Gigabit, 10-Gb, and Future Ethernet 339
Bit patterns from the MAC sublayer are converted into symbols, with symbols some-
times controlling information (such as start frame, end frame, and medium idle condi-
tions). The entire frame is broken up into control symbols and data symbols (data code
groups). All of this extra complexity is necessary to achieve the tenfold increase in net-
work speed over Fast Ethernet. For 1000BASE-T, the first part of the encoding uses a
technique called 8Bit-1Quinary quarter (8B1Q4); the second part of the encoding is
the actual line encoding specific to copper, called 4-dimensional 5 level pulse amplitude
modulation (4D-PAM5). The 8B1Q4 encoding followed by the 4D-PAM5 line encod-
ing provide the synchronization, bandwidth, and SNR characteristics needed to make
possible the four wire pairs (working in parallel) running full duplex on each wire pair
simultaneously. For 1000BASE-X, 8-bit/10-bit (8B/10B) encoding (similar to the 4B/
5B concept) is used, followed by the simple NRZ line encoding of light on optical fiber.
1000BASE-T
Goals for 1000BASE-T (introduced as 802.3ab-1999 1000BASE-T Gigabit Ethernet
over twisted-pair) included these:
■ Capability to function over existing Category 5 copper cable plants
■ Assurance that this cable would work by passing a Category 5e test, which
most cable can pass after a careful retermination
■ Interoperability with 10BASE-T and 100BASE-TX
■ Applications such as building backbones, interswitch links, wiring closet applica-
tions, server farms, and high-end desktop workstations
■ Provision of 10x bandwidth of Fast Ethernet, which became very widely installed
by end users, helping to necessitate more speed upstream in the network
To achieve this speed running over Category 5e copper cable, 1000BASE-T needed
to use all four pairs of wires. Category 5e cable reliably can carry up to 125 Mbps of
traffic. Using sophisticated circuitry, full-duplex transmissions on the same wire pair
allow 250 Mbps per pair; multiplied by four wire pairs, this gives a total of 1000 Mbps
(1 Gbps). For some purposes, it is helpful to think of these four wire pairs as “lanes”
over which the data travels simultaneously (to be reassembled carefully at the receiver).
The timing, frame format, and transmission were described previously in Chapter 5

and are common to all versions of 1000-Mbps Ethernet considered here.
1000BASE-T uses 8B1Q4 encoding with 4D-PAM5 line encoding on Cat 5e or better
UTP. Achieving the 1-Gbps rate required use of all four pairs in full-duplex simulta-
neously. This results in a permanent collision on the wire pairs, which is very different
from the first coaxial Ethernet systems. The “permanent” collisions—transmission and
receipt of data happens in both directions on the same wire at the same time—results
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340 Chapter 6: Ethernet Technologies and Ethernet Switching
in very complex voltage patterns. But using sophisticated integrated circuits, which,
among other things, use a technique called echo cancellation, works well.
Despite the constant collision of the signals, the system is capable of operating through
a careful selection of voltage levels and use of Layer 1 forward error correction (FEC).
Figure 6-17 shows the outbound (transmitting [Tx]) 1000BASE-T signal (the y-axis is
voltage; the x-axis is the time taken from an oscilloscope—voltage is a “differential sig-
nal” measured between two paired wires in one of the four pairs present in UTP cable).
Figure 6-17 Outbound (Tx) 1000BASE-T Signal
Figure 6-18 shows actually 1000BASE-T signal captured with a digital storage oscillo-
scope after several meters of cable. (the y-axis is voltage; the x-axis is time—voltage is
a “differential signal” measured between two paired wires in one of the four pairs
present in UTP cable).
Figure 6-18 Actual 1000BASE-T Signal
It is quite remarkable that the signal can be recovered at all when it is revealed that
during idle periods, there are nine voltage levels found on the cable, and during data
transmission periods, there are 17 voltage levels on the cable. Note the complex line
encoding to begin with. Then, in Figure 6-18, look at the actual signal on the wire with
constant collisions, as well as attenuation effects and noise. The signal looks analog.
The key here is that sophisticated circuitry is decoding all of this. However, the system
is susceptible to cable problems, termination problems, and noise unless standards are
followed. Gigabit Ethernet works very well if the cabling, termination, and noise
guidelines are followed.

Table 6-8 summarizes the use of all four pairs in the UTP cable. A, B, C, and D could
be considered “lanes” of data. The data from the sending station carefully is divided
into four parallel streams, encoded, transmitted and detected in parallel, and then reas-
sembled into one received bit stream.
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Gigabit, 10-Gb, and Future Ethernet 341
Figure 6-19 is a schematic representation of simultaneous full-duplex on four wire
pairs. Station-to-station, switch-to-switch, and station-to-switch cabling connections
are the same as in Fast Ethernet.
Figure 6-19 1000BASE-T Signal Transmission
Table 6-8 1000BASE-T Pinout
Pin Number Signal
1 BI_DA+ (bidirectional data, positive going)
2 BI_DA- (bidirectional data, negative going)
3 BI_DB+ (bidirectional data, positive going)
4 BI_DC+ (bidirectional data, positive going)
5 BI_DC- (bidirectional data, negative going)
6 BI_DB- (bidirectional data, negative going)
7 BI_DD+ (bidirectional data, positive going)
8 BI_DD- (bidirectional data, negative going)
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342 Chapter 6: Ethernet Technologies and Ethernet Switching
It is especially important to desktop, office, and wiring closet applications that there be
interoperability among Gigabit, Fast, and 10BASE-T Ethernet. This might seem to be a
hopeless affair. But upon close inspection, note that if the cabling installed in the walls
tests out (as it often does or easily can be made to by retermination) at Category 5e
and if all eight wires in the RJ-45 connectors and jacks are connected, the signal paths
exists for Gigabit, Fast, and 10BASE-T Ethernet to interoperate. Just as 10/100 devices
emerged in Fast Ethernet, 10/100/1000 interfaces have been developed for interopera-
bility. By using the same frame format, compatible wiring paths, and clever interface

engineering, it all works well.
For historical reasons, CSMA/CD and half duplex are options on 1000BASE-T. But
the overwhelming use of 1000BASE-T is full duplex. This is accomplished with sophis-
ticated hybrid circuits that can act as Tx and Rx at the same time for the same wires.
Before communications can begin, the two link partners must determine which will
source the master clock and which will use the data stream to recover the slave clock.
The master clock and slave clock are used as time markers for signal transmission.
This process usually is determined during autonegotiation, although it can be config-
ured manually. A number of other parameters also are determined in the same manner,
including duplex type. Autonegotiation usually determines that a multiport device (a
switch or hub) should become the master clock. The overall message here is that with
the 1 nanosecond bit-times, 1 billion bps data transfer rate, and four wire pairs simul-
taneously transmitting and receiving, synchronization is extremely important.
When the topic of 1000BASE-X (1000BASE-SX and 1000BASE-LX) is presented,
comparisons with 1000BASE-T architecture are included.
1000BASE-SX and 1000BASE-LX
Gigabit Ethernet over fiber is one of the most recommended backbone technologies. Its
benefits are tremendous:
■ 1000-Mbps data transfer, which can aggregate groupings of widely deployed
Fast Ethernet devices
■ Noise immunity
■ Lack of any ground potential problems between floors or buildings
■ An explosion in 1000BASE-X device options
■ Excellent distance characteristics
Gigabit Ethernet over fiber originally was introduced in the IEEE 802.3 supplement
entitled “802.3z-1998 1000BASE-X Gigabit Ethernet.” The only application for
which 1000BASE-SX and 1000BASE-LX has not caught on as rapidly is the office
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Gigabit, 10-Gb, and Future Ethernet 343
desktop—1000BASE-TX is considered more “user-proof” in terms of day-to-day wear,

and 10-/100-/1000-Mbps copper interfaces are common.
The timing, frame format, and transmission were described previously in Chapter 5
and are common to all versions of 1000-Mbps Ethernet considered here.
1000BASE-X uses 8B/10B encoding converted to NRZ line encoding, with either
lower-cost short-wavelength 850 nm laser (or sometimes LED) sources and multimode
optical fiber (1000BASE-SX, S for short), or long-wavelength 1310 nm laser sources
and single-mode optical fiber (1000BASE-LX, L for long).
NRZ encoding relies on the signal level found in the timing window to determine the
binary value for that bit period. Unlike most of the other encoding schemes described,
this encoding system is level-driven instead of edge-driven.
In the encoding example in Figure 6-20, one timing window is highlighted vertically
through all four waveform examples. The top waveform is low across the timing win-
dow. A low signal level represents a binary 0. A single 1 was introduced at the end of
the waveform to show the other signal level.
Figure 6-20 NRZ Encoding Example
The second waveform is high across the timing window. A high signal level represents
a binary 1. Again, a single 0 was introduced at the end of the waveform to show the
other signal level. Instead of a repeating sequence of the same binary value in the third
waveform, there is an alternating binary sequence. In this example, it is more obvious
that a low signal level indicates a binary 0 and a high signal indicates a binary 1.
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344 Chapter 6: Ethernet Technologies and Ethernet Switching
The fourth waveform example is random data. Three of these examples are good
examples of why this encoding scheme has the potential to cause dc voltage drift on
copper media. The second example is changing levels each bit period and would not
suffer from dc voltage drift. It is very easy for a string of the same binary signal to
cause a dc voltage bias on the cable, which has the potential of causing clocking errors.
On fiber media, this is not an issue.
The NRZ-encoded serialized bit stream is ready for transmission using pulsed light as
specified for 1000BASE-SX or 1000BASE-LX. Because of cycle time problems related

to turning the transmitter completely on and off each time, the light is pulsed using
low and high power. A logical 0 is represented by low power, and a logical 1 is repre-
sented by high power.
Table 6-9 shows the amazingly simple interface-to-interface interconnection for Gigabit
Ethernet over fiber. SC fiber-optic connectors most commonly are used.
Figure 6-21 shows the interface-to-interface connection for 1000BASE-SX. Short-
wavelength laser (or sometimes LED) sources typically are used with multimode
optical fiber.
Figure 6-21 1000BASE-SX Fiber Interface-to-Interface Connection
Figure 6-22 shows the interface-to-interface connection for 1000BASE-LX. Laser
sources typically are used with single-mode fiber to achieve distances of up to 5000m.
Table 6-9 Interface-to-Interface Interconnection for Gigabit Ethernet
Fiber Signal
1 Tx (laser transmitters)
2 Rx (high-speed photodiode detectors)
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Gigabit, 10-Gb, and Future Ethernet 345
Figure 6-22 1000BASE-LX Interface-to-Interface Connection
The MAC method used treats the link as point-to-point, and fiber is intrinsically full
duplex because separate Tx and Rx fibers. Gigabit Ethernet permits a single repeater
between two stations.
Gigabit Ethernet Architecture
Any device that adapts between different Ethernet speeds, such as between 100 Mbps
and 1000 Mbps, is an OSI Layer 2 bridge. It is not possible to adapt between speeds
and still be a repeater.
Full-duplex links can be substantially longer that what is shown in Tables 6-9 and 6-10
because they are limited only by the medium, not by the round-trip delay. Gigabit Ether-
net architecture is overwhelmingly station-to-station, station-to-switch, switch-to-switch,
and switch-to-router connections running at full duplex. 1000BASE-SX is specified for
multimode fiber. 1000BASE-LX is specified for multimode and single-mode fiber.

Tables 6-10 and 6-11 show distance limitations for 1000BASE-SX and 1000BASE-LX.
Because most Gigabit Ethernet is switched, these are the practical limits between devices.
Daisy-chaining, star, and extended star topologies all are allowed. The issue then
becomes one of logical topology and data flow, not timing or distance limitations.
Table 6-10 Maximum 1000BASE-SX Cable Distances
Medium
The maximum 1000BASE-SX cable distances at 805 nm (minimum overfilled launch).
Modal Bandwidth Maximum Distance
62.5 µm MMF 160 220m
62.5 µm MMF 200 275m
50 µm MMF 400 500m
50 µm MMF 500 550m
Fiber Interface A (NIC, Switch Port, …)
Multimode Fiber (MMF)
Sc or MTRJ
Connector
Tx
Laser
Rx
Detector
Single-Mode Fiber
Rx
Laser
Tx
Detector
Fiber Interface B (NIC, Switch Port, …)
Fiber Cable
1000BASE-LX
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346 Chapter 6: Ethernet Technologies and Ethernet Switching

A 1000BASE-T UTP cable is about the same as a 10BASE-T and 100BASE-TX cable,
except that link performance must meet the higher-quality Category 5e or ISO Class D
(2000) requirements.
As with 10-Mbps and 100-Mbps versions, it is possible to modify some of the architec-
ture rules slightly; however, there is virtually no allowance for additional delay in half
duplex. Modification of the architecture rules strongly is discouraged for 1000BASE-T.
At 100m, 1000BASE-T is operating close to the edge of the hardware’s capability to
recover the transmitted signal. Any cabling problems or environmental noise could
render an otherwise-compliant cable inoperable even at distances that are within the
specification. Refer to the technical timing descriptions in the current 802.3 standard
and the technical information about your hardware performance before attempting
any adjustments to the architecture rules.
Links operating in full-duplex links might be longer that what is indicated in Table 6-12
because they are limited only by the capability of the medium to deliver a robust enough
signal to decode the signaling; they are not limited by the round-trip delay. It is extremely
rare to find Gigabit Ethernet operating in half duplex. Half duplex is undesirable because
the signaling scheme is inherently full duplex, and forcing half-duplex communications
rules onto a full-duplex signaling system is not a wise use of resources. Operating under
half-duplex rules requires adherence to slot time round-trip delay limitations that
reduce the effective cable lengths, and there is also a substantial increase in overhead
introduced by the carrier extension. Furthermore, very few Gigabit repeaters are in
service, which means that the link is probably between a station and an OSI Layer 2
bridge, or between two bridges, so the collision domain would end at the bridge anyway.
It is recommended that all links between a station and a switch be configured for auto-
negotiation, to permit the highest common performance configuration to be established
without risking misconfiguration of the link, and to avoid accidental misconfiguration
of the other required parameters for proper Gigabit Ethernet operation.
Table 6-11 Maximum 1000BASE-LX Cable Distances
Medium Modal Bandwidth Maximum Distance
62.5 µm MMF 500 550m

50 µm MMF 400 550m
50 µm MMF 500 550m
10 µm SMF — 5000m
The maximum 1000BASE-LX cable distances at 805 nm (minimum overfilled launch).
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Gigabit, 10-Gb, and Future Ethernet 347
Table 6-12 shows the speed case for half-duplex operation. But because most Gigabit
Ethernet is switched, it is subject to link-by-link rules shown previously in Tables 6-10
and 6-11.
10-Gbps Versions of Ethernet
Most recently, in 2002, IEEE 802.3ae was adapted. This standard specifies 10-Gbps
full-duplex transmission over fiber-optic cable. Taken as a whole, the similarities
between 802.3ae and 802.3 (the original Ethernet) and all of the other varieties of
Ethernet are remarkable. Metcalfe’s original design has evolved, but it is still very
apparent in the modern Ethernet. Recently, 10-Gb Ethernet (10GbE) has emerged as
the latest example of the extensibility of the Ethernet system. Usable for LANs, storage-
area networks (SANs), metropolitan-area networks (MANs), and WANs, 10GbE offers
exciting new networking possibilities. What is 10GbE, and why should it be used?
Legacy Ethernet, Fast Ethernet, and Gigabit Ethernet now dominate the LAN market.
The next step in the evolution of Ethernet is to move to 10-Gb Ethernet (10GbE, oper-
ating at 10,000,000,000 bps). By maintaining the frame format and other Ethernet
Layer 2 specifications, increasing bandwidth needs can be accommodated with the
low-cost, easily implementable, and easily interoperable 10GbE. 10GbE runs only over
optical fiber media. End-to-end Ethernet networks become possible.
Because of massive growth in Internet- and intranet-based traffic, and the rapidly increas-
ing use of Gigabit Ethernet, even higher bandwidth interconnections are needed. Internet
service providers (ISPs) and network service providers (NSPs) can use 10GbE to create
high-speed, low-cost, easily interoperable connections between colocated carrier switches
and routers. Points of presence (POPs), intranet server farms comprised of Gigabit
Ethernet servers, digital video studios, SANs, and backbones already are envisaged

applications.
Perhaps most significantly, a major conceptual change comes with 10GbE. Ethernet
traditionally is thought of as a LAN technology. But 10GbE physical layer standards
allow both an extension in distance (to 40 km over single-mode fiber) and compatibility
Table 6-12 Architecture Configuration Cable Distances for Half-Duplex Operation
Architecture 1000BASE-T 1000BASE-SX/LX
1000BASE-SX/LX and
1000BASE-T
Station to station 100m 316m —
One repeater 200m 220m 100m 1000BASE-T (plus)
110m 1000BASE-SX/LX
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348 Chapter 6: Ethernet Technologies and Ethernet Switching
with Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH)
networks. Operation at a 40 km distance makes 10GbE a viable MAN technology.
Compatibility with SONET/SDH networks operating up to OC-192 speeds
(9.584640 Gps) makes 10GbE a viable WAN technology. 10GbE also might com-
pete with Asynchronous Transfer Mode (ATM) for certain applications.
The following summarizes how 10GbE compares to other varieties of Ethernet:
■ Frame format is the same, allowing interoperability among all varieties of legacy,
Fast, Gigabit, and 10-Gb Ethernet, with no reframing or protocol conversions.
■ Bit-time now at 0.1 nanoseconds. All other time variables scale accordingly.
■ No need for CSMA/CD because only full-duplex fiber connections are used.
■ IEEE 802.3 sublayers within OSI Layers 1 and 2 that are mostly preserved, with
a few additions to accommodate 40-km fiber links and interoperability with
SONET/SDH technologies.
■ Possibility of flexible, efficient, reliable, relatively low cost end-to-end Ethernet
networks.
■ Capability to run TCP/IP over LANs, MANs, and WANs with one Layer 2 trans-
port method

The basic standard governing CSMA/CD is IEEE 802.3. An IEEE 802.3 supplement,
entitled 802.3ae, governs the 10GBASE family. As is typical for new technologies, a
variety of implementations are being considered, including these:
■ 10GBASE-SR—Intended for short distances over already-installed multimode
fiber, supports a range between 26 m and 82 m.
■ 10GBASE-LX4—Uses wavelength-division multiplexing (WDM). Supports 240 m
to 300 m over already-installed multimode fiber, and 10 km over single-mode fiber.
■ 10GBASE-LR and 10GBASE-ER—Supports 10 km and 40 km over single-mode
fiber.
■ 10GBASE-SW, 10GBASE-LW, and 10GBASE-EW—Intended to work with
OC-192/STM SONET/SDH WAN equipment.
The IEEE 802.3ae task force and the 10-Gb Ethernet Alliance (10 GEA) are working
to standardize these emerging technologies.
10-Gb Ethernet (IEEE 802.3ae) was standardized in June 2002. It is a full-duplex
protocol that uses only fiber-optic fiber as a transmission medium. The maximum
transmission distances depend on the type of fiber being used. When using single-mode
fiber as the transmission medium, the maximum transmission distance is 40 km (25 miles).
Some discussions between IEEE members suggest the possibility of standards for 40-Gbps,
80-Gbps, and even 100-Gbps Ethernet. Given the history of Ethernet, there is no reason
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