Ethernet
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The Origin of Ethernet
– In the late 1960s, the University of Hawaii developed a WAN called ALOHA,
that extended LAN technology across a larger geographical area. The
university had a large geographical area and they wanted to connect
computers that were spread throughout its campus. One of the key
features of the network that they designed was the use of CSMA/CD as the
access method. This early network was the foundation for today's Ethernet.
In 1972, Robert Metcalfe and David Boggs invented a cabling and signaling
scheme at the Xerox Palo Alto Research Center (PARC), and in 1975
introduced the first Ethernet product. The original version of Ethernet was
designed as a 2.94-Mbps system to connect over 100 computers on a 1
kilometer cable. Xerox Ethernet was so successful that Xerox, Intel
Corporation, and Digital Equipment Corporation drew up a standard for a
10-Mbps Ethernet. Today it is a specification describing a method for
computers and data systems to connect and share cabling. The Ethernet
specification performs the same functions as the OSI Physical and Data
Link layers of data communications. This design is the basis for the IEEE's
802.3 specification.
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Ethernet Features
– Ethernet is currently the most popular network architecture. This baseband
architecture uses a bus topology, usually transmits at 10 Mbps, and relies
on CSMA/CD to regulate traffic on the main cable segment.
– The Ethernet media is passive, which means it draws power from the
computer and thus will not fail unless the media is physically cut or
improperly terminated.
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Ethernet Basics
– The following list summarizes Ethernet features.
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Traditional topology
linear bus
Other topologies
star bus
Type of architecture
baseband
Access method
CSMA/CD
Specifications
IEEE 802.3
Transfer speed
10 Mbps or 100 Mbps
Cable types
thicknet, thinnet, UTP
The Ethernet Frame Format
– Ethernet breaks data down into packages in a format that is different from
the packet used in other networks. Ethernet breaks data down into frames.
A frame is a package of information transmitted as a single unit. An
Ethernet frame can be between 64 and 1,518 bytes long, but the Ethernet
frame itself uses at least 18 bytes; therefore, the data in an Ethernet frame
can be between 46 and 1,500 bytes long. Every frame contains control
information and follows the same basic organization.
– For example, the Ethernet II frame, used for TCP/IP, that gets transmitted across the
network consists of the sections listed in the following table.
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Frame field
Preamble
Destination and source
Type
IPX).
• Cyclical redundancy check (CRC)
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Description
Marks the start of the frame
The origin and destination addresses.
Used to identify the Network layer protocol (IP or
Error checking field to determine if the, frame arrived
without being corrupted
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The 10 Mbps IEEE Standards
– Four different 10 Mbps Ethernet topologies:
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10BaseT
10Base2
10Base5
10BaseFL
– 10BaseT
• In 1990, the IEEE committee published the 802.3 specification for running
Ethernet over twisted-pair wiring. 10BaseT (10 Mbps, baseband, over twisted-pair
cable) is an Ethernet network that typically uses unshielded twisted-pair (UTP) to
connect computers. While 10BaseT normally uses UTP, shielded twisted-pair
(STP) will also work without changing any of the 10BaseT parameters. Most
networks of this type are configured in a star pattern but internally use a bus
signaling system like other Ethernet configurations. Typically, the hub of a
10BaseT network serves as a multiport repeater and often is located in a wiring
closet of the building. Each computer is located at the end point of a cable
connected to the hub. Each computer has two pairs of wire-one pair is used to
receive data and one pair is used to transmit data. The maximum length of a
10BaseT segment is 100 meters (328 feet). Repeaters can be used to extend this
maximum cable length. The minimum cable length between computers is 2.5
meters (about 8 feet). A 10BaseT LAN will serve 1,024 computers.
1OBaseT Summary
Category
Cable
Connectors
Transceiver
Transceiver to hub distance
Backbones for hubs
Total computers per LAN
without connectivity components
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Notes
Category 3, 4, or 5 UTP
RJ-45 at cable ends
Each computer needs one; some cards have
transceivers built in
100 meters maximum
Coaxial or fiber-optic to join a larger LAN
1024 by specification
10Base2
– This topology is called 10Base2 by the IEEE 802.3 specification because it
transmits at 10 Mbps over a baseband wire and can carry a signal roughly
two times 100 meters (the actual distance is 185 meters). This type of
network uses thin coaxial cable, or thinnet, which has a maximum segment
length of 185 meters. There is also a minimum cable length of at least 0.5
meters (20 inches). There is also a 30 computer maximum per 185 meter
segment. Thinnet cabling components include:
• BNC barrel connectors
• BNC T connectors
• BNC terminators
– Thinnet networks generally use a local bus topology. IEEE standards for
thinnet do not allow a transceiver cable to be used from the bus T connector
to a computer. Instead, a T connector fits directly on the network adapter
card.
– A BNC barrel connector may be used to connect thinnet cable segments
together, thus extending a length of cable. For example, if you need a
length of cable that is 30 feet long, but all you have is a 25-foot length and a
5-foot length of thinnet cable, a BNC barrel connector can be used to join
the two cable segments together. However, the use of barrel connectors
should be kept to a minimum because each connection in the cable reduces
the signal quality.
– A thinnet network is an economical way to support a small department or
workgroup. The cable used for this type of network is:
• Relatively inexpensive.
• Easy to install.
• Easy to configure.
– A single thinnet network can support a maximum of 30 nodes (computers
and repeaters) per cable segment as per the IEEE 802.3 specification.
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The 5-4-3 Rule
– A thinnet network can combine as many as five cable segments connected
by four repeaters, but only three segments can have stations attached.
Thus, two segments are untapped and are often referred to as interrepeater links. This is known as the 5-4-3 rule.
– Because normal Ethernet limits would be too confining for a large business,
repeaters can be used to join Ethernet segments and extend the network to
a total length of 925 meters.
1OBase2 Summary
Category
Maximum segment length
Connection to network adapter card
Trunk segments and repeaters
Computers per segment
Segments that can have computers
Maximum total network length
Maximum number of computers per
network without connectivity components
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Notes
185 meters (607 feet)
BNC T connector
Five segments may be joined using four
repeaters
30 computers per segment by specification
Three of the five segments may be
populated
925 meters (3,035 feet)
1,024 by specification
10Base5
– The IEEE specification for this topology is 10 Mbps, baseband, and 500meter (five 100-meter) segments. It is also called standard Ethernet. This
topology makes use of thick coaxial, or thicknet. Thicknet generally uses a
bus topology and can support as many as 100 nodes (stations, repeaters,
and so on) per backbone segment. The backbone, or trunk segment, is the
main cable from which transceiver cables are connected to stations and
repeaters. A thicknet segment can be 500 meters long for a total network
length of 2,500 meters (8,200 feet). The distances and tolerances for
thicknet are greater than those for thinnet.
– The thicknet cabling components include:
• Transceivers.
– Transceivers (transmit and receive) provide communications between the computer and
the main LAN cable and are located in the vampire taps attached to the cable.
• Transceiver cables.
– The transceiver cable (drop cable) connects the transceiver to the network adapter card.
• DIX or AUI connector.
– This is the connector on the transceiver cable.
• N-series connectors including N-series barrel connectors, and N-series
terminators.
– The thicknet components work the same way that the thinnet components
do.
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The 5-4-3 Rule in Thicknet
– One thicknet Ethernet network can have a maximum of five backbone
segments connected using repeaters (based on the IEEE 802.3
specification), of which three can accommodate computers. The length of
the transceiver cables is not used to measure the distance supported on the
thicknet cable; only the end-to-end length of the thicknet cable segment
itself is used.
– Between connections, the minimum thicknet cable segment is 2.5 meters
(about 8 feet). This measurement excludes transceiver cables. Thicknet
was designed to support a backbone for a large department or an entire
building.
10Base5 Summary
Category
Maximum segment length
Transceivers
Maximum computer-to-transceiver distance
Minimum distance between transceivers
Trunk segments and repeaters
Segments that can have computers
Maximum total length of joined segments
Maximum number of computers per
segment
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Notes
500 meters
Connected to the segment (in the tap)
50 meters (164 feet)
2.5 meters (8 feet)
Five segments may be joined using four
repeaters
Three of the five segments may be
populated
2,500 meters (8,200 feet)
100 by specification
Combining Thicknet and Thinnet
– It is common for larger networks to combine thick and thin Ethernet.
Thicknet is good for backbones with thinnet used for branch segments.
What this means is that the thicknet cable is the main cable covering the
long distances. You may remember that thicknet has a larger copper core
and can therefore carry signals for a longer distance than thinnet. The
transceiver attaches to the thicknet cable and the transceiver cable's AUI
connector plugs into a repeater. The branching segments of thinnet plug
into the repeater and connect the computers to the network.
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10BaseFL
– The IEEE committee published a specification for running Ethernet over
fiber-optic cable. 10BaseFL (10 Mbps, baseband, over fiber-optic cable) is
an Ethernet network that typically uses fiber-optic cable to connect
computers and repeaters. The primary reason for using 10BaseFL is for
long cable runs between repeaters, such as between buildings. The
maximum distance for a 10BaseFL segment is 2000 meters.
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The 100 Mbps IEEE Standard
– New Ethernet standards are pushing the traditional Ethernet limits beyond
the original 10 Mbps. These new capabilities are being developed to handle
such high-bandwidth applications as:
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CAD (computer aided design)
CAM (computer aided manufacturing)
Video
Imaging and document storage
– Two emerging Ethernet standards that can meet the increased demands
are:
• 100BaseVG-AnyLAN Ethernet
• 100BaseX Ethernet (Fast Ethernet)
– Both Fast Ethernet and 100BaseVG-AnyLAN are about five to 10 times
faster than standard Ethernet. They are also compatible with existing
10BaseT cabling systems. This means they will allow for Plug and Play
upgrades from existing 10BaseT installations.
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100VG-AnyLAN
– 100VG (Voice Grade) AnyLAN is an emerging networking technology that
combines elements of both Ethernet and Token Ring. Originally developed
by Hewlett-Packard, it is currently being refined and ratified by the IEEE
802.12 committee. The 802.12 specification is a standard for transmitting
802.3 Ethernet frames and 802.5 Token Ring packets. This technology goes
by any of the following names, all of which refer to the same type of
network:
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100VG-AnyLAN
100BaseVG
VG
AnyLAN
Specifications
– Some of the current 100VG-AnyLAN specifications include:
• A minimum data rate of 100 Mbps.
• Ability to support a cascaded star topology over Category 3, 4, and 5 twisted-pair
and fiber-optic cable.
• The demand priority access method which allows for two priority levels (low and
high).
• Ability to support an option for filtering individually addressed frames at the hub to
enhance privacy.
• Support for both Ethernet frames and Token Ring packets.
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Topology
– A 100VG-AnyLAN network is built on a star topology with all computers
attached to a hub. The network can be expanded by adding child hubs to
the central hub. The child hubs act as computers to their parent hubs. The
parent hubs control transmission of computers attached to their children.
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Considerations
– This topology requires its own hubs and cards. Also, the cable distances of
100BaseVG are limited when compared to 10BaseT and other
implementations of Ethernet. The two longest cables from the 100BaseT
hub to a computer cannot exceed 250 meters. Extending this requires
special equipment used to expand the size of a LAN. These cable length
limits mean that 100BaseT will require more wiring closets than 10BaseT.
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100BaseX Ethernet
– This standard, sometimes called Fast Ethernet, is an extension to the
existing Ethernet standard. It runs on UTP Category 5 data-grade cable
and uses CSMA/CD in a star wired bus, similar to 10BaseT where all cables
are attached to a hub. Media Specifications 100BaseX incorporates three
media specifications:
• 100BaseT4 (4-pair Category 3, 4, or 5 UTP)
• 100BaseTX (2-pair Category 5 UTP or STP)
• 100BaseFX (2-strand fiber-optic cable)
– These media are described further in the following table.
Value
100
Base
T4
Represents
Transmission speed
Signal type
Cable type
TX
Cable type
FX
Cable type
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Actual meaning
100 Mbps or 100 megabits per second
Baseband
Indicates twisted-pair cable using four
telephone-grade pairs
Indicates twisted-pair cable using two
data-grade pairs
Indicates fiber-optic link using two
strands of fiber-optic cable
Performance Considerations
– Ethernet can use several communication protocols including TCP/IP, which
works well in the UNIX environment. This makes Ethernet a favorite in the
scientific and academic communities.
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Segmentation
– Ethernet performance can be improved by dividing a crowded segment into
two less-populated segments and joining them with either a bridge or a
router. This reduces traffic on each segment. Because there are fewer
computers attempting to transmit onto the segment, access time improves.
Segment division is a good tactic if large numbers of new users are joining
the network or new, high-bandwidth applications such as database or video
program are being added to the network.
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Network Operating Systems on Ethernet
– Ethernet will work with most popular network operating systems.