Module
Overview
7.1 10-Mbps and
100-Mbps Ethernet
7.1.1 10-Mbps
Ethernet
7.1.2 10BASE5
7.1.3 10BASE2
7.1.4 10BASE-T
7.1.5 10BASE-T
wiring and architecture
7.1.6 100-Mbps
Ethernet
7.1.7 100BASE-TX
7.1.8 100BASE-FX
7.1.9 Fast
Ethernet architecture
7.2 Gigabit and
10-Gigabit Ethernet
7.2.1 1000-Mbps
Ethernet
7.2.2 1000BASE-T
7.2.3 1000BASE-SX
and LX
7.2.4 Gigabit
Ethernet architecture
7.2.5 10-Gigabit
Ethernet
7.2.6 10-Gigabit
Ethernet architectures
7.2.7 Future of
Ethernet
Module: Summary
Overview
Ethernet has been
the most successful LAN technology mainly because of how easy it is to
implement. Ethernet has also been successful because it is a flexible
technology that has evolved as needs and media capabilities have changed. This
module will provide details about the most important types of Ethernet. The
goal is to help students understand what is common to all forms of Ethernet.
Changes in
Ethernet have resulted in major improvements over the 10-Mbps Ethernet of the
early 1980s. The 10-Mbps Ethernet standard remained virtually unchanged until
1995 when IEEE announced a standard for a 100-Mbps Fast Ethernet. In recent
years, an even more rapid growth in media speed has moved the transition from
Fast Ethernet to Gigabit Ethernet. The standards for Gigabit Ethernet emerged
in only three years. A faster Ethernet version called 10-Gigabit Ethernet is
now widely available and faster versions will be developed.
MAC addresses,
CSMA/CD, and the frame format have not been changed from earlier versions of
Ethernet. However, other aspects of the MAC sublayer, physical layer, and
medium have changed. Copper-based NICs capable of 10, 100, or 1000 Mbps are now
common. Gigabit switch and router ports are becoming the standard for wiring
closets. Optical fiber to support Gigabit Ethernet is considered a standard for
backbone cables in most new installations.
This module
covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811
exams.
Students who
complete this module should be able to perform the following tasks:
- Describe the differences and
similarities among 10BASE5, 10BASE2, and 10BASE-T Ethernet
- Define Manchester encoding
- List the factors that affect
Ethernet timing limits
- List 10BASE-T wiring parameters
- Describe the key
characteristics and varieties of 100-Mbps Ethernet
- Describe the evolution of
Ethernet
- Explain the MAC methods, frame
formats, and transmission process of Gigabit Ethernet
- Describe the uses of specific
media and encoding with Gigabit Ethernet
- Identify the pinouts and wiring
typical to the various implementations of Gigabit Ethernet
- Describe the similarities and
differences between Gigabit and 10-Gigabit Ethernet
- Describe the basic architectural
considerations of Gigabit and 10-Gigabit Ethernet
7.1
10-Mbps and 100-Mbps Ethernet
7.1.1
10-Mbps Ethernet
This page will
discuss 10-Mbps Ethernet technologies.
10BASE5, 10BASE2,
and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet
are timing parameters, the frame format, transmission processes, and a basic
design rule.
Figure displays the parameters for 10-Mbps Ethernet
operation. 10-Mbps Ethernet and slower versions are asynchronous. Each
receiving station uses eight octets of timing information to synchronize its
receive circuit to the incoming data. 10BASE5, 10BASE2, and 10BASE-T all share
the same timing parameters. For example, 1 bit time at 10 Mbps = 100
nanoseconds (ns) = 0.1 microseconds = 1 10-millionth of a second. This means
that on a 10-Mbps Ethernet network, 1 bit at the MAC sublayer requires 100 ns
to transmit.
For all speeds of
Ethernet transmission 1000 Mbps or slower, transmission can be no slower than
the slot time. Slot time is just longer than the time it theoretically can take
to go from one extreme end of the largest legal Ethernet collision domain to
the other extreme end, collide with another transmission at the last possible
instant, and then have the collision fragments return to the sending station to
be detected.
10BASE5, 10BASE2,
and 10BASE-T also have a common frame format.
The Legacy
Ethernet transmission process is identical until the lower part of the OSI
physical layer. As the frame passes from the MAC sublayer to the physical
layer, other processes occur before the bits move from the physical layer onto
the medium. One important process is the signal quality error (SQE) signal. The
SQE is a transmission sent by a transceiver back to the controller to let the
controller know whether the collision circuitry is functional. The SQE is also
called a heartbeat. The SQE signal is designed to fix the problem in earlier
versions of Ethernet where a host does not know if a transceiver is connected.
SQE is always used in half-duplex. SQE can be used in full-duplex operation but
is not required. SQE is active in the following instances:
- Within 4 to 8 microseconds
after a normal transmission to indicate that the outbound frame was
successfully transmitted
- Whenever there is a collision
on the medium
- Whenever there is an improper
signal on the medium, such as jabber, or reflections that result from a
cable short
- Whenever a transmission has
been interrupted
All 10-Mbps forms
of Ethernet take octets received from the MAC sublayer and perform a process
called line encoding. Line encoding describes how the bits are actually
signaled on the wire. The simplest encodings have undesirable timing and
electrical characteristics. Therefore, line codes have been designed with
desirable transmission properties. This form of encoding used in 10-Mbps
systems is called Manchester encoding.
Manchester
encoding uses the transition in the middle of the timing window to determine
the binary value for that bit period. In Figure , the top waveform moves to a
lower position so it is interpreted as a binary zero. The second waveform moves
to a higher position and is interpreted as a binary one. The third waveform has
an alternating binary sequence. When binary data alternates, there is no need
to return to the previous voltage level before the next bit period. The wave
forms in the graphic show that the binary bit values are determined based on
the direction of change in a bit period. The voltage levels at the start or end
of any bit period are not used to determine binary values.
Legacy Ethernet
has common architectural features. Networks usually contain multiple types of
media. The standard ensures that interoperability is maintained. The overall
architectural design is most important in mixed-media networks. It becomes
easier to violate maximum delay limits as the network grows. The timing limits
are based on the following types of parameters:
- Cable length and propagation
delay
- Delay of repeaters
- Delay of transceivers
- Interframe gap shrinkage
- Delays within the station
10-Mbps Ethernet
operates within the timing limits for a series of up to five segments separated
by up to four repeaters. This is known as the 5-4-3 rule. No more than four
repeaters can be used in series between any two stations. There can also be no
more than three populated segments between any two stations.
The next page
will describe 10BASE5.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.2
10BASE5
This page will
discuss the original 1980 Ethernet product, which is 10BASE5. 10BASE5
transmitted 10 Mbps over a single thin coaxial cable bus.
10BASE5 is
important because it was the first medium used for Ethernet. 10BASE5 was part
of the original 802.3 standard. The primary benefit of 10BASE5 was length.
10BASE5 may be found in legacy installations. It is not recommended for new
installations. 10BASE5 systems are inexpensive and require no configuration.
Two disadvantages are that basic components like NICs are very difficult to
find and it is sensitive to signal reflections on the cable. 10BASE5 systems
also represent a single point of failure.
10BASE5 uses
Manchester encoding. It has a solid central conductor. Each segment of thick
coax may be up to 500 m (1640.4 ft) in length. The cable is large, heavy, and
difficult to install. However, the distance limitations were favorable and this
prolonged its use in certain applications.
When the medium
is a single coaxial cable, only one station can transmit at a time or a
collision will occur. Therefore, 10BASE5 only runs in half-duplex with a
maximum transmission rate of 10 Mbps.
Figure illustrates a configuration for an end-to-end
collision domain with the maximum number of segments and repeaters. Remember
that only three segments can have stations connected to them. The other two
repeated segments are used to extend the network.
The Lab Activity
will help students decode a waveform.
The Interactive
Media Activity will help students learn the features of 10BASE5 technology.
The next page
will discuss 10BASE2.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.3
10BASE2
This page covers
10BASE2, which was introduced in 1985.
Installation was
easier because of its smaller size, lighter weight, and greater flexibility.
10BASE2 still exists in legacy networks. Like 10BASE5, it is no longer
recommended for network installations. It has a low cost and does not require
hubs.
10BASE2 also uses
Manchester encoding. Computers on a 10BASE2 LAN are linked together by an
unbroken series of coaxial cable lengths. These lengths are attached to a
T-shaped connector on the NIC with BNC connectors.
10BASE2 has a
stranded central conductor. Each of the maximum five segments of thin coaxial
cable may be up to 185 m (607 ft) long and each station is connected directly
to the BNC T-shaped connector on the coaxial cable.
Only one station
can transmit at a time or a collision will occur. 10BASE2 also uses
half-duplex. The maximum transmission rate of 10BASE2 is 10 Mbps.
There may be up
to 30 stations on a 10BASE2 segment. Only three out of five consecutive
segments between any two stations can be populated.
The Interactive
Media Activity will help students learn the features of 10BASE2 technology.
The next page
will discuss 10BASE-T.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.4
10BASE-T
This page covers
10BASE-T, which was introduced in 1990.
10BASE-T used
cheaper and easier to install Category 3 UTP copper cable instead of coax
cable. The cable plugged into a central connection device that contained the
shared bus. This device was a hub. It was at the center of a set of cables that
radiated out to the PCs like the spokes on a wheel. This is referred to as a
star topology. As additional stars were added and the cable distances grew,
this formed an extended star topology. Originally 10BASE-T was a half-duplex
protocol, but full-duplex features were added later. The explosion in the
popularity of Ethernet in the mid-to-late 1990s was when Ethernet came to
dominate LAN technology.
10BASE-T also
uses Manchester encoding. A 10BASE-T UTP cable has a solid conductor for each
wire. The maximum cable length is 90 m (295 ft). UTP cable uses eight-pin RJ-45
connectors. Though Category 3 cable is adequate for 10BASE-T networks, new
cable installations should be made with Category 5e or better. All four pairs
of wires should be used either with the T568-A or T568-B cable pinout
arrangement. This type of cable installation supports the use of multiple
protocols without the need to rewire. Figure
shows the pinout arrangement for a 10BASE-T connection. The pair that
transmits data on one device is connected to the pair that receives data on the
other device.
Half duplex or
full duplex is a configuration choice. 10BASE-T carries 10 Mbps of traffic in
half-duplex mode and 20 Mbps in full-duplex mode.
The Interactive
Media Activity will help students learn the features of 10BASE-T technology.
The next page
describes the wiring and architecture of 10BASE-T.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.5
10BASE-T wiring and architecture
This page
explains the wiring and architecture of 10BASE-T.
A 10BASE-T link
generally connects a station to a hub or switch. Hubs are multi-port repeaters
and count toward the limit on repeaters between distant stations. Hubs do not
divide network segments into separate collision domains. Bridges and switches
divide segments into separate collision domains. The maximum distance between
bridges and switches is based on media limitations.
Although hubs may
be linked, it is best to avoid this arrangement. A network with linked hubs may
exceed the limit for maximum delay between stations. Multiple hubs should be
arranged in hierarchical order like a tree structure. Performance is better if
fewer repeaters are used between stations.
An architectural
example is shown in Figure . The distance from one end of the network to the
other places the architecture at its limit. The most important aspect to
consider is how to keep the delay between distant stations to a minimum,
regardless of the architecture and media types involved. A shorter maximum
delay will provide better overall performance.
10BASE-T links
can have unrepeated distances of up to 100 m (328 ft). While this may seem like
a long distance, it is typically maximized when wiring an actual building. Hubs
can solve the distance issue but will allow collisions to propagate. The
widespread introduction of switches has made the distance limitation less
important. If workstations are located within 100 m (328 ft) of a switch, the
100-m distance starts over at the switch.
The next page
will describe Fast Ethernet.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.6
100-Mbps Ethernet
This page will
discuss 100-Mbps Ethernet, which is also known as Fast Ethernet. The two
technologies that have become important are 100BASE-TX, which is a copper UTP
medium and 100BASE-FX, which is a multimode optical fiber medium.
Three
characteristics common to 100BASE-TX and 100BASE-FX are the timing parameters,
the frame format, and parts of the transmission process. 100BASE-TX and
100BASE-FX both share timing parameters. Note that one bit time at 100-Mbps =
10 ns = .01 microseconds = 1 100-millionth of a second.
The 100-Mbps
frame format is the same as the 10-Mbps frame.
Fast Ethernet is
ten times faster than 10BASE-T. The bits that are sent are shorter in duration
and occur more frequently. These higher frequency signals are more susceptible
to noise. In response to these issues, two separate encoding steps are used by
100-Mbps Ethernet. The first part of the encoding uses a technique called
4B/5B, the second part of the encoding is the actual line encoding specific to
copper or fiber.
The next page
will discuss the 100BASE-TX standard.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.7
100BASE-TX
This page will
describe 100BASE-TX.
In 1995,
100BASE-TX was the standard, using Category 5 UTP cable, which became
commercially successful.
The original
coaxial Ethernet used half-duplex transmission so only one device could
transmit at a time. In 1997, Ethernet was expanded to include a full-duplex
capability that allowed more than one PC on a network to transmit at the same
time. Switches replaced hubs in many networks. These switches had full-duplex
capabilities and could handle Ethernet frames quickly.
100BASE-TX uses
4B/5B encoding, which is then scrambled and converted to Multi-Level Transmit
(MLT-3) encoding. Figure shows four
waveform examples. The top waveform has no transition in the center of the
timing window. No transition indicates a binary zero. The second waveform shows
a transition in the center of the timing window. A transition represents a
binary one. The third waveform shows an alternating binary sequence. The fourth
wavelength shows that signal changes indicate ones and horizontal lines
indicate zeros.
Figure shows the pinout for a 100BASE-TX connection.
Notice that the two separate transmit-receive paths exist. This is identical to
the 10BASE-T configuration.
100BASE-TX
carries 100 Mbps of traffic in half-duplex mode. In full-duplex mode,
100BASE-TX can exchange 200 Mbps of traffic. The concept of full duplex will
become more important as Ethernet speeds increase.
The next page
will discuss the fiber optic version of Fast Ethernet.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.8
100BASE-FX
This page covers
100BASE-FX.
When copper-based
Fast Ethernet was introduced, a fiber version was also desired. A fiber version
could be used for backbone applications, connections between floors, buildings
where copper is less desirable, and also in high-noise environments. 100BASE-FX
was introduced to satisfy this desire. However, 100BASE-FX was never adopted
successfully. This was due to the introduction of Gigabit Ethernet copper and
fiber standards. Gigabit Ethernet standards are now the dominant technology for
backbone installations, high-speed cross-connects, and general infrastructure
needs.
The timing, frame
format, and transmission are the same in both copper and fiber versions of
100-Mbps Fast Ethernet. 100BASE-FX, however, uses NRZI encoding, which is shown
in Figure . The top waveform has no transition, which indicates a binary 0. In
the second waveform, the transition in the center of the timing window
indicates a binary 1. In the third waveform, there is an alternating binary
sequence. In the third and fourth waveforms it is more obvious that no
transition indicates a binary zero and the presence of a transition is a binary
one.
Figure summarizes a 100BASE-FX link and pinouts. A
fiber pair with either ST or SC connectors is most commonly used.
The separate
Transmit (Tx) and Receive (Rx) paths in 100BASE-FX optical fiber allow for
200-Mbps transmission.
The next page
will explain the Fast Ethernet architecture.
7.1
10-Mbps and 100-Mbps Ethernet
7.1.9
Fast Ethernet architecture
This page
describes the architecture of Fast Ethernet.
Fast Ethernet
links generally consist of a connection between a station and a hub or switch.
Hubs are considered multi-port repeaters and switches are considered multi-port
bridges. These are subject to the 100-m (328 ft) UTP media distance limitation.
A Class I
repeater may introduce up to 140 bit-times latency. Any repeater that changes
between one Ethernet implementation and another is a Class I repeater. A Class
II repeater is restricted to smaller timing delays, 92 bit times, because it
immediately repeats the incoming signal to all other ports without a
translation process. To achieve a smaller timing delay, Class II repeaters can
only connect to segment types that use the same signaling technique.
As with 10-Mbps
versions, it is possible to modify some of the architecture rules for 100-Mbps
versions. Modification of the architecture rules is strongly discouraged for
100BASE-TX. 100BASE-TX cable between Class II repeaters may not exceed 5 m (16
ft). Links that operate in half duplex are not uncommon in Fast Ethernet.
However, half duplex is undesirable because the signaling scheme is inherently
full duplex.
Figure shows architecture configuration cable distances.
100BASE-TX links can have unrepeated distances up to 100 m. Switches have made
this distance limitation less important. Most Fast Ethernet implementations are
switched.
This page
concludes this lesson. The next lesson will discuss Gigabit and 10-Gigabit
Ethernet. The first page describes 1000-Mbps Ethernet standards.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.1
1000-Mbps Ethernet
This page covers
the 1000-Mbps Ethernet or Gigabit Ethernet standards. These standards specify
both fiber and copper media for data transmissions. The 1000BASE-T standard, IEEE 802.3ab, uses
Category 5, or higher, balanced copper cabling. The 1000BASE-X standard, IEEE
802.3z, specifies 1 Gbps full duplex over optical fiber.
1000BASE-TX,
1000BASE-SX, and 1000BASE-LX use the same timing parameters, as shown in Figure
. They use a 1 ns, 0.000000001 of a second, or 1 billionth of a second bit
time. The Gigabit Ethernet frame has the same format as is used for 10 and
100-Mbps Ethernet. Some implementations of Gigabit Ethernet may use different
processes to convert frames to bits on the cable. Figure shows the Ethernet frame fields.
The differences
between standard Ethernet, Fast Ethernet and Gigabit Ethernet occur at the
physical layer. Due to the increased speeds of these newer standards, the
shorter duration bit times require special considerations. Since the bits are
introduced on the medium for a shorter duration and more often, timing is
critical. This high-speed transmission requires higher frequencies. This causes
the bits to be more susceptible to noise on copper media.
These issues
require Gigabit Ethernet to use two separate encoding steps. Data transmission
is more efficient when codes are used to represent the binary bit stream. The
encoded data provides synchronization, efficient usage of bandwidth, and
improved signal-to-noise ratio characteristics.
At the physical
layer, the bit patterns from the MAC layer are converted into symbols. The
symbols may also be control information such as start frame, end frame, and
idle conditions on a link. The frame is coded into control symbols and data
symbols to increase in network throughput.
Fiber-based
Gigabit Ethernet, or 1000BASE-X, uses 8B/10B encoding, which is similar to the
4B/5B concept. This is followed by the simple nonreturn to zero (NRZ) line
encoding of light on optical fiber. This encoding process is possible because
the fiber medium can carry higher bandwidth signals.
The next page
will discuss the 1000BASE-T standard.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.2
1000BASE-T
This page will
describe 1000BASE-T.
As Fast Ethernet
was installed to increase bandwidth to workstations, this began to create
bottlenecks upstream in the network. The 1000BASE-T standard, which is IEEE
802.3ab, was developed to provide additional bandwidth to help alleviate these
bottlenecks. It provided more throughput for devices 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 Category 5 copper cable that passes the Category
5e test. Most installed Category 5 cable can pass the Category 5e certification
if properly terminated. It is important for the 1000BASE-T standard to be
interoperable with 10BASE-T and 100BASE-TX.
Since Category 5e
cable can reliably carry up to 125 Mbps of traffic, 1000 Mbps or 1 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 requires complex circuitry that allows
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 Category 5e, or better, UTP.
That means 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 1-Gigabit 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. 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.
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 represents 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.
The next page
will introduce 1000BASE-SX and LX.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.3
1000BASE-SX and LX
This page will
discuss single-mode and multimode optical fiber.
The IEEE 802.3
standard recommends that Gigabit Ethernet over fiber be the preferred backbone
technology.
The timing, frame
format, and transmission are common to all versions of 1000 Mbps. Two
signal-encoding schemes are defined at the physical layer. The 8B/10B scheme is used for optical fiber
and shielded copper media, and the pulse amplitude modulation 5 (PAM5) is used
for UTP.
1000BASE-X uses 8B/10B
encoding converted to non-return to zero (NRZ) line encoding. 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. That is the
determination of whether a bit is a zero or a one is made by the level of the
signal rather than when the signal changes levels.
The NRZ signals
are then pulsed into the fiber using either short-wavelength or long-wavelength
light sources. The short-wavelength uses an 850 nm laser or LED source in
multimode optical fiber (1000BASE-SX). It is the lower-cost of the options but
has shorter distances. The long-wavelength 1310 nm laser source uses either
single-mode or multimode optical fiber (1000BASE-LX). Laser sources used with
single-mode fiber can achieve distances of up to 5000 meters. Because of the
length of time to completely turn the LED or laser on and off each time, the
light is pulsed using low and high power. A logic zero is represented by low
power, and a logic one by high power.
The Media Access
Control method treats the link as point-to-point. Since separate fibers are
used for transmitting (Tx) and receiving (Rx) the connection is inherently full
duplex. Gigabit Ethernet permits only a single repeater between two stations.
Figure is a 1000BASE Ethernet media
comparison chart.
The next page
describes the architecture of Gigabit Ethernet.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.4
Gigabit Ethernet architecture
This page will
discuss the architecture of Gigabit Ethernet.
The distance
limitations of full-duplex links are only limited by the medium, and not the
round-trip delay. Since most Gigabit Ethernet is switched, the values in
Figures and are the practical limits between devices.
Daisy-chaining, star, and extended star topologies are all allowed. The issue
then becomes one of logical topology and data flow, not timing or distance
limitations.
A 1000BASE-T UTP
cable is the same as 10BASE-T and 100BASE-TX cable, except that link
performance must meet the higher quality Category 5e or ISO Class D (2000)
requirements.
Modification of
the architecture rules is strongly discouraged for 1000BASE-T. At 100 meters,
1000BASE-T is operating close to the edge of the ability of the hardware 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.
It is recommended
that all links between a station and a hub or switch be configured for
Auto-Negotiation to permit the highest common performance. This will avoid
accidental misconfiguration of the other required parameters for proper Gigabit
Ethernet operation.
The next page
will discuss 10-Gigabit Ethernet.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.5
10-Gigabit Ethernet
This page will
describe 10-Gigabit Ethernet and compare it to other versions of Ethernet.
IEEE 802.3ae was
adapted to include 10 Gbps full-duplex transmission over fiber optic cable. The
basic similarities between 802.3ae and 802.3, the original Ethernet are
remarkable. This 10-Gigabit Ethernet (10GbE) is evolving for not only LANs, but
also MANs, and WANs.
With the frame
format and other Ethernet Layer 2 specifications compatible with previous
standards, 10GbE can provide increased bandwidth needs that are interoperable
with existing network infrastructure.
A major
conceptual change for Ethernet is emerging with 10GbE. Ethernet is
traditionally 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 with synchronous optical network (SONET) and synchronous
digital hierarchy (SDH) networks. Operation at 40 km distance makes 10GbE a
viable MAN technology. Compatibility with SONET/SDH networks operating up to
OC-192 speeds (9.584640 Gbps) make 10GbE a viable WAN technology. 10GbE may
also compete with ATM for certain applications.
To summarize, how
does 10GbE compare to other varieties of Ethernet?
- Frame format is the same,
allowing interoperability between all varieties of legacy, fast, gigabit,
and 10 gigabit, with no reframing or protocol conversions.
- Bit time is now 0.1 nanoseconds.
All other time variables scale accordingly.
- Since only full-duplex fiber
connections are used, CSMA/CD is not necessary.
- The IEEE 802.3 sublayers within
OSI Layers 1 and 2 are mostly preserved, with a few additions to
accommodate 40 km fiber links and interoperability with SONET/SDH
technologies.
- Flexible, efficient, reliable,
relatively low cost end-to-end Ethernet networks become possible.
- TCP/IP can run over LANs, MANs,
and WANs with one Layer 2 transport method.
The basic
standard governing CSMA/CD is IEEE 802.3. An IEEE 802.3 supplement, entitled
802.3ae, governs the 10GbE family. As is typical for new technologies, a
variety of implementations are being considered, including:
- 10GBASE-SR
– Intended for short distances over already-installed multimode fiber,
supports a range between 26 m to 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
– Support 10 km and 40 km over single-mode fiber
- 10GBASE-SW, 10GBASE-LW, and
10GBASE-EW – Known collectively as
10GBASE-W, intended to work with OC-192 synchronous transport module
SONET/SDH WAN equipment
The IEEE 802.3ae Task force and the 10-Gigabit Ethernet
Alliance (10 GEA) are working to standardize these emerging technologies.
10-Gbps Ethernet
(IEEE 802.3ae) was standardized in June 2002. It is a full-duplex protocol that
uses only 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 kilometers
(25 miles). Some discussions between IEEE members have begun that suggest the
possibility of standards for 40, 80, and even 100-Gbps Ethernet.
The next page
will discuss the architecture of 10-Gigabit Ethernet.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.6
10-Gigabit Ethernet architectures
This page
describes the 10-Gigabit Ethernet architectures.
As with the development
of Gigabit Ethernet, the increase in speed comes with extra requirements. The
shorter bit time duration because of increased speed requires special
considerations. For 10 GbE transmissions, each data bit duration is 0.1
nanosecond. This means there would be 1,000 GbE data bits in the same bit time
as one data bit in a 10-Mbps Ethernet data stream. Because of the short
duration of the 10 GbE data bit, it is often difficult to separate a data bit
from noise. 10 GbE data transmissions rely on exact bit timing to separate the
data from the effects of noise on the physical layer. This is the purpose of
synchronization.
In response to
these issues of synchronization, bandwidth, and Signal-to-Noise Ratio,
10-Gigabit Ethernet uses two separate encoding steps. By using codes to
represent the user data, transmission is made more efficient. The encoded data
provides synchronization, efficient usage of bandwidth, and improved
Signal-to-Noise Ratio characteristics.
Complex serial
bit streams are used for all versions of 10GbE except for 10GBASE-LX4, which
uses Wide Wavelength Division Multiplex (WWDM) to multiplex four bit
simultaneous bit streams as four wavelengths of light launched into the fiber
at one time.
Figure represents the particular case of using four
slightly different wavelength, laser sources. Upon receipt from the medium, the
optical signal stream is demultiplexed into four separate optical signal
streams. The four optical signal streams are then converted back into four
electronic bit streams as they travel in approximately the reverse process back
up through the sublayers to the MAC layer.
Currently, most
10GbE products are in the form of modules, or line cards, for addition to
high-end switches and routers. As the 10GbE technologies evolve, an increasing
diversity of signaling components can be expected. As optical technologies
evolve, improved transmitters and receivers will be incorporated into these
products, taking further advantage of modularity. All 10GbE varieties use
optical fiber media. Fiber types include 10µ single-mode Fiber, and 50µ and
62.5µ multimode fibers. A range of fiber attenuation and dispersion
characteristics is supported, but they limit operating distances.
Even though
support is limited to fiber optic media, some of the maximum cable lengths are
surprisingly short. No repeater is
defined for 10-Gigabit Ethernet since half duplex is explicitly not supported.
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. Due to dispersion of the signal
and other issues the light pulse becomes undecipherable beyond certain
distances.
The next page
will discuss the future of Ethernet.
7.2
Gigabit and 10-Gigabit Ethernet
7.2.7
Future of Ethernet
This page will
teach students about the future of Ethernet.
Ethernet has gone
through an evolution from Legacy —> Fast —> Gigabit —> MultiGigabit
technologies. While other LAN technologies are still in place (legacy
installations), Ethernet dominates new LAN installations. So much so that some
have referred to Ethernet as the LAN “dial tone”. Ethernet is now the standard
for horizontal, vertical, and inter-building connections. Recently developing
versions of Ethernet are blurring the distinction between LANs, MANs, and WANs.
While 1-Gigabit
Ethernet is now widely available and 10-Gigabit products becoming more
available, the IEEE and the 10-Gigabit Ethernet Alliance are working on 40,
100, or even 160 Gbps standards. The technologies that 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. The problem of
collisions with physical bus topologies of 10BASE5 and 10BASE2 and 10BASE-T and
100BASE-TX hubs is no longer common. Using UTP and optical fiber with separate
Tx and Rx paths, and the decreasing costs of switches make single shared media,
half-duplex media connections much less important.
The future of
networking media is three-fold:
- 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)
Copper and
wireless media have certain physical and practical limitations on the highest
frequency signals that can be transmitted. This is not a limiting factor for
optical fiber in the foreseeable future. The bandwidth limitations on optical
fiber are extremely large and are not yet being threatened. In fiber systems,
it is the electronics technology (such as emitters and detectors) and fiber
manufacturing processes that most limit the speed. Upcoming developments in
Ethernet are likely to be heavily weighted towards 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.
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 it is Ethernet, not ATM that is approaching this goal.
This page
concludes this lesson. The next page will summarize the main points from the
module.
Summary
This page
summarizes the topics discussed in this module.
Ethernet is a
technology that has increased in speed one thousand times, from 10 Mbps to
10,000 Mbps, in less than a decade. All forms of Ethernet share a similar frame
structure and this leads to excellent interoperability. Most Ethernet copper
connections are now switched full duplex, and the fastest copper-based Ethernet
is 1000BASE-T, or Gigabit Ethernet. 10 Gigabit Ethernet and faster are
exclusively optical fiber-based technologies.
10BASE5, 10BASE2,
and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features
of Legacy Ethernet are timing parameters, frame format, transmission process,
and a basic design rule.
Legacy Ethernet
encodes data on an electrical signal. The form of encoding used in 10 Mbps
systems is called Manchester encoding. Manchester encoding uses a change in
voltage to represent the binary numbers zero and one. An increase or decrease
in voltage during a timed period, called the bit period, determines the binary
value of the bit.
In addition to a
standard bit period, Ethernet standards set limits for slot time and interframe
spacing. Different types of media can affect transmission timing and timing
standards ensure interoperability. 10 Mbps Ethernet operates within the timing
limits offered by a series of no more than five segments separated by no more
than four repeaters.
A single thick
coaxial cable was the first medium used for Ethernet. 10BASE2, using a thinner
coax cable, was introduced in 1985. 10BASE-T, using twisted-pair copper wire,
was introduced in 1990. Because it used multiple wires 10BASE-T offered the
option of full-duplex signaling. 10BASE-T carries 10 Mbps of traffic in
half-duplex mode and 20 Mbps in full-duplex mode.
10BASE-T links
can have unrepeated distances up to 100 m. Beyond that network devices such as
repeaters, hub, bridges and switches are used to extend the scope of the LAN.
With the advent of switches, the 4-repeater rule is not so relevant. You can
extend the LAN indefinitely by daisy-chaining switches. Each switch-to-switch
connection, with maximum length of 100m, is essentially a point-to-point
connection without the media contention or timing issues of using repeaters and
hubs.
100-Mbps
Ethernet, also known as Fast Ethernet, can be implemented using twisted-pair
copper wire, as in 100BASE-TX, or fiber media, as in 100BASE-FX. 100 Mbps forms
of Ethernet can transmit 200 Mbps in full duplex.
Because the
higher frequency signals used in Fast Ethernet are more susceptible to noise,
two separate encoding steps are used by 100-Mbps Ethernet to enhance signal
integrity.
Gigabit Ethernet
over copper wire is accomplished by the following:
- Category 5e UTP cable and
careful improvements in electronics are used to boost 100 Mbps per wire
pair to 125 Mbps per wire pair.
- All four wire pairs instead of
just two. This allows 125 Mbps per wire pair, or 500 Mbps for the four
wire pairs.
- Sophisticated electronics allow
permanent collisions on each wire pair and run signals in full duplex,
doubling the 500 Mbps to 1000 Mbps.
On Gigabit Ethernet networks bit signals occur in one
tenth of the time of 100 Mbps networks and 1/100 of the time of 10 Mbps
networks. With signals occurring in less time the bits become more susceptible
to noise. The issue becomes how fast the network adapter or interface can
change voltage levels to signal bits and still be detected reliably one hundred
meters away at the receiving NIC or interface. At this speed encoding and
decoding data becomes even more complex.
The fiber
versions of Gigabit Ethernet, 1000BASE-SX and 1000BASE-LX offer the following
advantages: noise immunity, small size, and increased unrepeated distances and
bandwidth. The IEEE 802.3 standard recommends that Gigabit Ethernet over fiber
be the preferred backbone technology.
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