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7.2 | |
Gigabit and 10-Gigabit Ethernet | |
| 7.2.2 | |
1000BASE-T |
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As Fast Ethernet was installed to increase
bandwidth to workstations, this began to create bottlenecks upstream
in the network. 1000BASE-T (IEEE 802.3ab) was developed to provide
additional bandwidth to help alleviate these bottlenecks. It provided
more "speed" for applications such as intra-building backbones,
inter-switch links, server farms, and other wiring closet applications
as well as connections for high-end workstations. Fast Ethernet was
designed to function over existing Cat 5 copper cable and this
necessitated that cable would pass the Cat 5e test. Most installed Cat
5 cable can pass 5e certification if properly terminated. One of the
most important attributes of the 1000BASE-T standard is that it be
interoperable with 10BASE-T and 100BASE-TX.
Because Cat 5e cable can reliably carry up to 125 Mbps of traffic, getting 1000 Mbps (Gigabit) of bandwidth was a design challenge. The first step to accomplish 1000BASE-T is to use all four pairs of wires instead of the traditional two pairs of wires used by 10BASE-T and 100BASE-TX. This is done using complex circuitry to allow full duplex transmissions on the same wire pair. This provides 250 Mbps per pair. With all four-wire pairs, this provides the desired 1000 Mbps. Since the information travels simultaneously across the four paths, the circuitry has to divide frames at the transmitter and reassemble them at the receiver. The 1000BASE-T encoding with 4D-PAM5 line encoding is used on Cat 5e or better UTP. Achieving the 1 Gbps rate requires use of all four pairs in full duplex simultaneously. That is the transmission and reception of data happens in both directions on the same wire at the same time. As might be expected, this results in a permanent collision on the wire pairs. These collisions result in complex voltage patterns. With the complex integrated circuits using techniques such as echo cancellation, Layer 1 Forward Error Correction (FEC), and prudent selection of voltage levels, the system achieves the 1Gigabit throughput. In idle periods there are nine voltage
levels found on the cable, and during data transmission periods there
are 17 voltage levels found on the cable.
The data from the sending station is
carefully divided into four parallel streams, encoded, transmitted and
detected in parallel, and then reassembled into one received bit
stream.
Figure
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With this
large number of states and the effects of noise, the signal on the
wire looks more analog than digital. Like analog, the system is more
susceptible to noise due to cable and termination problems.
represents of the simultaneous full duplex on four-wire pairs. 1000BASE-T
supports both half-duplex as well as full-duplex operation. The use of
full-duplex 1000BASE-T is widespread.
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