Gigabit, 10-Gb, and Future Ethernet 349
to expect that its evolution will cease. The higher speeds and greater transmission dis-
tances that are making Ethernet both a LAN and a MAN protocol are not the only
additions to the Ethernet standard that we are likely to see. Because fiber is being used
as the transmission medium, the likelihood that an error in the data might occur during
the passage of the Ethernet packet across the network is very low, much lower than in
the original Ethernet. On a network with a very low error rate, it makes sense to trans-
mit larger packets of data.
The upper limit on the amount of data that can be carried in an Ethernet packet (a
frame) is 1500 bytes. Sending more data than that in a frame would make it an invalid
Ethernet frame and cause the network to discard it. This has been the standard since
Ethernet was created. Given a low likelihood of errors on a network, large files could
be moved over the network more efficiently if a larger amount of data could be carried
in each frame. The reason for this is that it takes time for computers to generate and to
process Ethernet headers and trailers. Each Ethernet frame must have a header and a
trailer. For example, if six times as much data could be sent per frame, there would be
fewer frames (only one sixth as many) needed to carry all the data in a file. This means
that fewer headers and trailers would have to be generated by the transmitter and pro-
cessed by the receiver. The result is a shorter amount of time needed to move a large
file over a network between two computers. WANs that use fiber as their transmission
medium routinely transmit large data packets.
For this reason, especially when multigigabit LANs are connected to WANs, it is likely
that we will see the use of Jumbo Ethernet frames. A Jumbo frame is any Ethernet frame
that is carrying more than 1500 bytes of data. The proposed upper limit for the amount
of data carried in a Jumbo frame is about 9,000 bytes. Jumbo frames are not currently
a part of the new IEEE 802.3ae standard. However, it is very likely that some vendors
of Ethernet networking equipment will allow Jumbo frames to be carried on Ethernet
networks built using only their equipment. This might force the IEEE 802.3 committee
to make support for larger frame sizes an option in new multigigabit standards.
Table 6-13 shows the parameters for 10-Gb Ethernet operation.
Table 6-13 Parameters for 10-Gbps Ethernet Operation

Parameter Value
Bit-time 0.1 nsec
Slot time —*
Interframe spacing 96 bits**
continues
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350 Chapter 6: Ethernet Technologies and Ethernet Switching
Amazingly, 10GbE uses the same frame format (with a few special case exceptions) as
10-, 100-, and 1000-Mbps Ethernet.
10GbE Media, Connections, and Architecture
10-Gb Ethernet is a tenfold increase in speed over Gigabit Ethernet. Just as with Gigabit
Ethernet, with this increase in speed comes extra requirements—the bits being sent get
shorter in duration (1 ns), occur more frequently, and require more careful timing. In
addition, their transmission requires frequencies closer to medium bandwidth limitations
and they become more susceptible to noise. In response to these issues of synchroniza-
tion, bandwidth, and SNR, two separate encoding steps are used by 10-Gb Ethernet.
The basic idea is to use codes—which can be engineered to have desirable properties—
to represent the user data in a way that is efficient to transmit, including synchronization,
efficient usage of bandwidth, and improved SNR characteristics.
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 Gigabit Ethernet. 8B/10B encoding (similar to the 4B/5B concept) is
used, followed by several different types of line encoding on the optical fiber.
Collision attempt limit —*
Collision backoff limit —*
Collision jam size —*
Maximum untagged frame size 1518 octets
Minimum frame size 512 bits (64 octets)

Burst limit —*
Interframe spacing stretch ratio 104 bits***
* 10-Gbps Ethernet does not permit half-duplex operation, so parameters related to slot timing
and collision handling do not apply.
** The value listed is the official interframe spacing.
*** The interframe spacing stretch ratio applies exclusively to 10GBASE-W definitions.
Table 6-13 Parameters for 10-Gbps Ethernet Operation (Continued)
Parameter Value
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Gigabit, 10-Gb, and Future Ethernet 351
Figure 6-23 represents what happens to the 8B-10B before it is line-encoded. 10-Gb
Ethernet uses a variety of complex encodings before line encoding, including 8B/10B
and 64B/66B. Bits from these codes then are converted to line signals: low power light
for binary 0 and higher power light for binary 1. Complex serial bit streams are used
for all versions of 10GbE except for 10GBASE-LX4, which uses wide wavelength-
division multiplexing (WWDM) to multiplex 4-bit simultaneous bit streams as four
wavelengths of light launched into the fiber at one time.
Figure 6-23 How 10GbE Converts MAC Frames to Four Lanes of Bits
Figure 6-23 shows how 10GbE converts MAC frames to four lanes of bits for parallel
transmission on four wire pairs of UTP or as a bit stream that is then serialized for
laser transmission on single-mode fiber.
Figure 6-24 represents the particular case of using four slightly different-colored laser
sources. Upon receipt from the medium, the optical signal stream is demultiplexed into
four separate optical signal streams. The four optical signal streams then are converted
back into four electronic bit streams as they travel in approximately the reverse pro-
cess back up through the sublayers to the MAC sublayer.
Currently, most 10GbE products are in the form of modules (line cards) for addition to
high-end switches and routers. As the 10GbE technologies evolve, an increasing diver-
sity of signaling components can be expected. As optical technologies involve, improved
transmitters and receivers will be incorporated into these products, further taking

advantage of modularity. All 10GbE varieties use optical-fiber media. Fiber types
include 10µm single-mode fiber, and 50µm and 62.5 µm multimode fibers. A range of
fiber attenuation and dispersion characteristics are supported, but they limit operating
distances.
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352 Chapter 6: Ethernet Technologies and Ethernet Switching
Figure 6-24 10GBASE-LX4 Signal Multiplexing
SC fiber optic connectors most commonly are used. Because optical fiber is the medium
used by 10GbE, typically a fiber pair connects Tx for device 1 to Rx for device 2, and
vice versa. The primary devices connecting currently via 10GbE are high-end modular
switches and routers. Table 6-14 lists the pinout options for 10GbE.
10-Gb Ethernet is available in full-duplex mode only and runs only over optical fiber.
Hence, collisions are nonexistent and CSMA/CD is unnecessary.
As 10GbE standards and products evolve, a wide range of architectures and applica-
tion guidelines is becoming possible. Most important to consider is that the addition of
10GbE, with its LAN, SAN, MAN, and WAN capabilities, enables network engineers
to consider very sophisticated end-to-end Ethernet networks. LAN, SAN, MAN, and
WAN topologies using Gigabit Ethernet all are being implemented.
10-Gb Ethernet is supported only over fiber-optic media. Support is available for 62.5 µm
and 50 µm multimode fiber, as well as 10 µm single-mode fiber. Even though support is
limited to fiber-optic media, some of the maximum cable lengths are surprisingly short.
No repeater is defined for 10-Gb Ethernet because half duplex explicitly is not supported.
Table 6-14 10GbE Pinout
Fiber Signal
1 Tx (laser transmitters)
2 Rx (high-speed photodiode detectors)
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Gigabit, 10-Gb, and Future Ethernet 353
As with 10-Mbps, 100,-Mbps and 1000-Mbps versions, it is possible to modify some
of the architecture rules slightly. Possible architecture adjustments are related to signal

loss and distortion along the medium. Because of dispersion of the signal and other
issues, the light pulse becomes undecipherable beyond certain distances. Refer to the
technical timing and spectral requirements in the current 802.3 standard, as well as
the technical information about your hardware performance, before attempting any
adjustments to the architecture rules.
Table 6-15 shows the 10-Gb Ethernet implementations. Both R and W specifications
are covered by each appropriate entry (for example, 10GBASE-E covers both
10GBASE-ER and 10GBASE-EW).
Note the versatility of 10GbE. A diverse set of fiber types and laser sources can be used
to achieve not only LAN, but also MAN and WAN distances.
Table 6-15 10-Gb Ethernet Implementations
Implementation Wavelength Medium
Minimum
Modal Bandwidth
Operating
Distance
10GBASE-LX4 1310 nm 62.5 µm MMF 500 MHz/km 2m to 300m
10GBASE-LX4 1310 nm 50 µm MMF 400 MHz/km 2m to 240m
10GBASE-LX4 1310 nm 50 µm MMF 500 MHz/km 2m to 300m
10GBASE-LX4 1310 nm 10 µm SMF — 2 km to 10 km
10GBASE-S 850 nm 62.5 µm MMF 160 MHz/km 2m to 26m
10GBASE-S 850 nm 62.5 µm MMF 200 MHz/km 2m to 33m
10GBASE-S 850 nm 50 µm MMF 400 MHz/km 2m to 66m
10GBASE-S 850 nm 50 µm MMF 500 MHz/km 2m to 82 m
10GBASE-S 850 nm 50 µm MMF 2000 MHz/km 2m to 300 m
10GBASE-L 1310 nm 10 µm SMF — 2 km to 10 km
10GBASE-E 1550 nm 10 µm SMF — 2 km to 30 km*
*The standard permits 40-km lengths if link attenuation is low enough.
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354 Chapter 6: Ethernet Technologies and Ethernet Switching

The Future of Ethernet
As the last several sections have documented, Ethernet has gone through an evolution
from legacy to Fast to Gigabit to multigigabit technologies. Although other LAN tech-
nologies are still in place (legacy installations), Ethernet dominates new LAN installa-
tions—so much so that some have referred to Ethernet as the LAN “dial tone.” Ethernet
is now the standard for horizontal, vertical, and interbuilding connections. In fact,
recently developing versions of Ethernet are blurring the distinction between LANs,
MANs, and WANs in terms of geographic distance covered as part of one network.
Figure 6-25 illustrates the expanding scope of Ethernet.
Figure 6-25 Ethernet’s Expanding Scope
Although Gigabit Ethernet is now widely available and 10-Gb products becoming
more available, the IEEE and the 10-Gb Ethernet Alliance currently have released
40-Gbps, 100-Gbps, and even 160-Gbps standards. Which technologies actually are
adopted will depend on a number of factors, including the rate of maturation of the
technologies and standards, the rate of adoption in the market, and cost.
Proposals for Ethernet arbitration schemes other than CSMA/CD have been made.
But the problem of collisions, so fundamental to physical bus topologies of 10BASE5,
10BASE2, 10BASE-T, and 100BASE-TX hubs, are no longer so common. Use of UTP
and optical fiber, both of which have separate Tx and Rx paths, and the decreasing
costs of switched instead of hubbed connections, make single shared-media, half-duplex
media connections much less important.
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Ethernet Switching 355
The future of networking media is threefold:
■ Copper (up to 1000 Mbps, perhaps more)
■ Wireless (approaching 100 Mbps, perhaps more)
■ Optical fiber (currently at 10,000 Mbps and soon to be more)
Unlike copper and wireless media, in which certain physical and practical limitations
on the highest-frequency signals that can be transmitted are being approached, the
bandwidth limitation on optical fiber is extremely large and is not a limiting factor for

the foreseeable future. In fiber systems, the electronics technology (such as emitters
and detectors) and the fiber-manufacturing processes most limit the speed. Therefore,
upcoming developments in Ethernet likely will be heavily weighted toward laser light
sources and single-mode optical fiber.
When Ethernet was slower, half duplex (subject to collisions and a “democratic” process
for prioritization) was not considered to have the quality of service (QoS) capabilities
required to handle certain types of traffic. This included such things as IP telephony
and video multicast.
However, the full-duplex, high-speed Ethernet technologies that now dominate the
market are proving to be sufficient at supporting even QoS-intensive applications. This
makes the potential applications of Ethernet even wider. Ironically, end-to-end QoS
capability helped drive a push for ATM to the desktop and to the WAN in the mid-
1990s, but now Ethernet, not ATM, approaching this goal.
At 30 years old, Ethernet technologies continue to grow and have a very bright future.
Ethernet Switching
As more nodes are added to an Ethernet physical segment, the contention for the
medium increases. The addition of more nodes increases the demands on the available
bandwidth and places additional loads on the medium. With the additional traffic, the
probability of collisions increases, resulting in more retransmissions. A solution to the
problem is to break the large segment into parts and separate it using Catalyst switches.
This isolates these newly segmented sections into isolated collision domains. This reduces
the number of collisions and increases the reliability of the network.
Bridging and switching are technologies that decrease congestion in LANs by reducing
traffic and increasing bandwidth. LAN switches and bridges, operating at Layer 2 of
the OSI reference model, forward frames based on the MAC addresses to perform the
switching function. If the Layer 2 MAC address is unknown, the device floods the
frame in an attempt to reach the desired destination. LAN switches and bridges also
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356 Chapter 6: Ethernet Technologies and Ethernet Switching
forward all broadcast frames. The result could be storms of traffic being looped end-

lessly through the network. The Spanning Tree Protocol (STP) is a loop-prevention
protocol; it is a technology that enables switches to communicate with each other to
discover physical loops in the network.
The sections that follow introduce Layer 2 bridging, Layer 2 switching, switching
modes, and the Spanning Tree Protocol (STP).
Layer 2 Bridging
A bridge is a Layer 2 device designed to create two or more LAN segments, each of
which is a separate collision domain. In other words, bridges were designed to create
more usable bandwidth. The purpose of a bridge is to filter traffic on a LAN to keep
local traffic local, yet allow connectivity to other parts (segments) of the LAN for traf-
fic that is directed there. To filter or selectively deliver network traffic, bridges build
tables of all MAC addresses located on a network segment and other networks, and
map them to associated ports. The process is as follows:
■ If data comes along the network medium, a bridge compares the destination
MAC address carried by the data to MAC addresses contained in its tables.
■ If the bridge determines that the destination MAC address of the data is from the
same network segment as the source, it does not forward the data to other seg-
ments of the network. This process is known as filtering. By performing this
process, bridges significantly can reduce the amount of traffic between network
segments by eliminating unnecessary traffic.
■ If the bridge determines that the destination MAC address of the data is not from
the same network segment as the source, it forwards the data to the appropriate
segment.
■ If the destination MAC address is unknown to the bridge, the bridge broadcasts
the data to all devices on a network except the one on which it was received. The
process is known as flooding.
Generally, a bridge has only two ports and divides a collision domain into two parts.
All decisions made by a bridge are based on MAC addresses or Layer 2 addressing,
and do not affect the logical or Layer 3 addressing. Thus, a bridge divides a collision
domain but not a logical or broadcast domain. No matter how many bridges are in a

network, unless a device such as a router works on Layer 3 addressing, all of the net-
work will share the same logical (broadcast) address space. A bridge will create more
(and smaller) collision domains but will not add broadcast domains. Because every
device on the network must pay attention to broadcasts, bridges always forward them.
Therefore, all segments in a bridged environment are considered to be in the same
broadcast domain.
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Ethernet Switching 357
Layer 2 Switching
LAN switches are essentially multiport bridges that use microsegmentation to reduce
the number of collisions in a LAN and increase the bandwidth. LAN switches also
support features such as full-duplex communication and multiple simultaneous con-
versations. Figure 6-30 shows a LAN with three workstations, a LAN switch, and the
LAN switch’s address table. The LAN switch has four interfaces (or network connec-
tions). Stations A and C are connected to the switch’s Interface 3, and Station B is on
Interface 4. As indicated in Figure 6-26, Station A needs to transmit data to Station B.
Figure 6-26 LAN Switch Operation
Remember that as this traffic goes through the network, the switch operates at Layer 2,
meaning that the switch can look at the MAC layer address. When Station A transmits
and the switch receives the frames, the switch assesses the traffic as it goes through to dis-
cover the source MAC address and store it in the address table, as shown in Figure 6-27.
Figure 6-27 Building an Address Table
AC
B
1
2
3
4
10 Mbps
10 Mbps

Interface
Stations
12
3
4
Data from A to B
AC
B
1
2
3
4
10 Mbps
10 Mbps
Interface
Stations
12
3
4
A
X
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358 Chapter 6: Ethernet Technologies and Ethernet Switching
As the traffic goes through the switch, an entry is made in the address table identifying
the source station and the interface that it is connected to on the switch. The switch
now knows where Station A is connected. When that frame of data is in the switch, it
floods to all ports because the destination station is unknown, as shown in Figure 6-28.
Figure 6-28 Flooding Data to All Switch Ports
After the address entry is made in the table, however, a response comes back from
Station B to Station A. The switch now knows that Station B is connected to Inter-

face 4, as shown in Figure 6-29.
Figure 6-29 Responding to the Flooding Message
The data is transmitted into the switch, but notice that the switch does not flood the
traffic this time. The switch sends the data out of only Interface 3 because it knows
where Station A is on the network, as shown in Figure 6-30.
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