channel
1
standard,
in the same manner that 100Base-X fiber modes were based on the FDDI physical
layer.
The choice of Fibre Channel signaling was made simply to expedite the intro-
duction of the Gigabit Ethernet technology. At the time of first consideration of
expanding Ethernet operation to the gigabit range, Fibre Channel existed as an
accepted standard. More importantly, integrated circuits existed to implement the
signaling, so it was very time- and cost-effective to utilize the same basic circuitry.
The signaling rate was modified to allow for an 8-line by 125 Mbps interface to
achieve a data rate of exactly 1000 Mbps. The actual medium-independent layer of
Gigabit Ethernet is the 8-bit-wide gigabit media independent interface (GMII), cor-
responding to the MII in Fast Ethernet and the AUI in standard Ethernet.
Gigabit Ethernet is topologically similar to Fast Ethernet (100BaseT). All
workstations connect directly to a switch or hub, and collision domains are tightly
controlled to allow the collision-detection mechanism to operate properly. As a mat-
ter of fact, shared-medium operation of Gigabit Ethernet is virtually absent, as
Gigabit Ethernet switches are used in most hub-type applications. In essence, the
switch is an OSI Layer 2 bridge that operates much like its lower speed, multiport
counterparts.
Gigabit Ethernet was originally specified as a fiber-optic topology, because the
bulk of installed cabling at the time was only Category 3. Cat 3 simply was not
capable of carrying the higher symbol rates needed to support 1 Gbps data. Today,
all of the newly installed cabling is Cat 5e or above, which makes a copper imple-
mentation possible.
1
Fibre Channel is spelled according to the European convention, as it is an international standard.
